Re: [PATCH] c++: detecting copy-init context during CTAD [PR102137]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 7 Mar 2022, Jason Merrill wrote:

> On 3/7/22 10:47, Patrick Palka wrote:
> > On Fri, 4 Mar 2022, Jason Merrill wrote:
> > 
> > > On 3/4/22 14:24, Patrick Palka wrote:
> > > > Here we're failing to communicate to cp_finish_decl from tsubst_expr
> > > > that we're in a copy-initialization context (via the
> > > > LOOKUP_ONLYCONVERTING
> > > > flag), which causes do_class_deduction to always consider explicit
> > > > deduction guides when performing CTAD for a templated variable
> > > > initializer.
> > > > 
> > > > We could fix this by passing LOOKUP_ONLYCONVERTING appropriately when
> > > > calling cp_finish_decl from tsubst_expr, but it seems do_class_deduction
> > > > can determine if we're in a copy-init context by simply inspecting the
> > > > initializer, and thus render its flags parameter unnecessary, which is
> > > > what this patch implements.  (If we were to fix this in tsubst_expr
> > > > instead, I think we'd have to inspect the initializer in the same way
> > > > in order to detect a copy-init context?)
> > > 
> > > Hmm, does this affect conversions as well?
> > > 
> > > Looks like it does:
> > > 
> > > struct A
> > > {
> > >    explicit operator int();
> > > };
> > > 
> > > template <class T> void f()
> > > {
> > >    T t = A();
> > > }
> > > 
> > > int main()
> > > {
> > >    f<int>(); // wrongly accepted
> > > }
> > > 
> > > The reverse, initializing via an explicit constructor, is caught by code
> > > in
> > > build_aggr_init much like the code your patch adds to do_auto_deduction;
> > > perhaps we should move/copy that code to cp_finish_decl?
> > 
> > Ah, makes sense.  Moving that code from build_aggr_init to
> > cp_finish_decl broke things, but using it in both spots seems to work
> > well.  And I suppose we might as well use it in do_class_deduction too,
> > since doing so lets us remove the flags parameter.
> 
> Before removing the flags parameter please try asserting that it now matches
> is_copy_initialization and see if anything breaks.

I added to do_class_deduction:

  gcc_assert (bool(flags & LOOKUP_ONLYCONVERTING) == is_copy_initialization (init));

Turns out removing the flags parameter breaks CTAD for new-expressions
of the form 'new TT(x)' because in this case build_new passes just 'x'
as the initializer to do_auto_deduction (as opposed to a single TREE_LIST),
for which is_copy_initialization returns true even though it's really
direct initalization.

Also turns out we're similarly not passing the right LOOKUP_* flags to
cp_finish_decl from instantiate_body, which breaks consideration of
explicit conversions/deduction guides when instantiating the initializer
of a static data member.  I added some xfailed testcases for these
situations.

Here's a patch that keeps the flags parameter of do_auto_deduction, and
only changes the call to cp_finish_decl from tsubst_expr:

-- >8 --

Subject: [PATCH] c++: detecting copy-init context during CTAD [PR102137]

Here we're failing to communicate to cp_finish_decl from tsubst_expr
that we're in a copy-initialization context (via the LOOKUP_ONLYCONVERTING
flag), which causes us to always consider explicit deduction guides when
performing CTAD for a templated variable initializer.

It turns out this bug also affects consideration of explicit conversion
operators for the same reason.  But consideration of explicit constructors
is unaffected and seems to work correctly thanks to code in build_aggr_init
that sets LOOKUP_ONLYCONVERTING when the initializer represents
copy-initialization.

This patch factors out the copy-initialization check from build_aggr_init
and reuses it in tsubst_expr for sake of cp_finish_decl.  This fixes
consideration of explicit dguides/conversion when instantiating the
initializer of block-scope variables, but the static data member case is
still similarly broken since those are handled from instantiate_body
not tsubst_expr.

