compiler: look through aliases for type compatibility

    
    Aliases are supposed to be identical to the type being aliased, so
    questions about type compatibility need to always ignore aliases,
    except for error messages involving the type name.
    
    The test case for this is https://golang.org/cl/94995.
    
    Fixes golang/go#23912
    
    Reviewed-on: https://go-review.googlesource.com/94996

From-SVN: r257845

gcc/go/gofrontend/MERGE

-cef3934fbc63f5e121abb8f88d3799961ac95b59
+459a8a94e04a19bde7173ef7cf2db369c2e62e2d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -597,10 +597,10 @@ Type::are_compatible_for_comparison(bool is_equality_op, const Type *t1,
 	  return false;
 	}
 
-      if (t1->named_type() != NULL)
-	return t1->named_type()->named_type_is_comparable(reason);
-      else if (t2->named_type() != NULL)
-	return t2->named_type()->named_type_is_comparable(reason);
+      if (t1->unalias()->named_type() != NULL)
+	return t1->unalias()->named_type()->named_type_is_comparable(reason);
+      else if (t2->unalias()->named_type() != NULL)
+	return t2->unalias()->named_type()->named_type_is_comparable(reason);
       else if (t1->struct_type() != NULL)
 	{
 	  if (t1->struct_type()->is_struct_incomparable())
@@ -678,6 +678,12 @@ Type::are_assignable(const Type* lhs, const Type* rhs, std::string* reason)
   if (Type::are_identical(lhs, rhs, true, reason))
     return true;
 
+  // Ignore aliases, except for error messages.
+  const Type* lhs_orig = lhs;
+  const Type* rhs_orig = rhs;
+  lhs = lhs->unalias();
+  rhs = rhs->unalias();
+
   // The types are assignable if they have identical underlying types
   // and either LHS or RHS is not a named type.
   if (((lhs->named_type() != NULL && rhs->named_type() == NULL)
@@ -740,15 +746,16 @@ Type::are_assignable(const Type* lhs, const Type* rhs, std::string* reason)
     {
       if (rhs->interface_type() != NULL)
 	reason->assign(_("need explicit conversion"));
-      else if (lhs->named_type() != NULL && rhs->named_type() != NULL)
+      else if (lhs_orig->named_type() != NULL
+	       && rhs_orig->named_type() != NULL)
 	{
-	  size_t len = (lhs->named_type()->name().length()
-			+ rhs->named_type()->name().length()
+	  size_t len = (lhs_orig->named_type()->name().length()
+			+ rhs_orig->named_type()->name().length()
 			+ 100);
 	  char* buf = new char[len];
 	  snprintf(buf, len, _("cannot use type %s as type %s"),
-		   rhs->named_type()->message_name().c_str(),
-		   lhs->named_type()->message_name().c_str());
+		   rhs_orig->named_type()->message_name().c_str(),
+		   lhs_orig->named_type()->message_name().c_str());
 	  reason->assign(buf);
 	  delete[] buf;
 	}
@@ -768,6 +775,10 @@ Type::are_convertible(const Type* lhs, const Type* rhs, std::string* reason)
   if (Type::are_assignable(lhs, rhs, reason))
     return true;
 
+  // Ignore aliases.
+  lhs = lhs->unalias();
+  rhs = rhs->unalias();
+
   // A pointer to a regular type may not be converted to a pointer to
   // a type that may not live in the heap, except when converting from
   // unsafe.Pointer.