fix(compiler)!: non-static extern methods are not supported (#4027)

It's not currently possible for extern method implementations to access class members (like fields, or other instance methods), yet it has still been possible to declare them both ways. This PR introduces an error message if you attempt to add a non-static extern method. In the future, this restriction can be lifted if the capability is supported through a new calling convention with extern JavaScript files or something like that.

## Checklist

- [x] Title matches [Winglang's style guide](https://www.winglang.io/contributing/start-here/pull_requests#how-are-pull-request-titles-formatted)
- [x] Description explains motivation and solution
- [x] 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)*.

Author: Chriscbr

Diff for: docs/docs/03-language-reference.md

@@ -1762,6 +1762,8 @@ exports.makeId = function () {
 Given a method of name X, the compiler will map the method to the JavaScript export with the 
 matching name (without any case conversion).
 
+Extern methods do not support access to class's members through `this`, so they must be declared `static`.
+
 ### 5.2.1 TypeScript
 
 It is possible to use TypeScript to write helpers, but at the moment this is not

Diff for: examples/tests/invalid/extern.w

@@ -2,5 +2,4 @@ class Foo {
   extern "./sad.js" static getNum(): num;
 //^ "./sad.js" not found
   extern "not-installed" static tooBad(): bool;
-//^ "not-installed" not found
-}
+}

Diff for: examples/tests/invalid/extern_static.w

@@ -0,0 +1,4 @@
+class Foo {
+  extern "../external_js.js" inflight getGreeting(name: str): str;
+                                    //^ Error: extern methods must be declared "static"
+}

Diff for: examples/tests/valid/dynamo.w

@@ -54,11 +54,11 @@ class DynamoTable {
     }
   }
 
-  extern "./dynamo.js" inflight _putItem(tableName: str, item: Json): void;
+  extern "./dynamo.js" static inflight _putItem(tableName: str, item: Json): void;
 
   inflight putItem(item: Map<Attribute>) {
     let json = this._itemToJson(item);
-    this._putItem(this.tableName, json);
+    DynamoTable._putItem(this.tableName, json);
   }
 
   inflight _itemToJson(item: Map<Attribute>): Json {

Diff for: examples/tests/valid/dynamo_awscdk.w

@@ -60,16 +60,16 @@ class DynamoTable {
     }
   }
 
-  extern "./dynamo.js" inflight _putItem(tableName: str, item: Json): void;
+  extern "./dynamo.js" static inflight _putItem(tableName: str, item: Json): void;
   inflight putItem(item: Map<Attribute>) {
     let json = this._itemToJson(item);
-    this._putItem(this.tableName, json);
+    DynamoTable._putItem(this.tableName, json);
   }
 
-  extern "./dynamo.js" inflight _getItem(tableName: str, key: Json): Json;
+  extern "./dynamo.js" static inflight _getItem(tableName: str, key: Json): Json;
   inflight getItem(key: Map<Attribute>): Json {
     let json = this._itemToJson(key);
-    return this._getItem(this.tableName, json);
+    return DynamoTable._getItem(this.tableName, json);
   }
 
   inflight _itemToJson(item: Map<Attribute>): Json {

Diff for: examples/tests/valid/extern_implementation.w

@@ -5,7 +5,7 @@ class Foo {
   extern "./external_js.js" static inflight regexInflight(pattern: str, text: str): bool;
   extern "./external_js.js" static inflight getUuid(): str;
   extern "./external_js.js" static inflight getData(): str;
-  extern "./external_js.js" inflight print(msg: str);
+  extern "./external_js.js" static inflight print(msg: str);
   extern "uuid" static v4(): str;
 
   inflight call() {
@@ -27,5 +27,5 @@ test "call" {
 }
 
 test "console" {
-  f.print("hey there");
+  Foo.print("hey there");
 }

Diff for: libs/wingc/src/type_check.rs

@@ -3941,6 +3941,15 @@ impl<'a> TypeChecker<'a> {
 			self.types.set_scope_env(scope, method_env);
 			self.inner_scopes.push(scope);
 		}
+
+		if let FunctionBody::External(_) = &method_def.body {
+			if !method_def.is_static {
+				self.spanned_error(
+					method_name,
+					"Extern methods must be declared \"static\" (they cannot access instance members)",
+				);
+			}
+		}
 	}
 
 	fn add_method_to_class_env(

Diff for: tools/hangar/__snapshots__/invalid.ts.snap

@@ -752,6 +752,21 @@ error: Failed to resolve extern \\"not-installed\\": Not Found
 
  
  
+Tests 1 failed (1)
+Test Files 1 failed (1)
+Duration <DURATION>"
+`;
+
+exports[`extern_static.w 1`] = `
+"error: Extern methods must be declared \\"static\\" (they cannot access instance members)
+  --> ../../../examples/tests/invalid/extern_static.w:2:39
+  |
+2 |   extern \\"../external_js.js\\" inflight getGreeting(name: str): str;
+  |                                       ^^^^^^^^^^^ Extern methods must be declared \\"static\\" (they cannot access instance members)
+
+
+ 
+ 
 Tests 1 failed (1)
 Test Files 1 failed (1)
 Duration <DURATION>"

Diff for: tools/hangar/__snapshots__/test_corpus/valid/extern_implementation.w_compile_tf-aws.md

@@ -20,15 +20,15 @@ module.exports = function({ $f }) {
 
 ## inflight.$Closure2-1.js
 ```js
-module.exports = function({ $f }) {
+module.exports = function({ $Foo }) {
   class $Closure2 {
     constructor({  }) {
       const $obj = (...args) => this.handle(...args);
       Object.setPrototypeOf($obj, this);
       return $obj;
     }
     async handle() {
-      (await $f.print("hey there"));
+      (await $Foo.print("hey there"));
     }
   }
   return $Closure2;
@@ -51,7 +51,7 @@ module.exports = function({  }) {
     static async getData() {
       return (require("<ABSOLUTE_PATH>/external_js.js")["getData"])()
     }
-    async print(msg) {
+    static async print(msg) {
       return (require("<ABSOLUTE_PATH>/external_js.js")["print"])(msg)
     }
     async call() {
@@ -348,7 +348,7 @@ class $Root extends $stdlib.std.Resource {
       static _toInflightType(context) {
         return `
           require("./inflight.$Closure2-1.js")({
-            $f: ${context._lift(f)},
+            $Foo: ${context._lift(Foo)},
           })
         `;
       }
@@ -368,7 +368,7 @@ class $Root extends $stdlib.std.Resource {
       }
       _registerBind(host, ops) {
         if (ops.includes("handle")) {
-          $Closure2._registerBindObject(f, host, ["print"]);
+          $Closure2._registerBindObject(Foo, host, ["print"]);
         }
         super._registerBind(host, ops);
       }