cgen: fix comptime is condition when using interface (#20952)

vlib/v/checker/if.v

@@ -10,6 +10,10 @@ fn (mut c Checker) check_compatible_types(left_type ast.Type, right ast.TypeNode
 	right_type := c.unwrap_generic(right.typ)
 	sym := c.table.sym(right_type)
 
+	if right_type.has_flag(.option) != left_type.has_flag(.option) {
+		return .skip
+	}
+
 	if sym.kind == .interface_ {
 		checked_type := c.unwrap_generic(left_type)
 		return if c.table.does_type_implement_interface(checked_type, right_type) {

vlib/v/checker/postfix.v

@@ -19,7 +19,8 @@ fn (mut c Checker) postfix_expr(mut node ast.PostfixExpr) ast.Type {
 			node.expr.pos())
 	}
 
-	if !c.inside_unsafe && is_non_void_pointer && !node.expr.is_auto_deref_var() {
+	if node.op != .question && !c.inside_unsafe && is_non_void_pointer
+		&& !node.expr.is_auto_deref_var() {
 		c.warn('pointer arithmetic is only allowed in `unsafe` blocks', node.pos)
 	}
 	if !(typ_sym.is_number() || ((c.inside_unsafe || c.pref.translated) && is_non_void_pointer)) {

vlib/v/gen/c/comptime.v

@@ -516,11 +516,11 @@ fn (mut g Gen) comptime_if_cond(cond ast.Expr, pkg_exist bool) (bool, bool) {
 							got_sym := g.table.sym(got_type)
 
 							if got_sym.kind == .interface_ && got_sym.info is ast.Interface {
-								is_true := g.table.does_type_implement_interface(exp_type,
-									got_type)
+								is_true := exp_type.has_flag(.option) == got_type.has_flag(.option)
+									&& g.table.does_type_implement_interface(exp_type, got_type)
 								if cond.op == .key_is {
 									if is_true {
-										g.write('1')
+										g.write('1 && ${exp_type.has_flag(.option)} == ${got_type.has_flag(.option)}')
 									} else {
 										g.write('0')
 									}

vlib/v/tests/comptime_is_interface_check_test.v

@@ -0,0 +1,37 @@
+module main
+
+interface TestInterface {
+	test_func() f64
+}
+
+struct Struct1 {
+	num f64
+}
+
+fn (o &Struct1) test_func() f64 {
+	return o.num
+}
+
+struct Struct2 {
+mut:
+	s1 ?&Struct1
+}
+
+fn do_thing[T](s1 Struct1) T {
+	mut t := T{}
+	t.s1 = &s1
+	$for field in T.fields {
+		$if field.typ is TestInterface {
+			i := TestInterface(t.$(field.name))
+			assert false
+		} $else $if field.typ is ?TestInterface {
+			i := TestInterface(t.$(field.name) ?)
+			assert true
+		}
+	}
+	return t
+}
+
+fn test_main() {
+	assert do_thing[Struct2](Struct1{1.23}).s1?.num == 1.23
+}