	PR c++/102137
	PR c++/87820

gcc/cp/ChangeLog:

	* cp-tree.h (is_copy_initialization): Declare.
	* init.cc (build_aggr_init): Split out copy-initialization
	check into ...
	(is_copy_initialization): ... here.
	(tsubst_expr) <case VAR_DECL>: Pass LOOKUP_ONLYCONVERTING
	to cp_finish_decl when is_copy_initialization is true.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/explicit15.C: New test.
	* g++.dg/cpp1z/class-deduction108.C: New test.
---
 gcc/cp/cp-tree.h                              |  1 +
 gcc/cp/init.cc                                | 20 +++--
 gcc/cp/pt.cc                                  |  6 +-
 gcc/testsuite/g++.dg/cpp0x/explicit15.C       | 83 +++++++++++++++++++
 .../g++.dg/cpp1z/class-deduction108.C         | 78 +++++++++++++++++
 5 files changed, 182 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/explicit15.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction108.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index ac723901098..fd76909ca75 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7039,6 +7039,7 @@ extern void emit_mem_initializers		(tree);
 extern tree build_aggr_init			(tree, tree, int,
                                                  tsubst_flags_t);
 extern int is_class_type			(tree, int);
+extern bool is_copy_initialization		(tree);
 extern tree build_zero_init			(tree, tree, bool);
 extern tree build_value_init			(tree, tsubst_flags_t);
 extern tree build_value_init_noctor		(tree, tsubst_flags_t);
diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index 545d904c0f9..d5faa0dc519 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -2019,11 +2019,7 @@ build_aggr_init (tree exp, tree init, int flags, tsubst_flags_t complain)
       return stmt_expr;
     }
 
-  if (init && init != void_type_node
-      && TREE_CODE (init) != TREE_LIST
-      && !(TREE_CODE (init) == TARGET_EXPR
-	   && TARGET_EXPR_DIRECT_INIT_P (init))
-      && !DIRECT_LIST_INIT_P (init))
+  if (is_copy_initialization (init))
     flags |= LOOKUP_ONLYCONVERTING;
 
   is_global = begin_init_stmts (&stmt_expr, &compound_stmt);
@@ -2331,6 +2327,20 @@ is_class_type (tree type, int or_else)
   return 1;
 }
 
+/* Returns true iff the variable initializer INIT represents
+   copy-initialization (and therefore we must set LOOKUP_ONLYCONVERTING
+   when processing it).  */
+
+bool
+is_copy_initialization (tree init)
+{
+  return (init && init != void_type_node
+	  && TREE_CODE (init) != TREE_LIST
+	  && !(TREE_CODE (init) == TARGET_EXPR
+	       && TARGET_EXPR_DIRECT_INIT_P (init))
+	  && !DIRECT_LIST_INIT_P (init));
+}
+
 /* Build a reference to a member of an aggregate.  This is not a C++
    `&', but really something which can have its address taken, and
    then act as a pointer to member, for example TYPE :: FIELD can have
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 402365285cf..18e2252c482 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -18606,7 +18606,11 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
 			TREE_TYPE (asmspec_tree) = char_array_type_node;
 		      }
 
-		    cp_finish_decl (decl, init, const_init, asmspec_tree, 0);
+
+		    int flags = LOOKUP_NORMAL;
+		    if (VAR_P (decl) && is_copy_initialization (init))
+		      flags |= LOOKUP_ONLYCONVERTING;
+		    cp_finish_decl (decl, init, const_init, asmspec_tree, flags);
 
