checker: fix comptime if T is interface (#12365)

vlib/v/checker/checker.v

@@ -6678,6 +6678,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 						is_comptime_type_is_expr = true
 					} else if branch.cond.right is ast.TypeNode && left is ast.TypeNode
 						&& sym.kind == .interface_ {
+						is_comptime_type_is_expr = true
 						// is interface
 						checked_type := c.unwrap_generic(left.typ)
 						should_skip = !c.table.does_type_implement_interface(checked_type,

vlib/v/tests/comptime_if_is_interface_test.v

@@ -0,0 +1,29 @@
+interface Something {
+	i int
+}
+
+struct Some {
+	i int
+}
+
+struct App {
+mut:
+	count u8
+}
+
+fn (mut self App) next<T>(input T) string {
+	$if T is Something {
+		return 'Something'
+	} $else $if T is f64 {
+		return 'f64'
+	} $else {
+		panic('${typeof(T.typ).name} is not supported')
+	}
+	panic('Unreachable')
+}
+
+fn test_comptime_if_is_interface() {
+	mut app := App{}
+	assert app.next(Something(Some{1})) == 'Something'
+	assert app.next(1.0) == 'f64'
+}