checker: check error of casting to interface (fix #17522) (#17531)

vlib/v/checker/checker.v

@@ -913,8 +913,8 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 				// TODO: remove once deprecation period for `IError` methods has ended
 				if inter_sym.idx == ast.error_type_idx
 					&& (imethod.name == 'msg' || imethod.name == 'code') {
-					// c.note("`$styp` doesn't implement method `$imethod.name` of interface `$inter_sym.name`. The usage of fields is being deprecated in favor of methods.",
-					// 	pos)
+					c.note("`${styp}` doesn't implement method `${imethod.name}` of interface `${inter_sym.name}`. The usage of fields is being deprecated in favor of methods.",
+						pos)
 					return false
 				}
 				// <<
@@ -2828,6 +2828,11 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 					final_to_sym = c.table.final_sym(to_type)
 				}
 			}
+		} else {
+			ft := c.table.type_to_str(from_type)
+			tt := c.table.type_to_str(to_type)
+			c.error('`${ft}` does not implement interface `${tt}`, cannot cast `${ft}` to interface `${tt}`',
+				node.pos)
 		}
 	} else if to_type == ast.bool_type && from_type != ast.bool_type && !c.inside_unsafe
 		&& !c.pref.translated && !c.file.is_translated {

vlib/v/checker/tests/cast_to_interface_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/cast_to_interface_err.vv:20:9: notice: `CustomError` doesn't implement method `msg` of interface `IError`. The usage of fields is being deprecated in favor of methods.
+   18 |
+   19 | fn my_error() IError {
+   20 |     return IError(CustomError{})
+      |            ~~~~~~~~~~~~~~~~~~~~~
+   21 | }
+vlib/v/checker/tests/cast_to_interface_err.vv:20:9: error: `CustomError` does not implement interface `IError`, cannot cast `CustomError` to interface `IError`
+   18 |
+   19 | fn my_error() IError {
+   20 |     return IError(CustomError{})
+      |            ~~~~~~~~~~~~~~~~~~~~~
+   21 | }

vlib/v/checker/tests/cast_to_interface_err.vv

@@ -0,0 +1,21 @@
+fn main() {
+	dump(foo()!)
+}
+
+fn foo() !int {
+	// do something
+	if true {
+		return 1
+	}
+	return my_error()
+}
+
+struct CustomError {
+pub:
+	msg  string
+	code int
+}
+
+fn my_error() IError {
+	return IError(CustomError{})
+}

vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.out

@@ -1,3 +1,17 @@
+vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:8:14: notice: `int` doesn't implement method `msg` of interface `IError`. The usage of fields is being deprecated in favor of methods.
+    6 |     if _ := f() {
+    7 |     } else {
+    8 |         _ = err is int
+      |                    ~~~
+    9 |     }
+   10 |
+vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:12:14: notice: `int` doesn't implement method `msg` of interface `IError`. The usage of fields is being deprecated in favor of methods.
+   10 |
+   11 |     _ = f() or {
+   12 |         _ = err is int
+      |                    ~~~
+   13 |         0
+   14 |     }
 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 {
@@ -6,7 +20,7 @@ vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:8:14: error:
     9 |     }
    10 |
 vlib/v/checker/tests/infix_is_notis_interface_unimplemented_err.vv:12:14: error: `int` doesn't implement interface `IError`
-   10 | 
+   10 |
    11 |     _ = f() or {
    12 |         _ = err is int
       |                    ~~~

vlib/v/checker/tests/mut_receiver_wrong_return_type.out

@@ -1,3 +1,16 @@
+vlib/v/checker/tests/mut_receiver_wrong_return_type.vv:4:2: notice: `&Test` doesn't implement method `msg` of interface `IError`. The usage of fields is being deprecated in favor of methods.
+    2 |
+    3 | fn (mut test Test) test() ?int {
+    4 |     return test
+      |     ~~~~~~~~~~~
+    5 | }
+    6 |
+vlib/v/checker/tests/mut_receiver_wrong_return_type.vv:8:2: notice: `&Test` doesn't implement method `msg` of interface `IError`. The usage of fields is being deprecated in favor of methods.
+    6 |
+    7 | fn (mut test Test) test2() int {
+    8 |     return test
+      |     ~~~~~~~~~~~
+    9 | }
 vlib/v/checker/tests/mut_receiver_wrong_return_type.vv:4:9: error: cannot use `Test` as type `?int` in return argument
     2 |
     3 | fn (mut test Test) test() ?int {