[PATCH] c++: ICE with template code in constexpr [PR104284]

[PATCH] c++: ICE with template code in constexpr [PR104284]

[Marek Polacek]

Since r9-6073 cxx_eval_store_expression preevaluates the value to
be stored, and that revealed a crash where a template code (here,
code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.

It happens because we're performing build_vec_init while processing
a template, which calls get_temp_regvar which creates an INIT_EXPR.
This INIT_EXPR's RHS contains an rvalue conversion so we create an
IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
which, with -fchecking=2, calls fold_non_dependent_expr.  At this
point the expression still has an IMPLICIT_CONV_EXPR, which ought to
be handled in instantiate_non_dependent_expr_internal.  However,
tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
tsubst_copy which does nothing when args is null.  So we fail to
replace the IMPLICIT_CONV_EXPR and ICE.

Eliding the IMPLICIT_CONV_EXPR in this particular case would be too
risky, so we could do

  if (TREE_CODE (t) == INIT_EXPR)
    t = TREE_OPERAND (t, 1);

in fold_non_dependent_expr, but that feels too ad hoc.  So it might
make sense to actually take care of INIT_EXPR in tsubst_c_and_b.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?

	PR c++/104284

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy_and_build): Handle INIT_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-104284.C: New test.
---
 gcc/cp/pt.cc                                  |  8 ++++++++
 gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C | 17 +++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index f7ee33a6dfd..e8920f98e4d 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -21289,6 +21289,14 @@ tsubst_copy_and_build (tree t,
 	 with constant operands.  */
       RETURN (t);
 
+    case INIT_EXPR:
+      {
+	tree op0 = RECUR (TREE_OPERAND (t, 0));
+	tree op1 = RECUR (TREE_OPERAND (t, 1));
+	RETURN (build2_loc (input_location, INIT_EXPR, TREE_TYPE (op0),
+			    op0, op1));
+      }
+
     case NON_LVALUE_EXPR:
     case VIEW_CONVERT_EXPR:
       if (location_wrapper_p (t))
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
new file mode 100644
index 00000000000..f60033069e4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
@@ -0,0 +1,17 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+auto x = [](auto) { constexpr S s[]{{}}; };
+
+template<class>
+constexpr void gn ()
+{
+  constexpr S s[]{{}};
+}
+
+static_assert ((gn<int>(), true), "");

base-commit: b5417a0ba7e26bec2abf05cad6c6ef840a9be41c
-- 
2.35.1

Re: [PATCH] c++: ICE with template code in constexpr [PR104284]

[Marek Polacek]

On Thu, Mar 10, 2022 at 05:04:59PM -0500, Marek Polacek via Gcc-patches wrote:
> Since r9-6073 cxx_eval_store_expression preevaluates the value to
> be stored, and that revealed a crash where a template code (here,
> code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
> 
> It happens because we're performing build_vec_init while processing
> a template, which calls get_temp_regvar which creates an INIT_EXPR.
> This INIT_EXPR's RHS contains an rvalue conversion so we create an
> IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
> INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
> which, with -fchecking=2, calls fold_non_dependent_expr.  At this
> point the expression still has an IMPLICIT_CONV_EXPR, which ought to
> be handled in instantiate_non_dependent_expr_internal.  However,
> tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
> tsubst_copy which does nothing when args is null.  So we fail to
> replace the IMPLICIT_CONV_EXPR and ICE.

Forgot to mention: without -fchecking=2 there's no problem because
digest_init will subst the IMPLICIT_CONV_EXPR:

#0  tsubst_copy_and_build (t=<implicit_conv_expr 0x7fffea4a9f20>, args=<tree 0x0>, complain=3, 
    in_decl=<tree 0x0>, function_p=false, integral_constant_expression_p=true)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:20063
#1  0x0000000000de1ae1 in instantiate_non_dependent_expr_internal (expr=<implicit_conv_expr 0x7fffea4a9f20>, 
    complain=3) at /home/mpolacek/src/gcc/gcc/cp/pt.cc:6358
#2  0x0000000000b702d4 in fold_non_dependent_expr_template (t=<implicit_conv_expr 0x7fffea4a9f20>, 
    complain=3, manifestly_const_eval=false, object=<tree 0x0>)
    at /home/mpolacek/src/gcc/gcc/cp/constexpr.cc:8050
#3  0x0000000000b706f0 in fold_non_dependent_init (t=<implicit_conv_expr 0x7fffea4a9f20>, complain=3, 
    manifestly_const_eval=false, object=<tree 0x0>) at /home/mpolacek/src/gcc/gcc/cp/constexpr.cc:8143
#4  0x0000000000f08f4f in massage_init_elt (type=<record_type 0x7fffea4b29d8 S>, 
    init=<implicit_conv_expr 0x7fffea4a9f20>, nested=0, flags=257, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck2.cc:1437
#5  0x0000000000f0949c in process_init_constructor_array (type=<array_type 0x7fffea4c9348>, 
    init=<constructor 0x7fffea4bfb10>, nested=0, flags=257, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck2.cc:1502
#6  0x0000000000f0aec1 in process_init_constructor (type=<array_type 0x7fffea4c9348>, 
    init=<constructor 0x7fffea4bfb10>, nested=0, flags=257, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck2.cc:1917
#7  0x0000000000f0890c in digest_init_r (type=<array_type 0x7fffea4c9348>, 
    init=<constructor 0x7fffea4bfb10>, nested=0, flags=257, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck2.cc:1324
#8  0x0000000000f08b1b in digest_init_flags (type=<array_type 0x7fffea4c9348>, 
    init=<constructor 0x7fffea4bfb10>, flags=257, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck2.cc:1370
#9  0x0000000000f06815 in store_init_value (decl=<var_decl 0x7fffea357cf0 s>, 
    init=<constructor 0x7fffea4bfb10>, cleanups=0x7fffffffba68, flags=257)
    at /home/mpolacek/src/gcc/gcc/cp/typeck2.cc:842
#10 0x0000000000bf56cc in check_initializer (decl=<var_decl 0x7fffea357cf0 s>, 
    init=<constructor 0x7fffea4bfb10>, flags=257, cleanups=0x7fffffffba68)
    at /home/mpolacek/src/gcc/gcc/cp/decl.cc:7337
#11 0x0000000000bfa8df in cp_finish_decl (decl=<var_decl 0x7fffea357cf0 s>, 
    init=<constructor 0x7fffea4bfac8>, init_const_expr_p=true, asmspec_tree=<tree 0x0>, flags=1)
    at /home/mpolacek/src/gcc/gcc/cp/decl.cc:8174
 
