checker: disallow nil in non-nil arrays and vice versa (vlang#21786)

vlib/v/checker/check_types.v

@@ -129,6 +129,20 @@ fn (mut c Checker) check_types(got ast.Type, expected ast.Type) bool {
 			return true
 		}
 	}
+
+	if (exp_idx == ast.nil_type_idx && got_idx == ast.string_type_idx)
+		|| (got_idx == ast.nil_type_idx && exp_idx == ast.string_type_idx) {
+		got_sym := c.table.sym(got)
+		exp_sym := c.table.sym(expected)
+
+		if expected.is_ptr() || got.is_ptr() {
+			return true
+		}
+		if got_sym.language != .c || exp_sym.language != .c {
+			return false
+		}
+	}
+
 	// allow direct int-literal assignment for pointers for now
 	// maybe in the future options should be used for that
 	if expected.is_any_kind_of_pointer() {

vlib/v/checker/tests/non_nil_array_with_nil_element_err.out

@@ -0,0 +1,8 @@
+vlib/v/checker/tests/non_nil_array_with_nil_element_err.vv:1:23: error: invalid array element: expected `string`, not `voidptr`
+    1 | const spnames = ['f', unsafe { nil }]!
+      |                       ~~~~~~
+    2 | const spnames1 = [unsafe { nil }, 'f']!
+vlib/v/checker/tests/non_nil_array_with_nil_element_err.vv:2:35: error: invalid array element: expected `voidptr`, not `string`
+    1 | const spnames = ['f', unsafe { nil }]!
+    2 | const spnames1 = [unsafe { nil }, 'f']!
+      |                                   ~~~

vlib/v/checker/tests/non_nil_array_with_nil_element_err.vv

@@ -0,0 +1,2 @@
+const spnames = ['f', unsafe { nil }]!
+const spnames1 = [unsafe { nil }, 'f']!

vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.out

@@ -1,3 +1,10 @@
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:15:2: error: cannot assign to field `name`: expected `string`, not `voidptr`
+   13 | 
+   14 | a := &Foo{
+   15 |     name: unsafe { nil }
+      |     ~~~~~~~~~~~~
+   16 |     bar: unsafe { nil }
+   17 |     name2: unsafe { nil }
 vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:15:8: error: cannot assign `nil` to struct field `name` with type `string`
    13 | 
    14 | a := &Foo{

vlib/v/tests/cast_fixed_array_to_ptr_ptr_test.v

@@ -5,7 +5,7 @@ struct Foo {
 fn test_main() {
 	foo := Foo{
 		bar: unsafe {
-			&&char(['a', 'b', nil]!)
+			&&char([c'a', c'b', nil]!)
 		}
 	}
 	println(foo)