checker: check generic sumtype declare error (#14367)

vlib/v/checker/checker.v

@@ -492,7 +492,13 @@ pub fn (mut c Checker) sum_type_decl(node ast.SumTypeDecl) {
 			c.error('sum type cannot hold an interface', variant.pos)
 		} else if sym.kind == .struct_ && sym.language == .js {
 			c.error('sum type cannot hold an JS struct', variant.pos)
+		} else if mut sym.info is ast.Struct {
+			if sym.info.is_generic && !variant.typ.has_flag(.generic) {
+				c.error('generic struct `$sym.name` must specify generic type names, e.g. Foo<T>',
+					variant.pos)
 			}
+		}
+
 		if sym.name.trim_string_left(sym.mod + '.') == node.name {
 			c.error('sum type cannot hold itself', variant.pos)
 		}

vlib/v/checker/tests/generic_sumtype_decl_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_sumtype_decl_err.vv:7:24: error: generic struct `Just` must specify generic type names, e.g. Foo<T>
+    5 | struct Nothing {}
+    6 |
+    7 | type Maybe = Nothing | Just
+      |                        ~~~~
+    8 |
+    9 | fn main() {

vlib/v/checker/tests/generic_sumtype_decl_err.vv

@@ -0,0 +1,10 @@
+struct Just<T> {
+  value T
+}
+
+struct Nothing {}
+
+type Maybe = Nothing | Just
+
+fn main() {
+}