fix(compiler): Json? can be provided where Json is expected (#6679)

fixes: #6061 

## Checklist

- [ ] Title matches [Winglang's style guide](https://www.winglang.io/contributing/start-here/pull_requests#how-are-pull-request-titles-formatted)
- [ ] Description explains motivation and solution
- [ ] Tests added (always)
- [ ] Docs updated (only required for features)
- [ ] Added `pr/e2e-full` label if this feature requires end-to-end testing

*By submitting this pull request, I confirm that my contribution is made under the terms of the [Wing Cloud Contribution License](https://github.com/winglang/wing/blob/main/CONTRIBUTION_LICENSE.md)*.

examples/tests/invalid/json_is_not_nil.test.w

@@ -0,0 +1,27 @@
+let innerOptional = (): Json? => {
+  return nil;
+};
+
+let outerJson = (x: Json) => {
+
+};
+
+
+let innerJson = (): Json => {
+  return {};
+};
+
+let outerOptional = (x: Json?) => {
+
+};
+
+
+// those are ok
+outerJson(innerJson());
+outerOptional(innerOptional());
+outerOptional(innerJson());
+
+// this is problematic as json? can be nil, and nil is not json
+outerJson(innerOptional());
+        //^^^^^^^^^^^^^^^ Expected type to be "Json", but got "Json?" instead
+

examples/tests/sdk_tests/resource/call.test.w

@@ -119,11 +119,11 @@ class MyResource {
   }
   pub inflight methodOptNum1(arg: num?): num? {
     // TODO: need a way to convert from Json to num?
-    return unsafeCast(this.backend.call("methodOptNum1", Json [arg]));
+    return unsafeCast(this.backend.call("methodOptNum1", Json [arg ?? 0]));
   }
   pub inflight methodOptNum2(arg: num?): num? {
     // TODO: need a way to convert from Json to num?
-    return unsafeCast(this.backend.call("methodOptNum2", Json [arg]));
+    return unsafeCast(this.backend.call("methodOptNum2", Json [arg ?? 0]));
   }
   pub inflight methodJson(arg: Json): Json {
     return this.backend.call("methodJson", [arg]);

examples/tests/sdk_tests/table/get.test.w

@@ -32,7 +32,7 @@ test "get" {
     table.get(NON_EXISTENT_KEY);
   });
 
-  let var result: Json = table.tryGet(VALID_KEY);
-  assert(result.get(COLUMN_NAME) == COLUMN_VALUE);
+  let var result: Json? = table.tryGet(VALID_KEY);
+  assert(result?.get(COLUMN_NAME) == COLUMN_VALUE);
   assert(table.tryGet(NON_EXISTENT_KEY) == nil);
 }

libs/wingc/src/type_check.rs

@@ -1253,7 +1253,6 @@ impl TypeRef {
 			Type::Inferred(..) => true,
 			Type::Array(v) => v.is_json_legal_value(),
 			Type::Map(v) => v.is_json_legal_value(),
-			Type::Optional(v) => v.is_json_legal_value(),
 			Type::Struct(ref s) => {
 				for (_, t) in s.fields(true) {
 					if !t.is_json_legal_value() {
@@ -3489,7 +3488,11 @@ new cloud.Function(@inflight("./handler.ts"), lifts: { bucket: ["put"] });
 			));
 		}
 
-		if matches!(**return_type.maybe_unwrap_option(), Type::Json(None) | Type::MutJson) {
+		if matches!(**return_type.maybe_unwrap_option(), Type::Json(None) | Type::MutJson)
+			&& !matches!(
+				**first_expected_type.maybe_unwrap_option(),
+				Type::Json(None) | Type::MutJson
+			) {
 			// known json data is statically known
 			hints.push(format!(
 				"use {first_expected_type}.fromJson() to convert dynamic Json\""

tools/hangar/__snapshots__/invalid.ts.snap

@@ -3135,6 +3135,21 @@ error: \\"Bucket\\" is not a legal JSON value
 
 
 
+Tests 1 failed (1)
+Snapshots 1 skipped
+Test Files 1 failed (1)
+Duration <DURATION>"
+`;
+
+exports[`json_is_not_nil.test.w 1`] = `
+"error: Expected type to be \\"Json\\", but got \\"Json?\\" instead
+   --> ../../../examples/tests/invalid/json_is_not_nil.test.w:25:11
+   |
+25 | outerJson(innerOptional());
+   |           ^^^^^^^^^^^^^^^
+
+
+
 Tests 1 failed (1)
 Snapshots 1 skipped
 Test Files 1 failed (1)