fix(compiler): cannot chain ?? expressions (#5987)

Fixes #1875

This fixes the issue by updating the precedent rules so the operation is right-associative. For now, expressions like `a ?? b` or `a ?? b ?? c` still have to return a non-optional value (like Rust's unwrap_or), but we can consider making it more flexible / open in the way that C# or Swift allows if appropriate in the future.

## 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)*.

examples/tests/valid/optionals.test.w

@@ -16,13 +16,14 @@ class Super {
 class Sub extends Super {
   new() { this.name = "Sub"; }
 }
-class Sub1 extends Super {
-  new() { this.name = "Sub"; }
+class SubSub extends Sub {
+  new() { this.name = "SubSub"; }
 }
 
 let optionalSup: Super? = new Super();
 let s = optionalSup ?? new Sub();
 assert(s.name == "Super");
+let s2 = optionalSup ?? optionalSup ?? new SubSub();
 
 struct Name {
   first: str;

libs/tree-sitter-wing/grammar.js

@@ -326,6 +326,7 @@ module.exports = grammar({
 
     expression: ($) =>
       choice(
+        $.unwrap_or,
         $.binary_expression,
         $.unary_expression,
         $.new_expression,
@@ -568,6 +569,14 @@ module.exports = grammar({
     _container_value_type: ($) =>
       seq("<", field("type_parameter", $._type), ">"),
 
+    unwrap_or: ($) => prec.right(PREC.UNWRAP_OR,
+      seq(
+        field("left", $.expression),
+        field("op", "??"),
+        field("right", $.expression)
+      )
+    ),
+
     optional_unwrap: ($) =>
       prec.right(PREC.OPTIONAL_UNWRAP, seq($.expression, "!")),
 
@@ -614,7 +623,6 @@ module.exports = grammar({
         //['<<', PREC.SHIFT],
         //['>>', PREC.SHIFT],
         //['>>>', PREC.SHIFT],
-        ["??", PREC.UNWRAP_OR],
       ];
 
       return choice(

libs/tree-sitter-wing/test/corpus/expressions.txt

@@ -298,29 +298,37 @@ Template string
 maybeVal ?? 2;
 maybeVal ?? 2 + 2;
 maybeVal ?? 2 == 2;
+maybeVal ?? "hi" ?? 2;
 
 --------------------------------------------------------------------------------
 
 (source
   (expression_statement
-    (binary_expression
+    (unwrap_or
       (reference
         (reference_identifier))
       (number)))
   (expression_statement
-    (binary_expression
+    (unwrap_or
       (reference
         (reference_identifier))
       (binary_expression
         (number)
         (number))))
   (expression_statement
     (binary_expression
-      (binary_expression
+      (unwrap_or
         (reference
           (reference_identifier))
         (number))
-      (number))))
+      (number)))
+  (expression_statement
+    (unwrap_or
+      (reference
+        (reference_identifier))
+      (unwrap_or
+        (string)
+        (number)))))
 
 ================================================================================
 Unwrap expression

libs/wingc/src/parser.rs

@@ -2021,7 +2021,7 @@ impl<'s> Parser<'s> {
 					expression_span,
 				))
 			}
-			"binary_expression" => Ok(Expr::new(
+			"binary_expression" | "unwrap_or" => Ok(Expr::new(
 				ExprKind::Binary {
 					left: Box::new(self.build_expression(&expression_node.child_by_field_name("left").unwrap(), phase)?),
 					right: Box::new(self.build_expression(&expression_node.child_by_field_name("right").unwrap(), phase)?),

tools/hangar/__snapshots__/test_corpus/valid/optionals.test.w_compile_tf-aws.md

@@ -52,19 +52,19 @@ module.exports = function({ $Super }) {
 //# sourceMappingURL=inflight.Sub-1.js.map
 ```
 
-## inflight.Sub1-1.js
+## inflight.SubSub-1.js
 ```js
 "use strict";
 const $helpers = require("@winglang/sdk/lib/helpers");
-module.exports = function({ $Super }) {
-  class Sub1 extends $Super {
+module.exports = function({ $Sub }) {
+  class SubSub extends $Sub {
     constructor({  }) {
       super({  });
     }
   }
-  return Sub1;
+  return SubSub;
 }
-//# sourceMappingURL=inflight.Sub1-1.js.map
+//# sourceMappingURL=inflight.SubSub-1.js.map
 ```
 
 ## inflight.Super-1.js
@@ -187,23 +187,23 @@ class $Root extends $stdlib.std.Resource {
         });
       }
     }
-    class Sub1 extends Super {
+    class SubSub extends Sub {
       constructor($scope, $id, ) {
         super($scope, $id);
-        this.name = "Sub";
+        this.name = "SubSub";
       }
       static _toInflightType() {
         return `
-          require("${$helpers.normalPath(__dirname)}/inflight.Sub1-1.js")({
-            $Super: ${$stdlib.core.liftObject(Super)},
+          require("${$helpers.normalPath(__dirname)}/inflight.SubSub-1.js")({
+            $Sub: ${$stdlib.core.liftObject(Sub)},
           })
         `;
       }
       _toInflight() {
         return `
           (await (async () => {
-            const Sub1Client = ${Sub1._toInflightType()};
-            const client = new Sub1Client({
+            const SubSubClient = ${SubSub._toInflightType()};
+            const client = new SubSubClient({
             });
             if (client.$inflight_init) { await client.$inflight_init(); }
             return client;
@@ -298,6 +298,7 @@ class $Root extends $stdlib.std.Resource {
     const optionalSup = new Super(this, "Super");
     const s = (optionalSup ?? new Sub(this, "Sub"));
     $helpers.assert($helpers.eq(s.name, "Super"), "s.name == \"Super\"");
+    const s2 = (optionalSup ?? (optionalSup ?? new SubSub(this, "SubSub")));
     let name = ({"first": "John", "last": "Doe"});
     {
       const $if_let_value = name;