[experiment] Skip incompatible discriminants when inferring from union sources by Andarist · Pull Request #64695 · microsoft/TypeScript

Mateusz Burzyński (Andarist) · 2026-10-08T17:21:28Z

I was kinda always wondering if a logic like this could be used to cut down on inference work when dealing with discriminated unions and the referenced PR pushed me to try it out.

I'd still like to do more work on this and recheck things. That's why I'm opening this as a draft, I'd be curious what extended user tests etc would show for this

Jake Bailey (jakebailey) · 2026-10-08T17:24:26Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-10-08T17:24:33Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`test top400`	✅ Started	✅ Results
`user test this`	✅ Started	✅ Results
`run dt`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

Copilot

🔵 Needs a closer look

The draft changes broad compiler inference semantics despite strong regression coverage and warrants human evaluation against extended user tests.

0 open findings

What changed in this PR

Adds discriminant-aware filtering to union-source inference, addressing #64690 while expanding compiler regression coverage.

Changes:

Skips source constituents with incompatible fixed discriminants.
Tests aliases, nesting, tag types, ordering, and optional-property semantics.
Adds diagnostics, type, and symbol baselines for both optional-property modes.

File	Description
`tsc/internal/checker/inference.go`	Implements discriminant-aware inference filtering.
`tsc/testdata/tests/cases/compiler/inferenceDiscriminatedUnionSource.ts`	Adds compiler regression cases.
`tsc/testdata/baselines/reference/compiler/inferenceDiscriminatedUnionSource(exactoptionalpropertytypes=true).types`	Records inferred types.
`tsc/testdata/baselines/reference/compiler/inferenceDiscriminatedUnionSource(exactoptionalpropertytypes=true).symbols`	Records symbols.
`tsc/testdata/baselines/reference/compiler/inferenceDiscriminatedUnionSource(exactoptionalpropertytypes=true).errors.txt`	Records diagnostics.
`tsc/testdata/baselines/reference/compiler/inferenceDiscriminatedUnionSource(exactoptionalpropertytypes=false).types`	Records inferred types.
`tsc/testdata/baselines/reference/compiler/inferenceDiscriminatedUnionSource(exactoptionalpropertytypes=false).symbols`	Records symbols.
`tsc/testdata/baselines/reference/compiler/inferenceDiscriminatedUnionSource(exactoptionalpropertytypes=false).errors.txt`	Records diagnostics.

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typescript-automation · 2026-10-08T17:48:44Z

Hey Jake Bailey (@jakebailey), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

typescript-automation · 2026-10-08T17:55:44Z

Jake Bailey (@jakebailey) Here are the results of running the user tests with tsc comparing baseline and pr:

Everything looks good!

