Fix TypeScript Arrow Function Disambiguation in Ternary Expressions

Summary

This pull request fixes a long-standing issue in the TypeScript parser where the ternary separator (:) was incorrectly interpreted as an arrow function return type annotation when the ternary consequent was a parenthesized expression.

Issue: #17140

The Problem

In valid TypeScript, the following code should parse correctly:

let _ = a ? (b) : c => 0;

However, the Babel parser's TypeScript plugin would misinterpret (b) : c => 0 as a single arrow function expression, leading to a "Unexpected token, expected ':'" error at the end of the ternary because it consumed the ternary separator as part of the arrow.

This was previously documented as a "known limitation" in Babel's test fixtures (specifically in arrow-ambiguity).

The Solution

The fix involves two key changes to the TypeScript parser plugin:

1. Context Sensitivity for shouldParseArrow: Mirroring the logic for async arrows, shouldParseArrow now checks if the parser is currently inside a ternary consequent. If so, a colon encountered after a parenthesized expression is treated as a ternary separator rather than a return type annotation.
2. State Management across Nesting: Since parseParenAndDistinguishExpression resets the inConditionalConsequent flag (which is necessary for async arrows inside nested expressions), we now track the state more precisely. We save the outer conditional state before resetting and check it specifically in shouldParseArrow. We also reset this tracking when entering an arrow function body, ensuring that typed arrows inside arrow bodies continue to be correctly parsed.

Changes

• @babel/parser: Modified packages/babel-parser/src/plugins/typescript/index.ts to implement the disambiguation logic in shouldParseArrow, parseParenAndDistinguishExpression, and parseArrowExpression.
• Tests:
  • Added a new test fixture for the reported issue in packages/babel-parser/test/fixtures/typescript/conditional/arrow-in-conditional-consequent/.
  • Updated 3 existing test fixtures that previously expected parse errors.
  • Updated 3 other fixtures to reflect the corrected AST structure for nested arrows in ternaries.

Verification

• Baseline Check: Confirmed that the original issue reproduces as a failure.
• Unit Tests: All 21,971 tests in @babel/parser pass with zero regressions.
• Integration: Verified that the change correctly handles all edge cases including nested ternaries and async calls.

Fixes #17140.