I don't think this holds for skipLibCheck: false

Since getSemanticDiagnostics(sourceFile) already appears to fast-return for .d.ts files under skipLibCheck, this change looks like an extra short-circuit in front of an existing short-circuit. I’m not seeing evidence of any win here, but we do take on extra code paths and another embedded assumption about TS diagnostic behavior. Unless there’s profiling showing this matters, I’d wouldn’t recommend moving forward with this change.

@atscott Measured on a real app (build time ~50s, ~N .d.ts files in node_modules, "N" because I don't know how many of them, when I log "continue" in that if-condition, it kills my terminal). NG_DIAGNOSTICS_TOTAL dropped from ~16s → ~12.9s (~19%). While TS does return emptyArray internally for these files, the call overhead across hundreds of declaration files is non-trivial. Please note I'm only learning in this direction, you can also point me whether I should do any additional profiling, I'm wondering whether ~20% is also a small win.

Also, DURATION[NG_DIAGNOSTICS_SEMANTIC] and other yield* expressions are NOT printed, is that a bug or expected?

That performance difference seems likely due to build time variance.

I modified the Angular build performance tracker to log individual durations for every getSemanticDiagnostics call. I then compared the unpatched (baseline) and patched compilations of the web application with parallel TypeScript compilation disabled (NG_BUILD_PARALLEL_TS=0) to ensure accurate measurement. The build was then performed on a project with ~7000 source files.

Results Summary

Key Findings

1. Call Count is Correctly Reduced

The patch correctly skips declaration files before calling getSemanticDiagnostics(). This avoided 1,238 redundant calls (dropping from 4,027 to 2,789).

2. TypeScript Fast Path makes Calls Extremely Cheap

When skipLibCheck is enabled (the default), TypeScript's getSemanticDiagnostics() has an internal fast path that returns an empty array almost instantly for declaration files:

• Average baseline call duration: ~19.6 microseconds (0.000019614s).
• Because these calls are so cheap, the total time spent on them is negligible.

3. Absolute Savings are Negligible

Skipping 1,238 calls saved a total of 16.8 milliseconds (0.0168s).
This is only ~0.08% of the total diagnostics phase (NG_DIAGNOSTICS_TOTAL ~20s), which is completely imperceptible.

While the patch is technically correct in avoiding redundant calls, the actual performance benefit is in the range of milliseconds, not seconds. The change does not hurt, but it does not provide the claimed speedup.