checker: fix sumtype generic checking (fix #23282) (#23309)

vlib/v/checker/infix.v

@@ -794,7 +794,8 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 					c.error('`${op}` can only be used with interfaces and sum types',
 						node.pos) // can be used in sql too, but keep err simple
 				} else if mut left_sym.info is ast.SumType {
-					if typ !in left_sym.info.variants {
+					if typ !in left_sym.info.variants
+						&& c.unwrap_generic(typ) !in left_sym.info.variants {
 						c.error('`${left_sym.name}` has no variant `${right_sym.name}`',
 							right_pos)
 					}

vlib/v/tests/sumtypes/sumtype_generic_checking_test.v

@@ -0,0 +1,20 @@
+type Sumtype = string | int
+
+fn generic_fn[T]() ?T {
+	a := Sumtype('123')
+	if a is T {
+		return a
+	}
+
+	b := Sumtype(123)
+	if b is T {
+		return b
+	}
+
+	return none
+}
+
+fn test_main() {
+	assert generic_fn[string]().str() == "Option('123')"
+	assert generic_fn[int]().str() == 'Option(123)'
+}