> Eliding the IMPLICIT_CONV_EXPR in this particular case would be too
> risky, so we could do
> 
>   if (TREE_CODE (t) == INIT_EXPR)
>     t = TREE_OPERAND (t, 1);
> 
> in fold_non_dependent_expr, but that feels too ad hoc.  So it might
> make sense to actually take care of INIT_EXPR in tsubst_c_and_b.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
> 
> 	PR c++/104284
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (tsubst_copy_and_build): Handle INIT_EXPR.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-104284.C: New test.
> ---
>  gcc/cp/pt.cc                                  |  8 ++++++++
>  gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C | 17 +++++++++++++++++
>  2 files changed, 25 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index f7ee33a6dfd..e8920f98e4d 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -21289,6 +21289,14 @@ tsubst_copy_and_build (tree t,
>  	 with constant operands.  */
>        RETURN (t);
>  
> +    case INIT_EXPR:
> +      {
> +	tree op0 = RECUR (TREE_OPERAND (t, 0));
> +	tree op1 = RECUR (TREE_OPERAND (t, 1));
> +	RETURN (build2_loc (input_location, INIT_EXPR, TREE_TYPE (op0),
> +			    op0, op1));
> +      }
> +
>      case NON_LVALUE_EXPR:
>      case VIEW_CONVERT_EXPR:
>        if (location_wrapper_p (t))
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
> new file mode 100644
> index 00000000000..f60033069e4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
> @@ -0,0 +1,17 @@
> +// PR c++/104284
> +// { dg-do compile { target c++14 } }
> +// { dg-additional-options "-fchecking=2" }
> +
> +struct S {
> +  char c{};
> +};
> +
> +auto x = [](auto) { constexpr S s[]{{}}; };
> +
> +template<class>
> +constexpr void gn ()
> +{
> +  constexpr S s[]{{}};
> +}
> +
> +static_assert ((gn<int>(), true), "");
> 
> base-commit: b5417a0ba7e26bec2abf05cad6c6ef840a9be41c
> -- 
> 2.35.1
> 

Marek

Re: [PATCH] c++: ICE with template code in constexpr [PR104284]

[Jason Merrill]

On 3/10/22 18:04, Marek Polacek wrote:
> Since r9-6073 cxx_eval_store_expression preevaluates the value to
> be stored, and that revealed a crash where a template code (here,
> code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
> 
> It happens because we're performing build_vec_init while processing
> a template

Hmm, that seems like the bug.  Where's that call coming from?

> which calls get_temp_regvar which creates an INIT_EXPR.
> This INIT_EXPR's RHS contains an rvalue conversion so we create an
> IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
> INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
> which, with -fchecking=2, calls fold_non_dependent_expr.  At this
> point the expression still has an IMPLICIT_CONV_EXPR, which ought to
> be handled in instantiate_non_dependent_expr_internal.  However,
> tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
> tsubst_copy which does nothing when args is null.  So we fail to
> replace the IMPLICIT_CONV_EXPR and ICE.
> 
> Eliding the IMPLICIT_CONV_EXPR in this particular case would be too
> risky, so we could do
> 
>    if (TREE_CODE (t) == INIT_EXPR)
>      t = TREE_OPERAND (t, 1);
> 
> in fold_non_dependent_expr, but that feels too ad hoc.  So it might
> make sense to actually take care of INIT_EXPR in tsubst_c_and_b.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
> 
> 	PR c++/104284
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (tsubst_copy_and_build): Handle INIT_EXPR.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-104284.C: New test.
> ---
>   gcc/cp/pt.cc                                  |  8 ++++++++
>   gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C | 17 +++++++++++++++++
>   2 files changed, 25 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index f7ee33a6dfd..e8920f98e4d 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -21289,6 +21289,14 @@ tsubst_copy_and_build (tree t,
>   	 with constant operands.  */
>         RETURN (t);
>   
> +    case INIT_EXPR:
> +      {
> +	tree op0 = RECUR (TREE_OPERAND (t, 0));
> +	tree op1 = RECUR (TREE_OPERAND (t, 1));
> +	RETURN (build2_loc (input_location, INIT_EXPR, TREE_TYPE (op0),
> +			    op0, op1));
> +      }
> +
>       case NON_LVALUE_EXPR:
>       case VIEW_CONVERT_EXPR:
>         if (location_wrapper_p (t))
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
> new file mode 100644
> index 00000000000..f60033069e4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284.C
> @@ -0,0 +1,17 @@
> +// PR c++/104284
> +// { dg-do compile { target c++14 } }
> +// { dg-additional-options "-fchecking=2" }
> +
> +struct S {
> +  char c{};
> +};
> +
> +auto x = [](auto) { constexpr S s[]{{}}; };
> +
> +template<class>
> +constexpr void gn ()
> +{
> +  constexpr S s[]{{}};
> +}
> +
> +static_assert ((gn<int>(), true), "");
> 
> base-commit: b5417a0ba7e26bec2abf05cad6c6ef840a9be41c