typescript-automation · 2026-10-08T17:55:47Z

Jake Bailey (@jakebailey)
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - native}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_115,276	_115,344	_{+68 (+ 0.06%)}	_~	_~	_{p=0.000 n=12}
_Types	_79,613	_79,825	_{+212 (+ 0.27%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{139,467k (± 0.05%)}	_{139,524k (± 0.07%)}	_~	_139,283k	_139,868k	_{p=0.291 n=12}
_{Memory Allocs}	_{2,190,609 (± 0.00%)}	_{2,191,450 (± 0.01%)}	_{+842 (+ 0.04%)}	_2,191,208	_2,191,918	_{p=0.000 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 4.59%)}	_{0.042s (± 3.81%)}	_~	_0.037s	_0.046s	_{p=0.466 n=12}
_{Bind Time}	_{0.013s (±11.42%)}	_{0.013s (±15.40%)}	_~	_0.010s	_0.018s	_{p=0.651 n=12}
_{Check Time}	_{0.496s (± 0.81%)}	_{0.499s (± 0.88%)}	_~	_0.490s	_0.512s	_{p=0.324 n=12}
_{Emit Time}	_{0.271s (± 1.78%)}	_{0.271s (± 1.74%)}	_~	_0.263s	_0.291s	_{p=0.743 n=12}
_{Total Time}	_{0.827s (± 0.53%)}	_{0.831s (± 0.89%)}	_~	_0.815s	_0.856s	_{p=0.522 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{845,646 (± 0.07%)}	_{846,217 (± 0.08%)}	_~	_844,344	_847,855	_{p=0.266 n=12}
_Types	_{248,123 (± 0.00%)}	_{251,330 (± 0.00%)}	_{+3,206 (+ 1.29%)}	_251,326	_251,340	_{p=0.000 n=12}
_{Memory Used}	_{788,084k (± 0.03%)}	_{789,200k (± 0.02%)}	_{+1,116k (+ 0.14%)}	_788,766k	_789,559k	_{p=0.000 n=12}
_{Memory Allocs}	_{12,920,468 (± 0.02%)}	_{12,931,329 (± 0.03%)}	_{+10,861 (+ 0.08%)}	_12,924,217	_12,946,521	_{p=0.000 n=12}
_{Config Time}	_{0.016s (± 1.97%)}	_{0.016s (± 1.75%)}	_~	_0.016s	_0.017s	_{p=0.667 n=12}
_{Parse Time}	_{0.263s (± 1.85%)}	_{0.253s (± 3.42%)}	_~	_0.225s	_0.271s	_{p=0.061 n=12}
_{Bind Time}	_{0.059s (± 2.22%)}	_{0.069s (±17.04%)}	_{🔻+0.011s (+18.07%)}	_0.059s	_0.119s	_{p=0.013 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.584s (± 0.76%)}	_{1.597s (± 1.43%)}	_~	_1.553s	_1.669s	_{p=0.523 n=12}
_{Total Time}	_{1.935s (± 0.72%)}	_{1.950s (± 1.46%)}	_~	_1.895s	_2.026s	_{p=0.580 n=12}
_{mui-docs - native}
_Errors	_11,405	_11,405	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_4,359,614	_4,359,688	_{+74 (+ 0.00%)}	_~	_~	_{p=0.000 n=12}
_Types	_1,376,379	_1,382,217	_{+5,838 (+ 0.42%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{2,954,247k (± 0.06%)}	_{2,955,020k (± 0.05%)}	_~	_2,951,708k	_2,960,703k	_{p=0.514 n=12}
_{Memory Allocs}	_{32,839,861 (± 0.07%)}	_{32,841,660 (± 0.04%)}	_~	_32,819,481	_32,866,395	_{p=0.977 n=12}
_{Config Time}	_{0.016s (± 1.97%)}	_{0.017s (± 1.45%)}	_~	_0.016s	_0.017s	_{p=0.089 n=12}
_{Parse Time}	_{0.502s (± 1.89%)}	_{0.492s (± 2.84%)}	_~	_0.452s	_0.516s	_{p=0.385 n=12}
_{Bind Time}	_{0.002s (±31.13%)}	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{8.681s (± 0.51%)}	_{8.637s (± 0.68%)}	_~	_8.528s	_8.788s	_{p=0.272 n=12}
_{Emit Time}	_{0.487s (±14.75%)}	_{0.528s (±14.10%)}	_~	_0.419s	_0.682s	_{p=0.172 n=12}
_{Total Time}	_{10.449s (± 0.31%)}	_{10.450s (± 0.52%)}	_~	_10.269s	_10.566s	_{p=0.767 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,390,229	_1,394,216	_{+3,987 (+ 0.29%)}	_~	_~	_{p=0.000 n=12}
_Types	_441,498	_450,921	_{+9,423 (+ 2.13%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{1,685,848k (± 0.99%)}	_{1,679,834k (± 0.85%)}	_~	_1,661,307k	_1,723,079k	_{p=0.590 n=12}
_{Memory Allocs}	_{92,117,106 (± 0.15%)}	_{92,360,812 (± 0.17%)}	_{+243,706 (+ 0.26%)}	_92,046,435	_92,717,754	_{p=0.020 n=12}
_{Config Time}	_{0.004s (±10.74%)}	_{0.003s (±18.62%)}	_{🟩-0.001s (-19.15%)}	_0.002s	_0.005s	_{p=0.048 n=12}
_{Parse Time}	_{0.199s (± 4.36%)}	_{0.193s (± 3.28%)}	_~	_0.179s	_0.214s	_{p=0.173 n=12}
_{Bind Time}	_{0.000s (±217.90%)}	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.813s (± 0.66%)}	_{1.820s (± 0.54%)}	_~	_1.801s	_1.849s	_{p=0.291 n=12}
_{Emit Time}	_{0.291s (± 6.21%)}	_{0.303s (± 2.82%)}	_~	_0.280s	_0.327s	_{p=0.417 n=12}
_{Total Time}	_{23.517s (± 0.73%)}	_{23.633s (± 0.69%)}	_~	_23.198s	_23.996s	_{p=0.347 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_334,953	_336,259	_{+1,306 (+ 0.39%)}	_~	_~	_{p=0.000 n=12}
_Types	_197,616	_201,520	_{+3,904 (+ 1.98%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{313,258k (± 0.02%)}	_{314,862k (± 0.03%)}	_{+1,604k (+ 0.51%)}	_314,659k	_315,126k	_{p=0.000 n=12}
