checker: forbid aliasing an alias (#6118)

vlib/v/checker/checker.v

@@ -266,6 +266,10 @@ pub fn (mut c Checker) type_decl(node ast.TypeDecl) {
 			typ_sym := c.table.get_type_symbol(node.parent_type)
 			if typ_sym.kind == .placeholder {
 				c.error("type `$typ_sym.name` doesn't exist", node.pos)
+			} else if typ_sym.kind == .alias {
+				orig_sym := c.table.get_type_symbol((typ_sym.info as table.Alias).parent_type)
+				c.error('type `$typ_sym.name` is an alias, use the original alias type `$orig_sym.name` instead',
+					node.pos)
 			}
 		}
 		ast.FnTypeDecl {

vlib/v/checker/tests/nested_aliases.out

@@ -0,0 +1,4 @@
+vlib/v/checker/tests/nested_aliases.v:2:1: error: type `MyInt` is an alias, use the original alias type `int` instead
+    1 | type MyInt = int
+    2 | type MyMyInt = MyInt
+      | ~~~~~~~~~~~~

vlib/v/checker/tests/nested_aliases.vv

@@ -0,0 +1,2 @@
+type MyInt = int
+type MyMyInt = MyInt