[PATCH v2] c++: ICE with template code in constexpr [PR104284]

[Marek Polacek]

On Fri, Mar 11, 2022 at 06:46:42PM -0500, Jason Merrill wrote:
> On 3/10/22 18:04, Marek Polacek wrote:
> > Since r9-6073 cxx_eval_store_expression preevaluates the value to
> > be stored, and that revealed a crash where a template code (here,
> > code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
> > 
> > It happens because we're performing build_vec_init while processing
> > a template
> 
> Hmm, that seems like the bug.  Where's that call coming from?

From build_aggr_init.  So we're handling e.g.

  template<class>
  constexpr void g () {
    constexpr S s2[]{{'a'}};
  }

cp_finish_decl (decl=s2, init={{'a'}}) sees we're in processing_template_decl,
but also that we have a constexpr var which is not dependent, nor is its
initializer:

      else if (init
               && (init_const_expr_p || DECL_DECLARED_CONSTEXPR_P (decl))
               && !TYPE_REF_P (type)
               && decl_maybe_constant_var_p (decl)
               && !(dep_init = value_dependent_init_p (init)))
        {
          /* This variable seems to be a non-dependent constant, so process
             its initializer.  If check_initializer returns non-null the
             initialization wasn't constant after all.  */
          tree init_code;
          cleanups = make_tree_vector ();
          init_code = check_initializer (decl, init, flags, &cleanups);

so we call check_initializer, where we go down this path:

  init_code = build_aggr_init_full_exprs (decl, init, flags);

build_aggr_init sees that the type of 's2' is ARRAY_TYPE, so it calls
build_vec_init.

I now recall that we've discussed build_vec_init in a template in the
past, for example in the context of c++/93676.  So I agree we ought to
make an effort to avoid calling build_vec_init in a template.  Perhaps
like this: use an INIT_EXPR.  With that, we should call build_vec_init
if needed while instantiating.  Does that make any sense?

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
Since r9-6073 cxx_eval_store_expression preevaluates the value to
be stored, and that revealed a crash where a template code (here,
code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.

It happens because we're performing build_vec_init while processing
a template, which calls get_temp_regvar which creates an INIT_EXPR.
This INIT_EXPR's RHS contains an rvalue conversion so we create an
IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
which, with -fchecking=2, calls fold_non_dependent_expr.  At this
point the expression still has an IMPLICIT_CONV_EXPR, which ought to
be handled in instantiate_non_dependent_expr_internal.  However,
tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
tsubst_copy which does nothing when args is null.  So we fail to
replace the IMPLICIT_CONV_EXPR and ICE.

The problem is that we call build_vec_init in a template in the
first place.  It should work to create an INIT_EXPR in a template
and only perform build_vec_init when instantiating.

	PR c++/104284

gcc/cp/ChangeLog:

	* init.cc (build_aggr_init): Don't call build_vec_init in
	a template, create an INIT_EXPR instead.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-104284-1.C: New test.
	* g++.dg/cpp1y/constexpr-104284-2.C: New test.
	* g++.dg/cpp1y/constexpr-104284-3.C: New test.
	* g++.dg/cpp1y/constexpr-104284-4.C: New test.
---
 gcc/cp/init.cc                                | 11 +++---
 .../g++.dg/cpp1y/constexpr-104284-1.C         | 34 ++++++++++++++++++
 .../g++.dg/cpp1y/constexpr-104284-2.C         | 33 +++++++++++++++++
 .../g++.dg/cpp1y/constexpr-104284-3.C         | 33 +++++++++++++++++
 .../g++.dg/cpp1y/constexpr-104284-4.C         | 35 +++++++++++++++++++
 5 files changed, 142 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C

diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index 7575597c8fd..58e66adbfe1 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -2006,10 +2006,13 @@ build_aggr_init (tree exp, tree init, int flags, tsubst_flags_t complain)
 	    }
 	}
 
