c++: Improve error recovery for =.

In a template we were happily embedding error_mark_node in a MODOP_EXPR,
leading to confusion later.

gcc/cp/ChangeLog:

	* typeck.c (build_x_modify_expr): Handle error_mark_node arguments.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-ice30.C: Adjust.
	* g++.dg/cpp0x/lambda/lambda-ice31.C: Adjust.
	* g++.dg/ext/fixed1.C: Adjust.
	* g++.dg/template/crash107.C: Adjust.
	* g++.dg/template/error35.C: Adjust.
	* g++.dg/template/sizeof-template-argument.C: Adjust.

gcc/cp/typeck.c

@@ -8768,6 +8768,9 @@ build_x_modify_expr (location_t loc, tree lhs, enum tree_code modifycode,
   tree overload = NULL_TREE;
   tree op = build_nt (modifycode, NULL_TREE, NULL_TREE);
 
+  if (lhs == error_mark_node || rhs == error_mark_node)
+    return cp_expr (error_mark_node, loc);
+
   if (processing_template_decl)
     {
       if (modifycode == NOP_EXPR

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-ice30.C

@@ -3,11 +3,11 @@
 
 template<int> void foo()
 {
-  int x[=];  // { dg-error "expected" }
+  int x[=];  // { dg-error "" }
   [&x]{};
 }
 
 void bar()
 {
-  foo<0>();
+  foo<0>();			// { dg-prune-output "not declared" }
 }

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-ice31.C

@@ -4,5 +4,5 @@
 template<typename T> void foo()
 {
   T x[=];  // { dg-error "expected" }
-  [&x]{};
+  [&x]{};  // { dg-prune-output "not declared" }
 }

gcc/testsuite/g++.dg/ext/fixed1.C

@@ -6,3 +6,5 @@ template<int> struct A {};
 template<typename> struct B : A<sizeof(0=0r)> {};	// { dg-error "not supported" }
 
 template<typename> struct C : A<sizeof(0=0r)> {};	// { dg-error "not supported" }
+
+// { dg-prune-output "template argument" }

gcc/testsuite/g++.dg/template/crash107.C

@@ -10,9 +10,9 @@ template<typename FP_> struct Vec { // { dg-message "note" "" { target c++17_dow
         X = y*rhs.z() - z*rhs.y(); // { dg-error "not declared|no member" }
     }
     Vec& operator^(Vec& rhs) {
-        return Vec(*this)^=rhs; // { dg-message "required" }
+        return Vec(*this)^=rhs;
     }
 };
 Vec<double> v(3,4,12); // { dg-error "no matching|too many initializers" }
 Vec<double> V(12,4,3);  // { dg-error "no matching|too many initializers" }
-Vec<double> c = v^V;   // { dg-message "required" }
+Vec<double> c = v^V;

gcc/testsuite/g++.dg/template/error35.C

@@ -1,3 +1,3 @@
 // PR c++/33494
 
-template<int> void foo(int(*f=0)()); // { dg-error "declared void|scope|erroneous-expression" }
+template<int> void foo(int(*f=0)()); // { dg-error "declared void|scope|cannot be used as a function" }

gcc/testsuite/g++.dg/template/sizeof-template-argument.C

@@ -3,9 +3,9 @@
 
 template<int> struct A {};
 
-template<typename> struct B : A <sizeof(=)> {}; /* { dg-error "expected primary-expression" } */
+template<typename> struct B : A <sizeof(=)> {}; /* { dg-error "" } */
 
-template<typename> struct C : A <sizeof(=)> {}; /* { dg-error "expected primary-expression" } */
+template<typename> struct C : A <sizeof(=)> {}; /* { dg-error "" } */
 
 int a;