c++: ICE with IMPLICIT_CONV_EXPR in array subscript [PR95508]

Since r10-7096 convert_like, when called in a template, creates an
IMPLICIT_CONV_EXPR when we're converting to/from array type.

In this test, we have e[f], and we're converting f (of type class A) to
int, so convert_like in build_new_op_1 created the IMPLICIT_CONV_EXPR
that got into cp_build_array_ref which calls maybe_constant_value.  My
patch above failed to adjust this spot to call fold_non_dependent_expr
instead, which can handle codes like I_C_E in a template.  Fixed by
using a new function maybe_fold_non_dependent_expr, which, if the expr
can't be evaluated to a constant, returns the original expression.

gcc/cp/ChangeLog:

	PR c++/95508
	* constexpr.c (maybe_fold_non_dependent_expr): New.
	* cp-tree.h (maybe_fold_non_dependent_expr): Declare.
	* typeck.c (cp_build_array_ref): Call maybe_fold_non_dependent_expr
	instead of maybe_constant_value.

gcc/testsuite/ChangeLog:

	PR c++/95508
	* g++.dg/template/conv16.C: New test.

1 files changed, 13 insertions, 0 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index c01c42b..f766abd 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -7093,6 +7093,19 @@ fold_non_dependent_expr (tree t,
   return maybe_constant_value (t, object, manifestly_const_eval);
 }
 
+/* Like fold_non_dependent_expr, but if EXPR couldn't be folded to a constant,
+   return the original expression.  */
+
+tree
+maybe_fold_non_dependent_expr (tree expr,
+			       tsubst_flags_t complain/*=tf_warning_or_error*/)
+{
+  tree t = fold_non_dependent_expr (expr, complain);
+  if (t && TREE_CONSTANT (t))
+    return t;
+
+  return expr;
+}
 
 /* Like maybe_constant_init but first fully instantiate the argument.  */