_{Memory Allocs}	_{4,623,651 (± 0.01%)}	_{4,637,069 (± 0.01%)}	_{+13,418 (+ 0.29%)}	_4,634,924	_4,637,871	_{p=0.000 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.114s (± 4.63%)}	_{0.114s (± 5.00%)}	_~	_0.100s	_0.130s	_{p=0.966 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.050s (± 0.52%)}	_{1.055s (± 0.36%)}	_~	_1.045s	_1.066s	_{p=0.246 n=12}
_{Emit Time}	_{0.140s (±11.74%)}	_{0.126s (±12.55%)}	_~	_0.100s	_0.168s	_{p=0.325 n=12}
_{Total Time}	_{1.357s (± 1.26%)}	_{1.352s (± 0.95%)}	_~	_1.323s	_1.383s	_{p=0.560 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_303,834	_305,049	_{+1,215 (+ 0.40%)}	_~	_~	_{p=0.000 n=12}
_Types	_181,663	_185,425	_{+3,762 (+ 2.07%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{273,936k (± 0.02%)}	_{275,311k (± 0.02%)}	_{+1,374k (+ 0.50%)}	_275,201k	_275,543k	_{p=0.000 n=12}
_{Memory Allocs}	_{1,615,929 (± 0.01%)}	_{1,629,712 (± 0.02%)}	_{+13,783 (+ 0.85%)}	_1,629,137	_1,630,454	_{p=0.000 n=12}
_{Config Time}	_0.000s	_{0.000s (±146.91%)}	_~	_0.000s	_0.001s	_{p=0.478 n=12}
_{Parse Time}	_{0.100s (± 3.02%)}	_{0.101s (± 3.96%)}	_~	_0.091s	_0.114s	_{p=0.659 n=12}
_{Bind Time}	_{0.037s (±13.63%)}	_{0.036s (±10.94%)}	_~	_0.028s	_0.047s	_{p=0.987 n=12}
_{Check Time}	_{0.828s (± 0.56%)}	_{0.830s (± 0.64%)}	_~	_0.821s	_0.846s	_{p=0.876 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.980s (± 0.80%)}	_{0.983s (± 1.05%)}	_~	_0.963s	_1.009s	_{p=0.639 n=12}
_{vscode - native}
_Errors	₃₈₀	₃₈₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_11,123,441	_11,130,334	_{+6,893 (+ 0.06%)}	_~	_~	_{p=0.000 n=12}
_Types	_3,686,393	_3,728,322	_{+41,929 (+ 1.14%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{7,127,150k (± 0.01%)}	_{7,141,723k (± 0.02%)}	_{+14,573k (+ 0.20%)}	_7,138,568k	_7,144,118k	_{p=0.000 n=12}
_{Memory Allocs}	_{54,647,398 (± 0.01%)}	_{54,766,078 (± 0.01%)}	_{+118,680 (+ 0.22%)}	_54,756,768	_54,789,920	_{p=0.000 n=12}
_{Config Time}	_{0.074s (± 0.53%)}	_{0.074s (± 0.49%)}	_~	_0.073s	_0.075s	_{p=0.870 n=12}
_{Parse Time}	_{1.810s (± 5.75%)}	_{1.806s (± 3.36%)}	_~	_1.666s	_2.004s	_{p=0.523 n=12}
_{Bind Time}	_{0.662s (±18.88%)}	_{0.588s (±15.39%)}	_~	_0.447s	_0.876s	_{p=0.541 n=12}
_{Check Time}	_{14.180s (± 1.50%)}	_{14.367s (± 1.55%)}	_~	_13.918s	_14.775s	_{p=0.062 n=12}
_{Emit Time}	_{5.327s (± 9.71%)}	_{4.941s (±11.75%)}	_~	_4.096s	_6.160s	_{p=0.319 n=12}
_{Total Time}	_{22.140s (± 1.53%)}	_{21.863s (± 1.88%)}	_~	_21.244s	_22.766s	_{p=0.319 n=12}
_{webpack - native}
_Errors	₅₈₉	₅₈₉	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,302,905	_1,303,729	_{+824 (+ 0.06%)}	_~	_~	_{p=0.000 n=12}
_Types	_618,533	_621,278	_{+2,745 (+ 0.44%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{1,010,981k (± 0.02%)}	_{1,011,922k (± 0.01%)}	_{+941k (+ 0.09%)}	_1,011,664k	_1,012,089k	_{p=0.000 n=12}
_{Memory Allocs}	_{6,595,437 (± 0.02%)}	_{6,606,104 (± 0.05%)}	_{+10,668 (+ 0.16%)}	_6,602,158	_6,621,750	_{p=0.000 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.290s (± 3.50%)}	_{0.297s (± 2.46%)}	_~	_0.271s	_0.315s	_{p=0.247 n=12}
_{Bind Time}	_{0.082s (±16.47%)}	_{0.071s (± 9.16%)}	_~	_0.062s	_0.099s	_{p=0.504 n=12}
_{Check Time}	_{2.023s (± 0.61%)}	_{2.032s (± 0.79%)}	_~	_1.976s	_2.055s	_{p=0.091 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.439s (± 0.67%)}	_{2.445s (± 0.81%)}	_~	_2.382s	_2.499s	_{p=0.809 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,055,452	_1,060,768	_{+5,316 (+ 0.50%)}	_~	_~	_{p=0.000 n=12}
_Types	_386,738	_389,848	_{+3,110 (+ 0.80%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{613,361k (± 0.03%)}	_{615,681k (± 0.03%)}	_{+2,320k (+ 0.38%)}	_615,295k	_616,271k	_{p=0.000 n=12}
_{Memory Allocs}	_{4,862,964 (± 0.04%)}	_{4,907,231 (± 0.11%)}	_{+44,267 (+ 0.91%)}	_4,896,581	_4,926,204	_{p=0.000 n=12}
_{Config Time}	_0.003s	_0.003s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.141s (± 3.53%)}	_{0.143s (± 2.59%)}	_~	_0.136s	_0.154s	_{p=0.485 n=12}
_{Bind Time}	_{0.047s (±18.37%)}	_{0.040s (±10.57%)}	_~	_0.032s	_0.051s	_{p=0.353 n=12}
_{Check Time}	_{1.138s (± 0.48%)}	_{1.157s (± 0.67%)}	_{+0.019s (+ 1.68%)}	_1.143s	_1.184s	_{p=0.000 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.339s (± 0.86%)}	_{1.355s (± 0.89%)}	_~	_1.335s	_1.397s	_{p=0.109 n=12}

