2008-04-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/35987
	* typeck.c (build_modify_expr) <case PREINCREMENT_EXPR>: Don't build
	COMPOUND_EXPR if the second argument would be error_mark_node.

	* g++.dg/other/error28.C: New test.

--- gcc/cp/typeck.c.jj	2008-04-18 17:00:44.000000000 +0200
+++ gcc/cp/typeck.c	2008-04-21 16:03:45.000000000 +0200
@@ -5667,10 +5667,11 @@ build_modify_expr (tree lhs, enum tree_c
 	lhs = build2 (TREE_CODE (lhs), TREE_TYPE (lhs),
 		      stabilize_reference (TREE_OPERAND (lhs, 0)),
 		      TREE_OPERAND (lhs, 1));
-      return build2 (COMPOUND_EXPR, lhstype,
-		     lhs,
-		     build_modify_expr (TREE_OPERAND (lhs, 0),
-					modifycode, rhs));
+      newrhs = build_modify_expr (TREE_OPERAND (lhs, 0),
+				  modifycode, rhs);
+      if (newrhs == error_mark_node)
+	return error_mark_node;
+      return build2 (COMPOUND_EXPR, lhstype, lhs, newrhs);
 
       /* Handle (a, b) used as an "lvalue".  */
     case COMPOUND_EXPR:
--- gcc/testsuite/g++.dg/other/error28.C.jj	2008-04-21 15:42:09.000000000 +0200
+++ gcc/testsuite/g++.dg/other/error28.C	2008-04-21 15:37:00.000000000 +0200
@@ -0,0 +1,8 @@
+// PR c++/35987
+// { dg-do compile }
+
+void
+foo (char *p)
+{
+  if (++p = true);	// { dg-error "cannot convert" }
+}