-      stmt_expr = build_vec_init (exp, NULL_TREE, init,
-				  /*explicit_value_init_p=*/false,
-				  from_array,
-                                  complain);
+      /* build_vec_init is not meant to be used in templates.  */
+      if (processing_template_decl)
+	stmt_expr = build2 (INIT_EXPR, itype, exp, init);
+      else
+	stmt_expr = build_vec_init (exp, NULL_TREE, init,
+				    /*explicit_value_init_p=*/false,
+				    from_array, complain);
       TREE_READONLY (exp) = was_const;
       TREE_THIS_VOLATILE (exp) = was_volatile;
       TREE_TYPE (exp) = type;
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
new file mode 100644
index 00000000000..809c26a6161
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
@@ -0,0 +1,34 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+auto x1 = [](auto) { constexpr S s[]{{}}; };
+auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
+#if __cpp_designated_initializers >= 201707L
+auto x3 = [](auto) { constexpr S s[]{{.c = 'a'}}; };
+#endif
+auto x4 = [](auto) { constexpr S s[]{'a'}; };
+auto x5 = [](auto) { constexpr S s[]{{{}}}; };
+
+template<class>
+constexpr void g ()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].c == '\0', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].c == 'a', "");
+#if __cpp_designated_initializers >= 201707L
+  constexpr S s3[]{{.c = 'a'}};
+  static_assert(s3[0].c == 'a', "");
+#endif
+  constexpr S s4[]{'a'};
+  static_assert(s4[0].c == 'a', "");
+  constexpr S s5[]{{{}}};
+  static_assert(s5[0].c == '\0', "");
+}
+
+static_assert ((g<int>(), true), "");
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
new file mode 100644
index 00000000000..704d37de129
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
@@ -0,0 +1,33 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char a;
+  constexpr S() : a{'a'} { }
+  constexpr S(char a_) : a{a_} { }
+};
+
+auto x1 = [](auto) { constexpr S s[]{{}}; };
+auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
+auto x3 = [](auto) { constexpr S s[]{'a'}; };
+auto x4 = [](auto) { constexpr S s[]{{{}}}; };
+
+template<typename>
+constexpr void g()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].a == 'a', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].a == 'a', "");
+  constexpr S s3[]{'a'};
+  static_assert(s3[0].a == 'a', "");
+  constexpr S s4[]{{{}}};
+  static_assert(s4[0].a == '\0', "");
+}
+
+void
+f ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
new file mode 100644
index 00000000000..6f23b255f9c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
@@ -0,0 +1,33 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+// Like constexpr-104284.C, but the function template is not
+// constexpr.  In that case, we were still calling build_vec_init
+// in a template, just not crashing.
+
+struct S {
+  char c{};
+};
+
+template<class>
+void g ()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].c == '\0', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].c == 'a', "");
+#if __cpp_designated_initializers >= 201707L
+  constexpr S s3[]{{.c = 'a'}};
+  static_assert(s3[0].c == 'a', "");
+#endif
+  constexpr S s4[]{'a'};
+  static_assert(s4[0].c == 'a', "");
+  constexpr S s5[]{{{}}};
+  static_assert(s5[0].c == '\0', "");
+}
+
+void
+f ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
new file mode 100644
index 00000000000..a99d3255a47
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
@@ -0,0 +1,35 @@
+// PR c++/104284
+// { dg-do run { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+template<class>
+constexpr void g ()
+{
+  S s1[]{{}};
+  if (s1[0].c != '\0')
+    __builtin_abort ();
+  S s2[]{{'a'}};
+  if (s2[0].c != 'a')
+    __builtin_abort ();
+#if __cpp_designated_initializers >= 201707L
+  S s3[]{{.c = 'a'}};
+  if (s3[0].c != 'a')
+    __builtin_abort ();
+#endif
+  S s4[]{'a'};
+  if (s4[0].c != 'a')
+    __builtin_abort ();
+  S s5[]{{{}}};
+  if (s5[0].c != '\0')
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  g<int>();
+}

base-commit: 0c016888ffd569c4b70722cf7df2efcc003f397b
-- 
2.35.1

Re: [PATCH v2] c++: ICE with template code in constexpr [PR104284]

[Jason Merrill]

On 3/18/22 17:55, Marek Polacek wrote:
> On Fri, Mar 11, 2022 at 06:46:42PM -0500, Jason Merrill wrote:
>> On 3/10/22 18:04, Marek Polacek wrote:
>>> Since r9-6073 cxx_eval_store_expression preevaluates the value to
>>> be stored, and that revealed a crash where a template code (here,
>>> code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
>>>
>>> It happens because we're performing build_vec_init while processing
>>> a template
>>
>> Hmm, that seems like the bug.  Where's that call coming from?
> 
>  From build_aggr_init.  So we're handling e.g.
> 
>    template<class>
>    constexpr void g () {
>      constexpr S s2[]{{'a'}};
>    }
> 
> cp_finish_decl (decl=s2, init={{'a'}}) sees we're in processing_template_decl,
> but also that we have a constexpr var which is not dependent, nor is its
> initializer:
> 
>        else if (init
>                 && (init_const_expr_p || DECL_DECLARED_CONSTEXPR_P (decl))
>                 && !TYPE_REF_P (type)
>                 && decl_maybe_constant_var_p (decl)
>                 && !(dep_init = value_dependent_init_p (init)))
>          {
>            /* This variable seems to be a non-dependent constant, so process
>               its initializer.  If check_initializer returns non-null the
>               initialization wasn't constant after all.  */
>            tree init_code;
>            cleanups = make_tree_vector ();
>            init_code = check_initializer (decl, init, flags, &cleanups);
> 
> so we call check_initializer, where we go down this path:
> 
>    init_code = build_aggr_init_full_exprs (decl, init, flags);
> 
> build_aggr_init sees that the type of 's2' is ARRAY_TYPE, so it calls
> build_vec_init.
> 
> I now recall that we've discussed build_vec_init in a template in the
> past, for example in the context of c++/93676.  So I agree we ought to
> make an effort to avoid calling build_vec_init in a template.  Perhaps
> like this: use an INIT_EXPR.  With that, we should call build_vec_init
> if needed while instantiating.  Does that make any sense?

