Fix compiler crash on generics overload resolution failure

[moznion (moznion)]

Fixes #61524

Replace Debug.fail() calls with proper error generation to prevent TypeScript compiler crashes when overload resolution fails for complex generic constraints, for example JSX elements.

Problem

The TypeScript compiler crashes with "Debug Failure. No error for last overload signature" when resolving certain complex generic constraints, e.g., JSX element types. This regression has existed since TypeScript 3.6 and affects many React/JSX projects.

Root Cause

When getSignatureApplicabilityError() returns no diagnostics for overload candidates, the compiler calls Debug.fail() instead of handling the edge case gracefully, causing an immediate crash.

Solution

• Replace Debug.fail() calls with fallback diagnostic generation
• Generate appropriate error messages for unhandled overload resolution cases
• Ensure compiler continues execution without crashing

Note

This change doesn’t fix the core logic of type inference. Ideally, that core should be fixed, but this pull request only addresses the compiler crash.

Related to #61524, #60229, #48636, #33133

[moznion (moznion)]

@microsoft-github-policy-service agree

[Jake Bailey (jakebailey)]

I think this is just papering over a bug somewhere else. IIRC the intent of these debug fails are to assert that the previous steps added an error.

[moznion (moznion)]

Jake Bailey (@jakebailey)
Thank you for your comment. Are you saying that the core issue lies in type inference, or somewhere else?
It makes sense from a language-processor perspective, but in our project the crash occurs during type checking, and it has been blocking us from upgrading from v5.6.3; this suggests the problem first appeared in v5.7.

[Jake Bailey (jakebailey)]

If it's a regression, I would consider bisecting with https://www.npmjs.com/package/every-ts

[moznion (moznion)]

I don't think this is a regression. It appears that the problem still occurs on the latest main branch.

[Jake Bailey (jakebailey)]

But you said it wasn't broken in 5.5, so there is a commit in between where this broke.

[Jake Bailey (jakebailey)]

But, #61524 (comment) seems to imply there's already a PR for this?

[TypeScript Bot (typescript-bot)]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate