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.

gcc/cp/ChangeLog

+2020-04-16  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94475
+	* cvt.c (ocp_convert): If the result of scalar_constant_value is
+	erroneous, ignore it and use the original expression.
+
 2020-04-16  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/94571

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;

gcc/testsuite/ChangeLog

+2020-04-16  Patrick Palka  <ppalka@redhat.com>
+
+	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.
+
 2020-04-16  Richard Sandiford  <richard.sandiford@arm.com>
 
 	PR rtl-optimization/94605

gcc/testsuite/g++.dg/conversion/err-recover2.C

+// PR c++/94475
+// { dg-do compile }
+
+unsigned char
+sr ()
+{
+  const unsigned char xz = EI; // { dg-error "not declared" }
+
+  return xz - (xz >> 1);
+}

gcc/testsuite/g++.dg/diagnostic/pr84138.C

@@ -5,4 +5,4 @@ foo()
 {
   const int i = 0 = 0; // { dg-error "lvalue required as left operand" }
   return 1 ? 0 : (char)i;
-} // { dg-warning "control reaches" }
+}

gcc/testsuite/g++.dg/warn/Wsign-compare-8.C

@@ -5,4 +5,4 @@ bool foo (char c)
 {
   const int i = 0 = 0; // { dg-error "lvalue" }
   return c = i;
-} // { dg-warning "control reaches" }
+}