Hmm.  If we do that, then we get back to

>           if (TREE_CODE (init_code) == INIT_EXPR)

in check_initializer, and pull out the same init again, and set 
LOOKUP_ALREADY_DIGESTED.  But I think that's wrong, we haven't digested 
it yet.

Maybe we could avoid entering the below block of check_initializer at 
all in this situation?

>       if (((type_build_ctor_call (type) || CLASS_TYPE_P (type))
>            && !(flags & LOOKUP_ALREADY_DIGESTED)
>            && !(init && BRACE_ENCLOSED_INITIALIZER_P (init)
>                 && CP_AGGREGATE_TYPE_P (type)
>                 && (CLASS_TYPE_P (type)

Maybe by adding || processing_template_decl here?

>                     || !TYPE_NEEDS_CONSTRUCTING (type)
>                     || type_has_extended_temps (type))))
>           || (DECL_DECOMPOSITION_P (decl) && TREE_CODE (type) == ARRAY_TYPE))

Jason

[PATCH v3] c++: ICE with template code in constexpr [PR104284]

[Marek Polacek]

On Thu, Mar 24, 2022 at 11:40:11AM -0400, Jason Merrill wrote:
> On 3/18/22 17:55, Marek Polacek wrote:
> > On Fri, Mar 11, 2022 at 06:46:42PM -0500, Jason Merrill wrote:
> > > On 3/10/22 18:04, Marek Polacek wrote:
> > > > Since r9-6073 cxx_eval_store_expression preevaluates the value to
> > > > be stored, and that revealed a crash where a template code (here,
> > > > code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
> > > > 
> > > > It happens because we're performing build_vec_init while processing
> > > > a template
> > > 
> > > Hmm, that seems like the bug.  Where's that call coming from?
> > 
> >  From build_aggr_init.  So we're handling e.g.
> > 
> >    template<class>
> >    constexpr void g () {
> >      constexpr S s2[]{{'a'}};
> >    }
> > 
> > cp_finish_decl (decl=s2, init={{'a'}}) sees we're in processing_template_decl,
> > but also that we have a constexpr var which is not dependent, nor is its
> > initializer:
> > 
> >        else if (init
> >                 && (init_const_expr_p || DECL_DECLARED_CONSTEXPR_P (decl))
> >                 && !TYPE_REF_P (type)
> >                 && decl_maybe_constant_var_p (decl)
> >                 && !(dep_init = value_dependent_init_p (init)))
> >          {
> >            /* This variable seems to be a non-dependent constant, so process
> >               its initializer.  If check_initializer returns non-null the
> >               initialization wasn't constant after all.  */
> >            tree init_code;
> >            cleanups = make_tree_vector ();
> >            init_code = check_initializer (decl, init, flags, &cleanups);
> > 
> > so we call check_initializer, where we go down this path:
> > 
> >    init_code = build_aggr_init_full_exprs (decl, init, flags);
> > 
> > build_aggr_init sees that the type of 's2' is ARRAY_TYPE, so it calls
> > build_vec_init.
> > 
> > I now recall that we've discussed build_vec_init in a template in the
> > past, for example in the context of c++/93676.  So I agree we ought to
> > make an effort to avoid calling build_vec_init in a template.  Perhaps
> > like this: use an INIT_EXPR.  With that, we should call build_vec_init
> > if needed while instantiating.  Does that make any sense?
> 
> Hmm.  If we do that, then we get back to
> 
> >           if (TREE_CODE (init_code) == INIT_EXPR)
> 
> in check_initializer, and pull out the same init again, and set
> LOOKUP_ALREADY_DIGESTED.  But I think that's wrong, we haven't digested it
> yet.

Yeah, that's probably no good :(
 
> Maybe we could avoid entering the below block of check_initializer at all in
> this situation?
> 
> >       if (((type_build_ctor_call (type) || CLASS_TYPE_P (type))
> >            && !(flags & LOOKUP_ALREADY_DIGESTED)
> >            && !(init && BRACE_ENCLOSED_INITIALIZER_P (init)
> >                 && CP_AGGREGATE_TYPE_P (type)
> >                 && (CLASS_TYPE_P (type)
> 
> Maybe by adding || processing_template_decl here?

That seems to work!  Thanks.

I've checked that we call build_vec_init when instantiating, so we
shouldn't be losing any of its effects.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
Since r9-6073 cxx_eval_store_expression preevaluates the value to
be stored, and that revealed a crash where a template code (here,
code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.

It happens because we're performing build_vec_init while processing
a template, which calls get_temp_regvar which creates an INIT_EXPR.
This INIT_EXPR's RHS contains an rvalue conversion so we create an
IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
which, with -fchecking=2, calls fold_non_dependent_expr.  At this
point the expression still has an IMPLICIT_CONV_EXPR, which ought to
be handled in instantiate_non_dependent_expr_internal.  However,
tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
tsubst_copy which does nothing when args is null.  So we fail to
replace the IMPLICIT_CONV_EXPR and ICE.

The problem is that we call build_vec_init in a template in the
first place.  We can avoid doing so by checking p_t_d before
calling build_aggr_init in check_initializer.

