fix(parser): correct precedence handling for private-in expression (#18169)

## Summary

- Fix incorrect parsing of `1 + #a in b` which was parsed as `1 + (#a in b)` instead of `(1 + #a) in b`
- Add precedence check before parsing `#a in` as `PrivateInExpression`
- Report "Unexpected private identifier" error when precedence doesn't allow `in` operator

## Details

The parser was greedily parsing `#a in` as a `PrivateInExpression` without checking operator precedence. For example, when parsing the RHS of `+` in `1 + #a in b`, the `in` operator's precedence (`Compare = 13`) is lower than `+`'s precedence (`Add = 15`), so `#a in` should not be parsed as a unit.

The fix extracts private-in parsing into `parse_private_in_expression` and adds a check: if `lhs_precedence >= Precedence::Compare`, we reject the private identifier with a syntax error, matching V8 and Babel behavior.

## Test plan

- `cargo test -p oxc_parser` passes
- Conformance improved: Negative Passed 1648 → 1649 for Babel tests
- `invalid-private-followed-by-in-2` test case now correctly reports syntax error

🤖 Generated with [Claude Code](https://claude.com/claude-code)

crates/oxc_parser/src/diagnostics.rs

@@ -730,6 +730,11 @@ pub fn private_in_private(span: Span) -> OxcDiagnostic {
     OxcDiagnostic::error("Unexpected right-hand side of private-in expression").with_label(span)
 }
 
+#[cold]
+pub fn unexpected_private_identifier(span: Span) -> OxcDiagnostic {
+    OxcDiagnostic::error("Unexpected private identifier").with_label(span)
+}
+
 #[cold]
 pub fn import_arguments(span: Span) -> OxcDiagnostic {
     OxcDiagnostic::error("Dynamic imports can only accept a module specifier and an optional set of attributes as arguments").with_label(span)

crates/oxc_parser/src/js/expression.rs

@@ -157,6 +157,27 @@ impl<'a> ParserImpl<'a> {
         self.ast.private_identifier(span, name)
     }
 
+    /// [+In] PrivateIdentifier in ShiftExpression[?Yield, ?Await]
+    fn parse_private_in_expression(
+        &mut self,
+        lhs_span: u32,
+        lhs_precedence: Precedence,
+    ) -> Expression<'a> {
+        let left = self.parse_private_identifier();
+        // Check if `in` operator precedence is allowed at current level.
+        // For `1 + #a in b`, when parsing RHS of `+`, lhs_precedence is `Add` which is
+        // higher than `Compare` (the precedence of `in`), so `#a in` cannot be parsed here.
+        if lhs_precedence >= Precedence::Compare {
+            return self.fatal_error(diagnostics::unexpected_private_identifier(left.span));
+        }
+        self.expect(Kind::In);
+        let right = self.parse_binary_expression_or_higher(Precedence::Compare);
+        if let Expression::PrivateInExpression(private_in_expr) = right {
+            return self.fatal_error(diagnostics::private_in_private(private_in_expr.span));
+        }
+        self.ast.expression_private_in(self.end_span(lhs_span), left, right)
+    }
+
     /// Section [Primary Expression](https://tc39.es/ecma262/#sec-primary-expression)
     /// `PrimaryExpression`[Yield, Await] :
     ///     this
@@ -1143,14 +1164,7 @@ impl<'a> ParserImpl<'a> {
         let lhs_parenthesized = self.at(Kind::LParen);
         // [+In] PrivateIdentifier in ShiftExpression[?Yield, ?Await]
         let lhs = if self.ctx.has_in() && self.at(Kind::PrivateIdentifier) {
-            let left = self.parse_private_identifier();
-            self.expect(Kind::In);
-            let right = self.parse_binary_expression_or_higher(Precedence::Compare);
-            if let Expression::PrivateInExpression(private_in_expr) = right {
-                let error = diagnostics::private_in_private(private_in_expr.span);
-                return self.fatal_error(error);
-            }
-            self.ast.expression_private_in(self.end_span(lhs_span), left, right)
+            self.parse_private_in_expression(lhs_span, lhs_precedence)
         } else {
             let has_pure_comment = self.lexer.trivia_builder.previous_token_has_pure_comment();
             let mut expr = self.parse_unary_expression_or_higher(lhs_span);

tasks/coverage/snapshots/parser_babel.snap

@@ -3,9 +3,7 @@ commit: fc58af40
 parser_babel Summary:
 AST Parsed     : 2224/2230 (99.73%)
 Positive Passed: 2207/2230 (98.97%)
-Negative Passed: 1648/1697 (97.11%)
-Expect Syntax Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2022/private-in/invalid-private-followed-by-in-2/input.js
-
+Negative Passed: 1649/1697 (97.17%)
 Expect Syntax Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2026/explicit-resource-management/invalid-for-using-of-no-initializer/input.js
 
