checker: fix the check of types not implemented by interfaces in infix key_is and not_is (fix #16282 #16298) (#16284)

vlib/v/checker/infix.v

@@ -554,7 +554,12 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 					c.error('`$op` can only be used with interfaces and sum types', node.pos)
 				} else if mut left_sym.info is ast.SumType {
 					if typ !in left_sym.info.variants {
-						c.error('`$left_sym.name` has no variant `$right_sym.name`', node.pos)
+						c.error('`$left_sym.name` has no variant `$right_sym.name`', right_pos)
+					}
+				} else if left_sym.info is ast.Interface {
+					if typ_sym.kind != .interface_ && !c.type_implements(typ, left_type, right_pos) {
+						c.error("`$typ_sym.name` doesn't implement interface `$left_sym.name`",
+							right_pos)
 					}
 				}
 			}

vlib/v/checker/tests/generic_sumtype_invalid_variant.out

@@ -1,14 +1,14 @@
-vlib/v/checker/tests/generic_sumtype_invalid_variant.vv:5:7: error: `MultiGeneric<bool, int, string>` has no variant `u64`
+vlib/v/checker/tests/generic_sumtype_invalid_variant.vv:5:10: error: `MultiGeneric<bool, int, string>` has no variant `u64`
     3 | fn main() {
     4 |     mut m := MultiGeneric<bool, int, string>(true)
     5 |     if m is u64 {
-      |          ~~
+      |             ~~~
     6 |         println('hi')
     7 |     }
-vlib/v/checker/tests/generic_sumtype_invalid_variant.vv:8:7: error: `MultiGeneric<bool, int, string>` has no variant `X`
+vlib/v/checker/tests/generic_sumtype_invalid_variant.vv:8:10: error: `MultiGeneric<bool, int, string>` has no variant `X`
     6 |         println('hi')
     7 |     }
     8 |     if m is X {
-      |          ~~
+      |             ^
     9 |         println('hi again')
    10 |     }

vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:8:14: error: `int` doesn't implement interface `IError`
+    6 |     if _ := f() {
+    7 |     } else {
+    8 |         _ = err is int
+      |                    ~~~
+    9 |     }
+   10 |
+vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:12:14: error: `int` doesn't implement interface `IError`
+   10 | 
+   11 |     _ = f() or {
+   12 |         _ = err is int
+      |                    ~~~
+   13 |         0
+   14 |     }
+vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:27:17: error: `int` doesn't implement method `some_method` of interface `IAbc`
+   25 | fn type_unimplemented_interface() {
+   26 |     ivalue := IAbc(Struct{})
+   27 |     _ := ivalue is int
+      |                    ~~~
+   28 | }

vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv

@@ -0,0 +1,28 @@
+fn f() ?int {
+	return none
+}
+
+fn type_unimplemented_ierror() {
+	if _ := f() {
+	} else {
+		_ = err is int
+	}
+
+	_ = f() or {
+		_ = err is int
+		0
+	}
+}
+
+interface IAbc {
+	some_method()
+}
+
+struct Struct {}
+
+fn (s Struct) some_method() {}
+
+fn type_unimplemented_interface() {
+	ivalue := IAbc(Struct{})
+	_ := ivalue is int
+}

vlib/v/checker/tests/is_type_invalid.out

@@ -1,14 +1,14 @@
-vlib/v/checker/tests/is_type_invalid.vv:14:12: error: `IoS` has no variant `u8`
+vlib/v/checker/tests/is_type_invalid.vv:14:15: error: `IoS` has no variant `u8`
    12 | 
    13 | fn main() {
    14 |     if IoS(1) is u8 {
-      |               ~~
+      |                  ~~
    15 |         println('not cool')
    16 |     }
-vlib/v/checker/tests/is_type_invalid.vv:18:7: error: `Cat` doesn't implement method `speak` of interface `Animal`
+vlib/v/checker/tests/is_type_invalid.vv:18:10: error: `Cat` doesn't implement method `speak` of interface `Animal`
    16 |     }
    17 |     a := Animal(Dog{})
    18 |     if a is Cat {
-      |          ~~
+      |             ~~~
    19 |         println('not cool either')
    20 |     }

vlib/v/tests/generics_array_typedef_test.v

@@ -1,12 +1,12 @@
 struct Node<T> {
 mut:
 	data T
-	next &Node<T> = 0
+	next &Node<T> = unsafe { nil }
 }
 
 struct SinglyLinkedList<T> {
 mut:
-	first_node &Node<T> = 0
+	first_node &Node<T> = unsafe { nil }
 }
 
 fn init_singlylinkedlist<T>(nodes []Node<T>) SinglyLinkedList<T> {

vlib/v/tests/generics_assign_reference_generic_struct_test.v

@@ -1,12 +1,12 @@
 pub struct List<T> {
 pub mut:
-	head &ListNode<T> = 0
+	head &ListNode<T> = unsafe { nil }
 }
 
 pub struct ListNode<T> {
 pub mut:
 	value T
-	next  &ListNode<T> = 0
+	next  &ListNode<T> = unsafe { nil }
 }
 
 pub fn list_new<T>() List<T> {