Go compiler patch: issue receive type errors earlier

[Ian Lance Taylor]

This patch by Chris Manghane changes the Go frontend to issue type
errors earlier for a receive operation.  This fixes
https://golang.org/issue/12323 .  Bootstrapped and ran Go testsuite on
x86_64-unknown-linux-gnu.  Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 227834)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-1cb26dc898bda1e85f4dd2ee204adbce792e4813
+e069d4417a692c1261df99fe3323277e1a0193d2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: gcc/go/gofrontend/expressions.cc
===================================================================
--- gcc/go/gofrontend/expressions.cc    (revision 227834)
+++ gcc/go/gofrontend/expressions.cc    (working copy)
@@ -13502,9 +13502,14 @@ Expression::make_heap_expression(Express
 Type*
 Receive_expression::do_type()
 {
+  if (this->is_error_expression())
+    return Type::make_error_type();
   Channel_type* channel_type = this->channel_->type()->channel_type();
   if (channel_type == NULL)
-    return Type::make_error_type();
+    {
+      this->report_error(_("expected channel"));
+      return Type::make_error_type();
+    }
   return channel_type->element_type();
 }
 
@@ -13516,6 +13521,7 @@ Receive_expression::do_check_types(Gogo*
   Type* type = this->channel_->type();
   if (type->is_error())
     {
+      go_assert(saw_errors());
       this->set_is_error();
       return;
     }
Index: gcc/go/gofrontend/statements.cc
===================================================================
--- gcc/go/gofrontend/statements.cc     (revision 227696)
+++ gcc/go/gofrontend/statements.cc     (working copy)
@@ -3856,7 +3856,10 @@ Switch_statement::do_lower(Gogo*, Named_
   if (this->val_ != NULL
       && (this->val_->is_error_expression()
          || this->val_->type()->is_error()))
-    return Statement::make_error_statement(loc);
+    {
+      go_assert(saw_errors());
+      return Statement::make_error_statement(loc);
+    }
 
   if (this->val_ != NULL
       && this->val_->type()->integer_type() != NULL