re PR c++/84138 (ICE folding broken constant)

	PR c++/84138
	* cp-gimplify.c (cp_fold): Check if X is an error node before
	calling useless_type_conversion_p.

	* g++.dg/diagnostic/pr84138.C: New test.

From-SVN: r257240

gcc/cp/ChangeLog

+2018-01-31  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/84138
+	* cp-gimplify.c (cp_fold): Check if X is an error node before
+	calling useless_type_conversion_p.
+
 2018-01-30  Jason Merrill  <jason@redhat.com>
 
 	PR c++/84091 - ICE with local class in lambda in template.

gcc/cp/cp-gimplify.c

@@ -2356,6 +2356,7 @@ cp_fold (tree x)
       /* A COND_EXPR might have incompatible types in branches if one or both
 	 arms are bitfields.  If folding exposed such a branch, fix it up.  */
       if (TREE_CODE (x) != code
+	  && x != error_mark_node
 	  && !useless_type_conversion_p (TREE_TYPE (org_x), TREE_TYPE (x)))
 	x = fold_convert (TREE_TYPE (org_x), x);
 

gcc/testsuite/ChangeLog

+2018-01-31  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/84138
+	* g++.dg/diagnostic/pr84138.C: New test.
+
 2018-01-31  Janne Blomqvist  <jb@gcc.gnu.org>
 
 	PR fortran/78534

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

+// PR c++/84138
+
+char
+foo()
+{
+  const int i = 0 = 0; // { dg-error "lvalue required as left operand" }
+  return 1 ? 0 : (char)i;
+} // { dg-warning "control reaches" }