System info unknown

Hosts

native

Scenarios

Compiler-Unions - native
angular-1 - native
mui-docs - native
strada-build-src - native
strada-compiler - native
ts-pre-modules - native
vscode - native
webpack - native
xstate-main - native

_Benchmark	_Name	_Iterations
_Current	_pr	₁₂
_Baseline	_baseline	₁₂

Developer Information:

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Wesley Wigham (weswigham) · 2026-10-08T18:19:11Z

Man, when we tried this in the JS codebase, the type comparison on every possible inference target ended up blowing up any savings we got from skipping deeper inferences on less appropriate members. But the perf suite shows it as good now, so I'm a fan. Maybe the optional prop fastpath and objecty type carve-outs are saving enough low-value comparisons it's at least perf neutral again? I'd be happier if we used discriminateTypeByDiscriminableItems (and maybe optimized it a bit to match), though, so "the method of discrimination" always matches. areTypesComparable is interesting, considering we normally discriminate using assignability - thought I guess we're kinda inverting the normal check here - "skip incompatible" instead of "select only compatible". In theory those are compliments, though.

typescript-automation · 2026-10-08T18:37:44Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing baseline and pr:

Everything looks good!

Skip incompatible discriminants when inferring from union sources

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Copilot AI balanced review requested due to automatic review settings October 8, 2026 17:21

typescript-automation Bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Oct 8, 2026

Copilot started reviewing on behalf of Mateusz Burzyński (Andarist) October 8, 2026 17:22 View session

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[experiment] Skip incompatible discriminants when inferring from union sources - #64695

[experiment] Skip incompatible discriminants when inferring from union sources#64695
Mateusz Burzyński (Andarist) wants to merge 1 commit into
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Andarist:fix-go/contextual-inference-discriminants

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