Speed up narrowing of literal unions by jakebailey · Pull Request #64044 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-08-26T22:59:07Z

Version	User CPU	Wall time
Base	10.8–12.1 s	6.9–8.4 s
`fix-55948`	0.08–0.09 s	0.07–0.10 s

We'll see if this hurts too much elsewhere

Narrowing a union by a type predicate maps every source constituent over every candidate constituent through the general type relation machinery. Large generated literal unions therefore take quadratic time even though matching literals only requires equality. Intersect unions made entirely of non-enum literals using keyed sets. Keep enum and mixed unions on the existing relation path because they require broader assignability semantics. Also bypass relation work for individual identical literals encountered by that fallback path. Add coverage for narrowing mixed string and number literal unions, including equal-valued literals of different primitive kinds. Fixes microsoft#55948

Copilot

Pull request overview

Optimizes literal-union narrowing for type predicates using direct literal comparisons and lookup-based filtering.

Changes:

Adds a fast path for non-enum literal unions.
Adds direct literal comparison during narrowing.
Adds a compiler regression test and baselines.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`tsc/internal/checker/flow.go`	Implements literal-union narrowing optimizations.
`tsc/testdata/tests/cases/compiler/narrowLiteralUnionByTypePredicate.ts`	Adds the regression test.
`tsc/testdata/baselines/reference/compiler/narrowLiteralUnionByTypePredicate.types`	Records narrowed types.
`tsc/testdata/baselines/reference/compiler/narrowLiteralUnionByTypePredicate.symbols`	Records symbol resolution.
`tsc/testdata/baselines/reference/compiler/narrowLiteralUnionByTypePredicate.js`	Records emitted JavaScript.

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+				if tLiteralFlags != 0 && tLiteralFlags == nLiteralFlags && !(t.flags&TypeFlagsEnumLiteral != 0 && n.flags&TypeFlagsEnumLiteral != 0) {
+					if t.AsLiteralType().value == n.AsLiteralType().value {
+						return t
+					}
+					return c.neverType
+				}


The per-literal fast path compared values and retained the source constituent. When a plain string literal was narrowed by a predicate whose candidate was the matching enum member, this lost the enum type and made the result unassignable to that string enum member. Retain the candidate when it carries enum identity and the source does not. Continue using the existing relation machinery when both sides are enum members, where equal underlying values are not sufficient. Add a string-enum predicate regression that requires the narrowed value to remain assignable to the asserted enum member.