	PR c++/104284

gcc/cp/ChangeLog:

	* decl.cc (check_initializer): Don't call build_aggr_init in
	a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-104284-1.C: New test.
	* g++.dg/cpp1y/constexpr-104284-2.C: New test.
	* g++.dg/cpp1y/constexpr-104284-3.C: New test.
	* g++.dg/cpp1y/constexpr-104284-4.C: New test.
---
 gcc/cp/decl.cc                                |  4 +++
 .../g++.dg/cpp1y/constexpr-104284-1.C         | 34 ++++++++++++++++++
 .../g++.dg/cpp1y/constexpr-104284-2.C         | 33 +++++++++++++++++
 .../g++.dg/cpp1y/constexpr-104284-3.C         | 33 +++++++++++++++++
 .../g++.dg/cpp1y/constexpr-104284-4.C         | 35 +++++++++++++++++++
 5 files changed, 139 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 68741bbf5d2..69f60a6dc0f 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7332,6 +7332,10 @@ check_initializer (tree decl, tree init, int flags, vec<tree, va_gc> **cleanups)
 	   && !(init && BRACE_ENCLOSED_INITIALIZER_P (init)
 		&& CP_AGGREGATE_TYPE_P (type)
 		&& (CLASS_TYPE_P (type)
+		    /* The call to build_aggr_init below could end up
+		       calling build_vec_init, which may break when we
+		       are processing a template.  */
+		    || processing_template_decl
 		    || !TYPE_NEEDS_CONSTRUCTING (type)
 		    || type_has_extended_temps (type))))
 	  || (DECL_DECOMPOSITION_P (decl) && TREE_CODE (type) == ARRAY_TYPE))
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
new file mode 100644
index 00000000000..809c26a6161
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
@@ -0,0 +1,34 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+auto x1 = [](auto) { constexpr S s[]{{}}; };
+auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
+#if __cpp_designated_initializers >= 201707L
+auto x3 = [](auto) { constexpr S s[]{{.c = 'a'}}; };
+#endif
+auto x4 = [](auto) { constexpr S s[]{'a'}; };
+auto x5 = [](auto) { constexpr S s[]{{{}}}; };
+
+template<class>
+constexpr void g ()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].c == '\0', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].c == 'a', "");
+#if __cpp_designated_initializers >= 201707L
+  constexpr S s3[]{{.c = 'a'}};
+  static_assert(s3[0].c == 'a', "");
+#endif
+  constexpr S s4[]{'a'};
+  static_assert(s4[0].c == 'a', "");
+  constexpr S s5[]{{{}}};
+  static_assert(s5[0].c == '\0', "");
+}
+
+static_assert ((g<int>(), true), "");
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
new file mode 100644
index 00000000000..704d37de129
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
@@ -0,0 +1,33 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char a;
+  constexpr S() : a{'a'} { }
+  constexpr S(char a_) : a{a_} { }
+};
+
+auto x1 = [](auto) { constexpr S s[]{{}}; };
+auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
+auto x3 = [](auto) { constexpr S s[]{'a'}; };
+auto x4 = [](auto) { constexpr S s[]{{{}}}; };
+
+template<typename>
+constexpr void g()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].a == 'a', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].a == 'a', "");
+  constexpr S s3[]{'a'};
+  static_assert(s3[0].a == 'a', "");
+  constexpr S s4[]{{{}}};
+  static_assert(s4[0].a == '\0', "");
+}
+
+void
+f ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
new file mode 100644
index 00000000000..6f23b255f9c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
@@ -0,0 +1,33 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+// Like constexpr-104284.C, but the function template is not
+// constexpr.  In that case, we were still calling build_vec_init
+// in a template, just not crashing.
+
+struct S {
+  char c{};
+};
+
+template<class>
+void g ()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].c == '\0', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].c == 'a', "");
+#if __cpp_designated_initializers >= 201707L
+  constexpr S s3[]{{.c = 'a'}};
+  static_assert(s3[0].c == 'a', "");
+#endif
+  constexpr S s4[]{'a'};
+  static_assert(s4[0].c == 'a', "");
+  constexpr S s5[]{{{}}};
+  static_assert(s5[0].c == '\0', "");
+}
+
+void
+f ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
new file mode 100644
index 00000000000..a99d3255a47
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
@@ -0,0 +1,35 @@
+// PR c++/104284
+// { dg-do run { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+template<class>
+constexpr void g ()
+{
+  S s1[]{{}};
+  if (s1[0].c != '\0')
+    __builtin_abort ();
+  S s2[]{{'a'}};
+  if (s2[0].c != 'a')
+    __builtin_abort ();
+#if __cpp_designated_initializers >= 201707L
+  S s3[]{{.c = 'a'}};
+  if (s3[0].c != 'a')
+    __builtin_abort ();
+#endif
+  S s4[]{'a'};
+  if (s4[0].c != 'a')
+    __builtin_abort ();
+  S s5[]{{{}}};
+  if (s5[0].c != '\0')
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  g<int>();
+}

base-commit: 346ab5a54a831ad9c78afcbd8dfe98e0e07e3070
-- 
2.35.1

Re: [PATCH v3] c++: ICE with template code in constexpr [PR104284]

[Jason Merrill]

