[PATCH] c++: fix ICE with constexpr ARRAY_REF [PR110382]

[Marek Polacek <polacek@redhat.com>]

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/13?

-- >8 --

This code in cxx_eval_array_reference has been hard to get right.
In r12-2304 I added some code; in r13-5693 I removed some of it.

Here the problematic line is "S s = arr[0];" which causes a crash
on the assert in verify_ctor_sanity:

  gcc_assert (!ctx->object || !DECL_P (ctx->object)
              || ctx->global->get_value (ctx->object) == ctx->ctor);

ctx->object is the VAR_DECL 's', which is correct here.  The second
line points to the problem: we replaced ctx->ctor in
cxx_eval_array_reference:

  new_ctx.ctor = build_constructor (elem_type, NULL); // #1

which I think we shouldn't have; the CONSTRUCTOR we created in
cxx_eval_constant_expression/DECL_EXPR

  new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL);

had the right type.

We still need #1 though.  E.g., in constexpr-96241.C, we never
set ctx.ctor/object before calling cxx_eval_array_reference, so
we have to build a CONSTRUCTOR there.  And in constexpr-101371-2.C
we have a ctx.ctor, but it has the wrong type, so we need a new one.

	PR c++/110382

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_array_reference): Create a new constructor
	only when we don't already have a matching one.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-110382.C: New test.
---
 gcc/cp/constexpr.cc                           |  5 ++++-
 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index fb94f3cefcb..518b7c7a2d5 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4291,7 +4291,10 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
   else
     val = build_value_init (elem_type, tf_warning_or_error);
 
-  if (!SCALAR_TYPE_P (elem_type))
+  if (!SCALAR_TYPE_P (elem_type)
+      /* Create a new constructor only if we don't already have one that
+	 is suitable.  */
+      && !(ctx->ctor && same_type_p (elem_type, TREE_TYPE (ctx->ctor))))
     {
       new_ctx = *ctx;
       new_ctx.ctor = build_constructor (elem_type, NULL);
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
new file mode 100644
index 00000000000..317c5ecfcd5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
@@ -0,0 +1,17 @@
+// PR c++/110382
+// { dg-do compile { target c++14 } }
+
+struct S {
+  double a = 0;
+};
+
+constexpr double
+g ()
+{
+  S arr[1];
+  S s = arr[0];
+  (void) arr[0];
+  return s.a;
+}
+
+int main() { return  g (); }

base-commit: 87516efcbe28884c39a8c68e600d11cc91ed96c7
-- 
2.41.0