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

[Marek Polacek <polacek at redhat dot com>]

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.

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

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 93dd8ae049c..f95aacf2580 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -4902,6 +4902,14 @@ cxx_constant_value (tree t, tree decl)
   return cxx_eval_outermost_constant_expr (t, false, true, decl);
 }
 
+/* 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);
+}
+
 /* Helper routine for fold_simple function.  Either return simplified
    expression T, otherwise NULL_TREE.
    In contrast to cp_fully_fold, and to maybe_constant_value, we try to fold
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index a53f4fd9c03..9f973305fbb 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -7417,6 +7417,7 @@ 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..b4abc54f537 100644
--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -830,7 +830,7 @@ 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);
       if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
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