 		    if (ndecl != error_mark_node)
 		      cp_finish_decomp (ndecl, first, cnt);
diff --git a/gcc/testsuite/g++.dg/cpp0x/explicit15.C b/gcc/testsuite/g++.dg/cpp0x/explicit15.C
new file mode 100644
index 00000000000..48563c8d0b2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/explicit15.C
@@ -0,0 +1,83 @@
+// PR c++/87820
+// { dg-do compile { target c++11 } }
+
+struct A {
+  constexpr explicit operator int() const { return 0; }
+};
+
+template<class T>
+void f() {
+  A a;
+  T t1 = a; // { dg-error "cannot convert" }
+  T t2 = {a}; // { dg-error "cannot convert" }
+  T t3(a);
+  T t4{a};
+  T t5 = T(a);
+  T t6 = T{a};
+  new T(a);
+  new T{a};
+}
+
+template<class T>
+void g() {
+  T t;
+  int n1 = t; // { dg-error "cannot convert" }
+  int n2 = {t}; // { dg-error "cannot convert" }
+  int n3(t);
+  int n4{t};
+  int n5 = int(t);
+  int n6 = int{t};
+  new int(t);
+  new int{t};
+}
+
+template void f<int>();
+template void g<A>();
+
+template<class T>
+struct B {
+  static constexpr A a{};
+  static constexpr T t1 = a; // { dg-error "cannot convert" "" { xfail c++17 } }
+  static constexpr T t2 = {a}; // { dg-error "cannot convert" "" { xfail c++17 } }
+  static constexpr T t4{a}; // { dg-bogus "cannot convert" "" { xfail c++14_down } }
+  static constexpr T t5 = T(a);
+  static constexpr T t6 = T{a};
+};
+
+template<class T>
+struct C {
+  static constexpr T t{};
+  static constexpr int n1 = t; // { dg-error "cannot convert" "" { xfail c++17 } }
+  static constexpr int n2 = {t}; // { dg-error "cannot convert" "" { xfail c++17 } }
+  static constexpr int n4{t}; // { dg-bogus "cannot convert" "" { xfail c++14_down } }
+  static constexpr int n5 = int(t);
+  static constexpr int n6 = int{t};
+};
+
+template struct B<int>;
+template struct C<A>;
+
+#if __cpp_inline_variables
+template<class T>
+struct D {
+  static inline A a;
+  static inline T t1 = a; // { dg-error "cannot convert" "" { target c++17  xfail *-*-* } }
+  static inline T t2 = {a}; // { dg-error "cannot convert" "" { target c++17  xfail *-*-* } }
+  static inline T t4{a};
+  static inline T t5 = T(a);
+  static inline T t6 = T{a};
+};
+
+template<class T>
+struct E {
+  static inline T t;
+  static inline int n1 = t; // { dg-error "cannot convert" "" { target c++17 xfail *-*-* } }
+  static inline int n2 = {t}; // { dg-error "cannot convert" "" { target c++17 xfail *-*-* } }
+  static inline int n4{t};
+  static inline int n5 = int(t);
+  static inline int n6 = int{t};
+};
+
+template struct D<int>;
+template struct E<A>;
+#endif
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction108.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction108.C
new file mode 100644
index 00000000000..5c9f85652ed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction108.C
@@ -0,0 +1,78 @@
+// PR c++/102137
+// { dg-do compile { target c++17 } }
+
+template<class T>
+struct A {
+  constexpr A() { }
+  constexpr A(int) { }
+};
+
+explicit A(...) -> A<int>;
+
+template<template<class> class TT>
+void f() {
+  TT x1 = 0; // { dg-error "deduction|no match" }
+  TT x2 = {0}; // { dg-error "explicit deduction guide" }
+  TT x3(0);
+  TT x4{0};
+  TT x5;
+  new TT(0);
+  new TT{0};
+  new TT();
+  new TT{};
+  new TT;
+}
+
+template<class T>
+void g(T t) {
+  A a1 = t; // { dg-error "deduction|no match" }
+  A a2 = {t}; // { dg-error "explicit deduction guide" }
+  A a3(t);
+  A a4{t};
+  A a5;
+  new A(t);
+  new A{t};
+}
+
+template void f<A>();
+template void g(int);
+
+template<template<class> class TT>
+struct B {
+  static inline TT x1 = 0; // { dg-error "deduction|no match" "" { xfail *-*-* } }
+  static inline TT x2 = {0}; // { dg-error "explicit deduction guide" "" { xfail *-*-* } }
+  static inline TT x4{0};
+  static inline TT x5;
+};
+
+template<class T>
+struct C {
+  static inline T t;
+  static inline A a1 = t; // { dg-error "deduction|no match" "" { xfail *-*-* } }
+  static inline A a2 = {t}; // { dg-error "explicit deduction guide" "" { xfail *-*-* } }
+  static inline A a4{t};
+  static inline A a5{};
+};
+
+template struct B<A>;
+template struct C<int>;
+
+template<template<class> class TT>
+struct E {
+  static constexpr TT x1 = 0; // { dg-error "deduction|no match" "" { xfail *-*-* } }
+  static constexpr TT x2 = {0}; // { dg-error "explicit deduction guide" "" { xfail *-*-* } }
+  static constexpr TT x4{0};
+  static constexpr TT x5{};
+};
+
+template<class T>
+struct F {
+  static constexpr T t{};
+  static constexpr A a1 = t; // { dg-error "deduction|no match" "" { xfail *-*-* } }
+  static constexpr A a2 = {t}; // { dg-error "explicit deduction guide" "" { xfail *-*-* } }
+  static constexpr A a4{t};
+  static constexpr A a5{};
+};
+
+template struct E<A>;
+template struct F<int>;
-- 
2.35.1.415.gc2162907e9