 Expect Syntax Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/estree/class-private-property/typescript-invalid-abstract/input.ts
@@ -7370,12 +7368,11 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
    ·  ────────────────
    ╰────
 
-  × Expected `in` but found `(`
-   ╭─[babel/packages/babel-parser/test/fixtures/es2022/class-private-methods/asi-failure-generator/input.js:3:7]
+  × Unexpected private identifier
+   ╭─[babel/packages/babel-parser/test/fixtures/es2022/class-private-methods/asi-failure-generator/input.js:3:4]
  2 │   p = x
  3 │   *#m () {}
-   ·       ┬
-   ·       ╰── `in` expected
+   ·    ──
  4 │ }
    ╰────
 
@@ -8538,14 +8535,22 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
    ╰────
   help: Did you mean `export { "吾道一以貫之。" as "忠恕。" } from 'some-module'`?
 
-  × Unexpected right-hand side of private-in expression
+  × Unexpected private identifier
    ╭─[babel/packages/babel-parser/test/fixtures/es2022/private-in/invalid-private-followed-by-in-1/input.js:5:11]
  4 │   method() {
  5 │     #a in #b in c
-   ·           ───────
+   ·           ──
  6 │   }
    ╰────
 
+  × Unexpected private identifier
+   ╭─[babel/packages/babel-parser/test/fixtures/es2022/private-in/invalid-private-followed-by-in-2/input.js:4:9]
+ 3 │   method() {
+ 4 │     1 + #a in b
+   ·         ──
+ 5 │   }
+   ╰────
+
   × Unexpected token
    ╭─[babel/packages/babel-parser/test/fixtures/es2022/private-in/invalid-private-followed-by-in-3/input.js:5:18]
  4 │   method() {
@@ -8571,12 +8576,11 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
  5 │   }
    ╰────
 
-  × Expected `in` but found `;`
-   ╭─[babel/packages/babel-parser/test/fixtures/es2022/private-in/private-binary-expression-right/input.js:4:11]
+  × Unexpected private identifier
+   ╭─[babel/packages/babel-parser/test/fixtures/es2022/private-in/private-binary-expression-right/input.js:4:9]
  3 │   test() {
  4 │     1 + #x;
-   ·           ┬
-   ·           ╰── `in` expected
+   ·         ──
  5 │   }
    ╰────
 

tasks/coverage/snapshots/parser_test262.snap

@@ -18123,11 +18123,11 @@ Negative Passed: 4581/4581 (100.00%)
     · ───────────
     ╰────
 
-  × Unexpected right-hand side of private-in expression
+  × Unexpected private identifier
     ╭─[test262/test/language/expressions/in/private-field-in-nested.js:26:15]
  25 │   constructor() {
  26 │     #field in #field in this;
-    ·               ──────────────
+    ·               ──────
  27 │   }
     ╰────
 

tasks/coverage/snapshots/parser_typescript.snap

@@ -14345,12 +14345,12 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/statements/Va
  26 │ 
     ╰────
 
-  × Cannot assign to this expression
-    ╭─[typescript/tests/cases/conformance/classes/members/privateNames/privateNameInInExpressionTransform.ts:29:9]
- 28 │     invalidLHS(v: any) {
- 29 │         'prop' in v = 10;
-    ·         ───────────
- 30 │         #field in v = 10;
+  × Unexpected private identifier
+    ╭─[typescript/tests/cases/conformance/classes/members/privateNames/privateNameInInExpressionTransform.ts:20:14]
+ 19 │ 
+ 20 │         v << #field in v << v; // Good precedence (SyntaxError): (v << #field) in (v << v)
+    ·              ──────
+ 21 │ 
     ╰────
 
   × Unexpected token