compiler: don't use sink as parameter in method expression thunk

Also fix a couple of cases where the error led to a later compiler crash.

Test case is https://go.dev/cl/414336.

Fixes golang/go#52871

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/414354

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-6edae0ef6521569e8f949aaaafa9dc1139825051
+927528cdc112fc51e0d07ee79e7a1254b586eabe
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -1426,7 +1426,12 @@ Sink_expression::do_get_backend(Translate_context* context)
   Gogo* gogo = context->gogo();
   if (this->bvar_ == NULL)
     {
-      go_assert(this->type_ != NULL && !this->type_->is_sink_type());
+      if (this->type_ == NULL || this->type_->is_sink_type())
+	{
+	  go_assert(saw_errors());
+	  return gogo->backend()->error_expression();
+	}
+
       Named_object* fn = context->function();
       go_assert(fn != NULL);
       Bfunction* fn_ctx = fn->func_value()->get_or_make_decl(gogo, fn);
@@ -15235,7 +15240,7 @@ Selector_expression::lower_method_expression(Gogo* gogo)
 	   p != method_parameters->end();
 	   ++p, ++i)
 	{
-	  if (!p->name().empty())
+	  if (!p->name().empty() && !Gogo::is_sink_name(p->name()))
 	    parameters->push_back(*p);
 	  else
 	    {

gcc/go/gofrontend/types.cc

@@ -4654,8 +4654,11 @@ class Sink_type : public Type
   { return false; }
 
   Btype*
-  do_get_backend(Gogo*)
-  { go_unreachable(); }
+  do_get_backend(Gogo* gogo)
+  {
+    go_assert(saw_errors());
+    return gogo->backend()->error_type();
+  }
 
   Expression*
   do_type_descriptor(Gogo*, Named_type*)