c++: Error recovery with erroneous DECL_INITIAL [PR94475]

Here we're ICE'ing in do_narrow during error-recovery, because ocp_convert
returns error_mark_node after it attempts to reduce a const decl to its
erroneous DECL_INITIAL via scalar_constant_value, and we later pass this
error_mark_node to fold_build2 which isn't prepared to handle error_mark_nodes.

We could fix this ICE in do_narrow by checking if ocp_convert returns
error_mark_node, but for the sake of consistency and for better error recovery
it seems it'd be preferable if ocp_convert didn't care that a const decl's
initializer is erroneous and would instead proceed as if the decl was not const,
which is the approach that this patch takes.

gcc/cp/ChangeLog:

	PR c++/94475
	* cvt.c (ocp_convert): If the result of scalar_constant_value is
	erroneous, ignore it and use the original expression.

gcc/testsuite/ChangeLog:

	PR c++/94475
	* g++.dg/conversion/err-recover2.C: New test.
	* g++.dg/diagnostic/pr84138.C: Remove now-bogus warning.
	* g++.dg/warn/Wsign-compare-8.C: Remove now-bogus warning.

1 files changed, 3 insertions, 1 deletions

diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index a3b8096..656e7fd 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -723,7 +723,9 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
   if (!CLASS_TYPE_P (type))
     {
       e = mark_rvalue_use (e);
-      e = scalar_constant_value (e);
+      tree v = scalar_constant_value (e);
+      if (!error_operand_p (v))
+	e = v;
     }
   if (error_operand_p (e))
     return error_mark_node;