cgen: fix `$if T.typ is Type {` (#13135)

vlib/v/gen/c/comptime.v

@@ -375,7 +375,11 @@ fn (mut g Gen) comptime_if_cond(cond ast.Expr, pkg_exist bool) bool {
 						}
 					} else if left is ast.SelectorExpr {
 						name = '${left.expr}.$left.field_name'
-						exp_type = g.comptime_var_type_map[name]
+						if left.gkind_field == .typ {
+							exp_type = g.unwrap_generic(left.name_type)
+						} else {
+							exp_type = g.comptime_var_type_map[name]
+						}
 					} else if left is ast.TypeNode {
 						// this is only allowed for generics currently, otherwise blocked by checker
 						exp_type = g.unwrap_generic(left.typ)

vlib/v/tests/comptime_if_expr_generic_typ_is_type_test.v

@@ -0,0 +1,18 @@
+module main
+
+fn write<T>(out T) string {
+	$if T.typ is bool {
+		println('FOO')
+		return 'FOO'
+	} $else $if T.typ !is bool {
+		println('BAR')
+		return 'BAR'
+	}
+	return 'EMPTY'
+}
+
+fn test_comptime_if_expr_generic_typ_is_type() {
+	mut val := false
+	ret := write<bool>(val)
+	assert ret == 'FOO'
+}