Critical null pointer bug in incremental builder: affectedFilesIndex undefined access

[DrHazemAli]

🔎 Search Terms

There is a critical null pointer issue in TypeScript's incremental compilation builder system where state.affectedFilesIndex is accessed with non-null assertion (!) despite being defined as number | undefined in the BuilderProgramState interface.

incremental builder, affectedFilesIndex, null pointer, builder.ts, GH#18217

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about incremental compilation, builder state management, and null pointer access patterns
• The fix has been provided in PR Fixed (Critical null pointer bug) in incremental builder #61807
• This affects the current main branch
• Related to TODO comment: // TODO: GH#18217

⏯ Playground Link

N/A - compiler issue

💻 Code

Problematic code locations:

• src/compiler/builder.ts line 643: let affectedFilesIndex = state.affectedFilesIndex!; // TODO: GH#18217
• src/compiler/builder.ts line 2147: Similar pattern with undefined access

Interface definition:

export interface BuilderProgramState extends BuilderState, ReusableBuilderProgramState {
    /**
     * Current index to retrieve affected file from
     */
    affectedFilesIndex: number | undefined; // ← Can be undefined
}

🙁 Actual behavior

The code uses non-null assertion (!) when accessing affectedFilesIndex, which can cause runtime crashes when the value is actually undefined.

🙂 Expected behavior

The code should properly handle the undefined case using appropriate null-checking or default values.

Additional information about the issue

This issue also affects diagnostic property mappings between Diagnostic and ReusableDiagnostic types where property names differ (reportsDeprecated vs reportDeprecated).

Impact

• Potential runtime crashes during incremental builds
• Type safety violations
• Affects complex project builds with multiple file dependencies

Proposed Solution

I have a fix ready in PR #61807 that:

1. Replaces state.affectedFilesIndex! with state.affectedFilesIndex ?? 0
2. Fixes diagnostic property mapping inconsistencies
3. Maintains backward compatibility while improving type safety

Risk Assessment: Low risk - only affects error-prone access patterns without changing core functionality.

[sheetalkamat]

These are invariants and hence asserts. If you have repro steps then pls file the issue with repro steps