c++: ICE with enum and conversion fn in template [PR115657]

Here we initialize an enumerator with a class prvalue with a conversion
function.  When we fold it in build_enumerator, we create a TARGET_EXPR
for the object, and subsequently crash in tsubst_expr, which should not
see such a code.

Normally, we fix similar problems by using an IMPLICIT_CONV_EXPR but here
I may get away with not using the result of fold_non_dependent_expr unless
the result is a constant.  A TARGET_EXPR is not constant.

	PR c++/115657

gcc/cp/ChangeLog:

	* decl.cc (build_enumerator): Call maybe_fold_non_dependent_expr
	instead of fold_non_dependent_expr.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-recursion2.C: New test.
	* g++.dg/template/conv21.C: New test.

1 files changed, 8 insertions, 2 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index f23b635..12139e1 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -17387,9 +17387,15 @@ build_enumerator (tree name, tree value, tree enumtype, tree attributes,
   tree type;
 
   /* scalar_constant_value will pull out this expression, so make sure
-     it's folded as appropriate.  */
+     it's folded as appropriate.
+
+     Creating a TARGET_EXPR in a template breaks when substituting, and
+     here we would create it for instance when using a class prvalue with
+     a user-defined conversion function.  So don't use such a tree.  We
+     instantiate VALUE here to get errors about bad enumerators even in
+     a template that does not get instantiated.  */
   if (processing_template_decl)
-    value = fold_non_dependent_expr (value);
+    value = maybe_fold_non_dependent_expr (value);
 
   /* If the VALUE was erroneous, pretend it wasn't there; that will
      result in the enum being assigned the next value in sequence.  */