Re: C++ PATCH to fix rejects-valid with constexpr ctor in C++17 (PR c++/83692)

[Marek Polacek <polacek@redhat.com>]

On Fri, Feb 16, 2018 at 04:10:20PM -0500, Jason Merrill wrote:
> On Mon, Feb 5, 2018 at 1:45 PM, Jason Merrill <jason@redhat.com> wrote:
> > On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <polacek@redhat.com> wrote:
> >> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:
> >>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:
> >>> > This is a similar problem to 83116: we'd cached a constexpr call, but after a
> >>> > store the result had become invalid, yet we used the wrong result again when
> >>> > encountering the same call later.  This resulted in evaluating a THROW_EXPR
> >>> > which doesn't work.  Details in
> >>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5
> >>> >
> >>> > The fix for 83116 didn't work here, because when evaluating the body of the
> >>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do
> >>> > cache.
> >>>
> >>> > It seems that we may no longer rely on the constexpr call table when we
> >>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the
> >>> > value of an object.  Might be too big a hammer again, but I couldn't think
> >>> > of how I could guard the caching of a constexpr call.
> >>>
> >>> > This doesn't manifest in C++14 because build_special_member_call in C++17 is
> >>> > more aggressive with copy elisions (as required by P0135 which changed how we
> >>> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so
> >>> > expand_default_init calls maybe_constant_init, for which STRICT is false, so
> >>> > we avoid caching as per 83116.
> >>>
> >>> So it sounds like the problem is using cxx_constant_value for the
> >>> diagnostic when it has different semantics from the
> >>> maybe_constant_init that follows right after.  I guess we want a
> >>> cxx_constant_init function that is a hybrid of the two.
> >>
> >> So like the following?  Thanks,
> >>
> >> Bootstrapped/regtested on x86_64-linux, ok for trunk?
> >>
> >> 2018-02-04  Marek Polacek  <polacek@redhat.com>
> >>
> >>         PR c++/83692
> >>         * constexpr.c (cxx_constant_init): New function.
> >>         * cp-tree.h (cxx_constant_init): Declare.
> >>         * typeck2.c (store_init_value): Call cxx_constant_init instead of
> >>         cxx_constant_value.
> >>
> >> +/* Like cxx_constant_value, but non-strict mode.  */
> >> +
> >> +tree
> >> +cxx_constant_init (tree t, tree decl)
> >> +{
> >> +  return cxx_eval_outermost_constant_expr (t, false, false, decl);
> >> +}
> >
> > Hmm, that doesn't do the TARGET_EXPR stripping that
> > maybe_constant_init does.  I was thinking of a version of
> > maybe_constant_init that passes false to allow_non_constant.  Probably
> > by making "maybe_constant_init" and cxx_constant_init both call the
> > current function with an additional parameter.  And then the existing
> > call to maybe_constant_init can move under an 'else' to avoid
> > redundant constexpr evaluation.
> 
> Want me to take this over?

Sorry again for the delay.

I tried to do what you suggested.  There was one twist: it regressed
constexpr-nullptr-2.C, in particular we lost diagnostics for

constexpr int* pj0 = &((S*)0)->j;       // { dg-error "not a constant expression" }
constexpr int* pj1 = &((S*)nullptr)->j;  // { dg-error "not a constant expression" }

because when maybe_constant_init_1 saw a constant:

5142   else if (CONSTANT_CLASS_P (t))
5143     /* No evaluation needed.  */;

so it didn't call cxx_eval_outermost_constant_expr which is supposed to give
the error.  I fixed it by adding "&& allow_non_constant" so now it gives the
proper diagnostics.

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

2018-02-23  Marek Polacek  <polacek@redhat.com>

	PR c++/83692
	* constexpr.c (maybe_constant_init_1): New function.
	(maybe_constant_init): Make it a wrapper around maybe_constant_init_1.
	(cxx_constant_init): New function.
	* cp-tree.h (cxx_constant_init): Declare.
	* typeck2.c (store_init_value): Call cxx_constant_init instead of
	cxx_constant_value.  Move the maybe_constant_init call under an 'else'.

	* g++.dg/cpp1z/constexpr-83692.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 47ff90cb055..26d0d099a05 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -5123,8 +5123,8 @@ fold_non_dependent_expr (tree t)
 /* Like maybe_constant_value, but returns a CONSTRUCTOR directly, rather
    than wrapped in a TARGET_EXPR.  */
 
-tree
-maybe_constant_init (tree t, tree decl)
+static tree
+maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant)
 {
   if (!t)
     return t;
@@ -5139,10 +5139,10 @@ maybe_constant_init (tree t, tree decl)
     t = TARGET_EXPR_INITIAL (t);
   if (!is_nondependent_static_init_expression (t))
     /* Don't try to evaluate it.  */;
-  else if (CONSTANT_CLASS_P (t))
+  else if (CONSTANT_CLASS_P (t) && allow_non_constant)
     /* No evaluation needed.  */;
   else
-    t = cxx_eval_outermost_constant_expr (t, true, false, decl);
+    t = cxx_eval_outermost_constant_expr (t, allow_non_constant, false, decl);
   if (TREE_CODE (t) == TARGET_EXPR)
     {
       tree init = TARGET_EXPR_INITIAL (t);
@@ -5152,6 +5152,22 @@ maybe_constant_init (tree t, tree decl)
   return t;
 }
 
+/* Wrapper for maybe_constant_init_1 which permits non constants.  */
+
+tree
+maybe_constant_init (tree t, tree decl)
+{
+  return maybe_constant_init_1 (t, decl, true);
+}
+
+/* Wrapper for maybe_constant_init_1 which does not permit non constants.  */
+
+tree
+cxx_constant_init (tree t, tree decl)
+{
+  return maybe_constant_init_1 (t, decl, false);
+}
+
 #if 0
 /* FIXME see ADDR_EXPR section in potential_constant_expression_1.  */
 /* Return true if the object referred to by REF has automatic or thread
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index 9038d677b2d..04c7b7ce3a9 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -7411,6 +7411,7 @@ extern bool require_potential_constant_expression (tree);
 extern bool require_constant_expression (tree);
 extern bool require_potential_rvalue_constant_expression (tree);
 extern tree cxx_constant_value			(tree, tree = NULL_TREE);
+extern tree cxx_constant_init			(tree, tree = NULL_TREE);
 extern tree maybe_constant_value		(tree, tree = NULL_TREE);
 extern tree maybe_constant_init			(tree, tree = NULL_TREE);
 extern tree fold_non_dependent_expr		(tree);
diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
index 899d60e8535..153b46cca77 100644
--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -830,9 +830,10 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
 	  if (!require_constant_expression (value))
 	    value = error_mark_node;
 	  else
-	    value = cxx_constant_value (value, decl);
+	    value = cxx_constant_init (value, decl);
 	}
-      value = maybe_constant_init (value, decl);
+      else
+	value = maybe_constant_init (value, decl);
       if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
 	/* Poison this CONSTRUCTOR so it can't be copied to another
 	   constexpr variable.  */
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
index e69de29bb2d..f6b61eeab85 100644
--- gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
@@ -0,0 +1,21 @@
+// PR c++/83692
+// { dg-options -std=c++17 }
+
+struct integer {
+  constexpr int value() const { return m_value;	}
+  int m_value;
+};
+
+struct outer {
+  integer m_x{0};
+  constexpr outer()
+    {
+      if (m_x.value() != 0)
+	throw 0;
+      m_x.m_value = integer{1}.value();
+      if (m_x.value() != 1)
+	throw 0;
+    }
+};
+
+constexpr outer o{};

	Marek