On 3/24/22 17:53, Marek Polacek wrote:
> On Thu, Mar 24, 2022 at 11:40:11AM -0400, Jason Merrill wrote:
>> On 3/18/22 17:55, Marek Polacek wrote:
>>> On Fri, Mar 11, 2022 at 06:46:42PM -0500, Jason Merrill wrote:
>>>> On 3/10/22 18:04, Marek Polacek wrote:
>>>>> Since r9-6073 cxx_eval_store_expression preevaluates the value to
>>>>> be stored, and that revealed a crash where a template code (here,
>>>>> code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
>>>>>
>>>>> It happens because we're performing build_vec_init while processing
>>>>> a template
>>>>
>>>> Hmm, that seems like the bug.  Where's that call coming from?
>>>
>>>   From build_aggr_init.  So we're handling e.g.
>>>
>>>     template<class>
>>>     constexpr void g () {
>>>       constexpr S s2[]{{'a'}};
>>>     }
>>>
>>> cp_finish_decl (decl=s2, init={{'a'}}) sees we're in processing_template_decl,
>>> but also that we have a constexpr var which is not dependent, nor is its
>>> initializer:
>>>
>>>         else if (init
>>>                  && (init_const_expr_p || DECL_DECLARED_CONSTEXPR_P (decl))
>>>                  && !TYPE_REF_P (type)
>>>                  && decl_maybe_constant_var_p (decl)
>>>                  && !(dep_init = value_dependent_init_p (init)))
>>>           {
>>>             /* This variable seems to be a non-dependent constant, so process
>>>                its initializer.  If check_initializer returns non-null the
>>>                initialization wasn't constant after all.  */
>>>             tree init_code;
>>>             cleanups = make_tree_vector ();
>>>             init_code = check_initializer (decl, init, flags, &cleanups);
>>>
>>> so we call check_initializer, where we go down this path:
>>>
>>>     init_code = build_aggr_init_full_exprs (decl, init, flags);
>>>
>>> build_aggr_init sees that the type of 's2' is ARRAY_TYPE, so it calls
>>> build_vec_init.
>>>
>>> I now recall that we've discussed build_vec_init in a template in the
>>> past, for example in the context of c++/93676.  So I agree we ought to
>>> make an effort to avoid calling build_vec_init in a template.  Perhaps
>>> like this: use an INIT_EXPR.  With that, we should call build_vec_init
>>> if needed while instantiating.  Does that make any sense?
>>
>> Hmm.  If we do that, then we get back to
>>
>>>            if (TREE_CODE (init_code) == INIT_EXPR)
>>
>> in check_initializer, and pull out the same init again, and set
>> LOOKUP_ALREADY_DIGESTED.  But I think that's wrong, we haven't digested it
>> yet.
> 
> Yeah, that's probably no good :(
>   
>> Maybe we could avoid entering the below block of check_initializer at all in
>> this situation?
>>
>>>        if (((type_build_ctor_call (type) || CLASS_TYPE_P (type))
>>>             && !(flags & LOOKUP_ALREADY_DIGESTED)
>>>             && !(init && BRACE_ENCLOSED_INITIALIZER_P (init)
>>>                  && CP_AGGREGATE_TYPE_P (type)
>>>                  && (CLASS_TYPE_P (type)
>>
>> Maybe by adding || processing_template_decl here?
> 
> That seems to work!  Thanks.
> 
> I've checked that we call build_vec_init when instantiating, so we
> shouldn't be losing any of its effects.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> Since r9-6073 cxx_eval_store_expression preevaluates the value to
> be stored, and that revealed a crash where a template code (here,
> code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.
> 
> It happens because we're performing build_vec_init while processing
> a template, which calls get_temp_regvar which creates an INIT_EXPR.
> This INIT_EXPR's RHS contains an rvalue conversion so we create an
> IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
> INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
> which, with -fchecking=2, calls fold_non_dependent_expr.  At this
> point the expression still has an IMPLICIT_CONV_EXPR, which ought to
> be handled in instantiate_non_dependent_expr_internal.  However,
> tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
> tsubst_copy which does nothing when args is null.  So we fail to
> replace the IMPLICIT_CONV_EXPR and ICE.
> 
> The problem is that we call build_vec_init in a template in the
> first place.  We can avoid doing so by checking p_t_d before
> calling build_aggr_init in check_initializer.
> 
> 	PR c++/104284
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (check_initializer): Don't call build_aggr_init in
> 	a template.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-104284-1.C: New test.
> 	* g++.dg/cpp1y/constexpr-104284-2.C: New test.
> 	* g++.dg/cpp1y/constexpr-104284-3.C: New test.
> 	* g++.dg/cpp1y/constexpr-104284-4.C: New test.
> ---
>   gcc/cp/decl.cc                                |  4 +++
>   .../g++.dg/cpp1y/constexpr-104284-1.C         | 34 ++++++++++++++++++
>   .../g++.dg/cpp1y/constexpr-104284-2.C         | 33 +++++++++++++++++
>   .../g++.dg/cpp1y/constexpr-104284-3.C         | 33 +++++++++++++++++
