checker: add check for unknown generic types in type alias decl (#16377)

vlib/v/checker/checker.v

@@ -444,6 +444,16 @@ pub fn (mut c Checker) alias_type_decl(node ast.AliasTypeDecl) {
 		orig_sym := c.table.type_to_str(node.parent_type)
 		c.error('type `$typ_sym.str()` is an alias, use the original alias type `$orig_sym` instead',
 			node.type_pos)
+	} else if typ_sym.kind == .struct_ {
+		if mut typ_sym.info is ast.Struct {
+			// check if the generic param types have been defined
+			for ct in typ_sym.info.concrete_types {
+				ct_sym := c.table.sym(ct)
+				if ct_sym.kind == .placeholder {
+					c.error('unknown type `$ct_sym.name`', node.type_pos)
+				}
+			}
+		}
 	}
 }
 

vlib/v/checker/tests/type_alias_struct_generic_unknown_name_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/type_alias_struct_generic_unknown_name_err.vv:7:16: error: unknown type `UnknownType`
+    5 | }
+    6 | 
+    7 | type NewType = Foo<UnknownType>
+      |                ~~~~~~~~~~~~~~~~
+    8 | 
+    9 | fn main() {

vlib/v/checker/tests/type_alias_struct_generic_unknown_name_err.vv

@@ -0,0 +1,10 @@
+module main
+
+struct Foo<T> {
+	value T
+}
+
+type NewType = Foo<UnknownType>
+
+fn main() {
+}