>   .../g++.dg/cpp1y/constexpr-104284-4.C         | 35 +++++++++++++++++++
>   5 files changed, 139 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
> 
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 68741bbf5d2..69f60a6dc0f 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -7332,6 +7332,10 @@ check_initializer (tree decl, tree init, int flags, vec<tree, va_gc> **cleanups)
>   	   && !(init && BRACE_ENCLOSED_INITIALIZER_P (init)
>   		&& CP_AGGREGATE_TYPE_P (type)
>   		&& (CLASS_TYPE_P (type)
> +		    /* The call to build_aggr_init below could end up
> +		       calling build_vec_init, which may break when we
> +		       are processing a template.  */
> +		    || processing_template_decl
>   		    || !TYPE_NEEDS_CONSTRUCTING (type)
>   		    || type_has_extended_temps (type))))
>   	  || (DECL_DECOMPOSITION_P (decl) && TREE_CODE (type) == ARRAY_TYPE))
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
> new file mode 100644
> index 00000000000..809c26a6161
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
> @@ -0,0 +1,34 @@
> +// PR c++/104284
> +// { dg-do compile { target c++14 } }
> +// { dg-additional-options "-fchecking=2" }
> +
> +struct S {
> +  char c{};
> +};
> +
> +auto x1 = [](auto) { constexpr S s[]{{}}; };
> +auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
> +#if __cpp_designated_initializers >= 201707L
> +auto x3 = [](auto) { constexpr S s[]{{.c = 'a'}}; };
> +#endif
> +auto x4 = [](auto) { constexpr S s[]{'a'}; };
> +auto x5 = [](auto) { constexpr S s[]{{{}}}; };
> +
> +template<class>
> +constexpr void g ()
> +{
> +  constexpr S s1[]{{}};
> +  static_assert(s1[0].c == '\0', "");
> +  constexpr S s2[]{{'a'}};
> +  static_assert(s2[0].c == 'a', "");
> +#if __cpp_designated_initializers >= 201707L
> +  constexpr S s3[]{{.c = 'a'}};
> +  static_assert(s3[0].c == 'a', "");
> +#endif
> +  constexpr S s4[]{'a'};
> +  static_assert(s4[0].c == 'a', "");
> +  constexpr S s5[]{{{}}};
> +  static_assert(s5[0].c == '\0', "");
> +}
> +
> +static_assert ((g<int>(), true), "");
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
> new file mode 100644
> index 00000000000..704d37de129
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
> @@ -0,0 +1,33 @@
> +// PR c++/104284
> +// { dg-do compile { target c++14 } }
> +// { dg-additional-options "-fchecking=2" }
> +
> +struct S {
> +  char a;
> +  constexpr S() : a{'a'} { }
> +  constexpr S(char a_) : a{a_} { }
> +};
> +
> +auto x1 = [](auto) { constexpr S s[]{{}}; };
> +auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
> +auto x3 = [](auto) { constexpr S s[]{'a'}; };
> +auto x4 = [](auto) { constexpr S s[]{{{}}}; };
> +
> +template<typename>
> +constexpr void g()
> +{
> +  constexpr S s1[]{{}};
> +  static_assert(s1[0].a == 'a', "");
> +  constexpr S s2[]{{'a'}};
> +  static_assert(s2[0].a == 'a', "");
> +  constexpr S s3[]{'a'};
> +  static_assert(s3[0].a == 'a', "");
> +  constexpr S s4[]{{{}}};
> +  static_assert(s4[0].a == '\0', "");
> +}
> +
> +void
> +f ()
> +{
> +  g<int>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
> new file mode 100644
> index 00000000000..6f23b255f9c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
> @@ -0,0 +1,33 @@
> +// PR c++/104284
> +// { dg-do compile { target c++14 } }
> +// { dg-additional-options "-fchecking=2" }
> +// Like constexpr-104284.C, but the function template is not
> +// constexpr.  In that case, we were still calling build_vec_init
> +// in a template, just not crashing.
> +
> +struct S {
> +  char c{};
> +};
> +
> +template<class>
> +void g ()
> +{
> +  constexpr S s1[]{{}};
> +  static_assert(s1[0].c == '\0', "");
> +  constexpr S s2[]{{'a'}};
> +  static_assert(s2[0].c == 'a', "");
> +#if __cpp_designated_initializers >= 201707L
> +  constexpr S s3[]{{.c = 'a'}};
> +  static_assert(s3[0].c == 'a', "");
> +#endif
> +  constexpr S s4[]{'a'};
> +  static_assert(s4[0].c == 'a', "");
> +  constexpr S s5[]{{{}}};
> +  static_assert(s5[0].c == '\0', "");
> +}
> +
> +void
> +f ()
> +{
> +  g<int>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
> new file mode 100644
> index 00000000000..a99d3255a47
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
> @@ -0,0 +1,35 @@
> +// PR c++/104284
> +// { dg-do run { target c++14 } }
> +// { dg-additional-options "-fchecking=2" }
> +
> +struct S {
> +  char c{};
> +};
> +
> +template<class>
> +constexpr void g ()
> +{
> +  S s1[]{{}};
> +  if (s1[0].c != '\0')
> +    __builtin_abort ();
> +  S s2[]{{'a'}};
> +  if (s2[0].c != 'a')
> +    __builtin_abort ();
> +#if __cpp_designated_initializers >= 201707L
> +  S s3[]{{.c = 'a'}};
> +  if (s3[0].c != 'a')
> +    __builtin_abort ();
> +#endif
> +  S s4[]{'a'};
> +  if (s4[0].c != 'a')
> +    __builtin_abort ();
> +  S s5[]{{{}}};
> +  if (s5[0].c != '\0')
> +    __builtin_abort ();
> +}
> +
> +int
> +main ()
> +{
> +  g<int>();
> +}
> 
> base-commit: 346ab5a54a831ad9c78afcbd8dfe98e0e07e3070