Jake Bailey (jakebailey) · 2026-08-28T15:57:03Z

TypeScript Bot (@typescript-bot) perf test this faster

typescript-automation · 2026-08-28T15:57:09Z

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

Command	Status	Results
`perf test this faster`	✅ Started	👀 Results

typescript-automation · 2026-08-28T16:21:44Z

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	_115,480	_115,480	_~	_~	_~	_{p=1.000 n=12}
_Types	_96,945	_96,945	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{148,422k (± 0.59%)}	_{148,748k (± 0.46%)}	_~	_146,386k	_150,769k	_{p=0.198 n=12}
_{Memory Allocs}	_{2,271,244 (± 0.01%)}	_{2,271,214 (± 0.00%)}	_~	_2,270,985	_2,271,571	_{p=0.932 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 3.31%)}	_{0.041s (± 3.90%)}	_~	_0.035s	_0.045s	_{p=0.592 n=12}
_{Bind Time}	_{0.013s (±13.18%)}	_{0.013s (±12.23%)}	_~	_0.010s	_0.018s	_{p=0.476 n=12}
_{Check Time}	_{0.491s (± 0.97%)}	_{0.487s (± 0.78%)}	_~	_0.479s	_0.496s	_{p=0.270 n=12}
_{Emit Time}	_{0.271s (± 2.48%)}	_{0.269s (± 1.82%)}	_~	_0.256s	_0.278s	_{p=0.765 n=12}
_{Total Time}	_{0.823s (± 0.64%)}	_{0.816s (± 0.53%)}	_{-0.007s (- 0.82%)}	_0.810s	_0.830s	_{p=0.029 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,505 (± 0.11%)}	_{847,558 (± 0.10%)}	_~	_844,254	_849,242	_{p=0.078 n=12}
_Types	_{249,852 (± 0.00%)}	_{249,851 (± 0.00%)}	_~	_249,850	_249,853	_{p=0.213 n=12}
_{Memory Used}	_{793,483k (± 0.07%)}	_{793,412k (± 0.06%)}	_~	_792,302k	_794,625k	_{p=0.713 n=12}
_{Memory Allocs}	_{12,952,351 (± 0.02%)}	_{12,958,405 (± 0.08%)}	_~	_12,949,063	_13,008,939	_{p=0.887 n=12}
_{Config Time}	_{0.016s (± 1.75%)}	_0.016s	_~	_~	_~	_{p=0.217 n=12}
_{Parse Time}	_{0.271s (± 2.21%)}	_{0.275s (± 2.75%)}	_~	_0.254s	_0.293s	_{p=0.486 n=12}
_{Bind Time}	_{0.066s (±14.55%)}	_{0.063s (±12.23%)}	_~	_0.058s	_0.101s	_{p=0.165 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.619s (± 0.79%)}	_{1.622s (± 0.71%)}	_~	_1.605s	_1.672s	_{p=0.660 n=12}
_{Total Time}	_{1.989s (± 0.99%)}	_{1.992s (± 0.96%)}	_~	_1.964s	_2.078s	_{p=0.899 n=12}
_{mui-docs - native}
_Errors	_{11,276 (± 0.03%)}	_{11,277 (± 0.02%)}	_~	_11,268	_11,281	_{p=0.721 n=12}
_Symbols	_4,268,528	_4,268,528	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,593,260	_1,593,260	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{4,728,409k (± 0.04%)}	_{4,728,116k (± 0.03%)}	_~	_4,723,833k	_4,731,836k	_{p=0.811 n=12}
_{Memory Allocs}	_{47,622,280 (± 0.04%)}	_{47,631,652 (± 0.03%)}	_~	_47,594,353	_47,665,398	_{p=0.319 n=12}
_{Config Time}	_0.016s	_{0.016s (± 1.55%)}	_~	_0.015s	_0.016s	_{p=0.478 n=12}
_{Parse Time}	_{0.557s (± 2.08%)}	_{0.547s (± 2.70%)}	_~	_0.498s	_0.576s	_{p=0.213 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{16.310s (± 0.11%)}	_{16.389s (± 0.42%)}	_{+0.079s (+ 0.49%)}	_16.295s	_16.669s	_{p=0.004 n=12}
_{Emit Time}	_{0.435s (± 1.76%)}	_{0.432s (± 0.44%)}	_~	_0.429s	_0.439s	_{p=0.876 n=12}
_{Total Time}	_{18.069s (± 0.21%)}	_{18.128s (± 0.50%)}	_~	_17.876s	_18.449s	_{p=0.203 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,394,252	_1,394,252	_~	_~	_~	_{p=1.000 n=12}
_Types	_443,418	_443,418	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,650,175k (± 0.28%)}	_{1,649,850k (± 0.28%)}	_~	_1,636,294k	_1,661,047k	_{p=0.977 n=12}
_{Memory Allocs}	_{97,080,221 (± 0.03%)}	_{97,068,909 (± 0.04%)}	_~	_96,982,250	_97,194,752	_{p=0.514 n=12}
_{Config Time}	_{0.004s (±14.46%)}	_{0.004s (± 9.48%)}	_~	_0.003s	_0.005s	_{p=0.985 n=12}
_{Parse Time}	_{0.199s (± 2.30%)}	_{0.200s (± 2.81%)}	_~	_0.183s	_0.218s	_{p=0.943 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.808s (± 0.56%)}	_{1.789s (± 0.42%)}	_{-0.019s (- 1.04%)}	_1.773s	_1.808s	_{p=0.008 n=12}
_{Emit Time}	_{0.274s (± 5.67%)}	_{0.280s (± 5.44%)}	_~	_0.244s	_0.313s	_{p=0.560 n=12}
_{Total Time}	_{24.759s (± 0.77%)}	_{24.632s (± 0.59%)}	_~	_24.217s	_25.024s	_{p=0.219 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_337,531	_337,531	_~	_~	_~	_{p=1.000 n=12}
_Types	_198,830	_198,830	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{319,502k (± 0.01%)}	_{319,547k (± 0.03%)}	_~	_319,279k	_319,797k	_{p=0.198 n=12}
_{Memory Allocs}	_{4,673,094 (± 0.01%)}	_{4,672,921 (± 0.01%)}	_~	_4,671,914	_4,673,958	_{p=0.514 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.117s (± 4.18%)}	_{0.117s (± 4.43%)}	_~	_0.104s	_0.130s	_{p=0.787 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.042s (± 0.61%)}	_{1.038s (± 0.71%)}	_~	_1.018s	_1.054s	_{p=0.385 n=12}
_{Emit Time}	_{0.129s (±13.91%)}	_{0.147s (±11.09%)}	_~	_0.105s	_0.184s	_{p=0.124 n=12}
_{Total Time}	_{1.344s (± 0.96%)}	_{1.356s (± 0.72%)}	_~	_1.336s	_1.387s	_{p=0.091 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_305,179	_305,179	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,876	_181,876	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{277,180k (± 0.03%)}	_{277,212k (± 0.03%)}	_~	_277,012k	_277,355k	_{p=0.452 n=12}
_{Memory Allocs}	_{1,642,076 (± 0.01%)}	_{1,641,657 (± 0.01%)}	_{-420 (- 0.03%)}	_1,641,269	_1,642,136	_{p=0.008 n=12}
_{Config Time}	_{0.000s (±146.91%)}	_{0.000s (±217.90%)}	_~	_0.000s	_0.001s	_{p=1.000 n=12}
_{Parse Time}	_{0.102s (± 4.67%)}	_{0.101s (± 4.01%)}	_~	_0.091s	_0.111s	_{p=0.619 n=12}
_{Bind Time}	_{0.038s (±12.12%)}	_{0.038s (±11.28%)}	_~	_0.030s	_0.053s	_{p=0.680 n=12}
_{Check Time}	_{0.823s (± 0.51%)}	_{0.823s (± 0.61%)}	_~	_0.810s	_0.838s	_{p=0.765 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.977s (± 0.89%)}	_{0.976s (± 0.69%)}	_~	_0.956s	_0.992s	_{p=0.788 n=12}
_{vscode - native}
_Errors	₃₄₇	₃₄₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_8,129,562	_8,129,562	_~	_~	_~	_{p=1.000 n=12}
_Types	_2,798,292	_2,798,292	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{5,493,055k (± 0.02%)}	_{5,492,558k (± 0.01%)}	_~	_5,491,071k	_5,494,419k	_{p=0.551 n=12}
_{Memory Allocs}	_{42,302,535 (± 0.06%)}	_{42,285,996 (± 0.04%)}	_{-16,540 (- 0.04%)}	_42,272,965	_42,371,366	_{p=0.033 n=12}
_{Config Time}	_{0.064s (± 0.42%)}	_{0.064s (± 0.51%)}	_~	_0.063s	_0.065s	_{p=0.832 n=12}
_{Parse Time}	_{1.647s (± 2.74%)}	_{1.668s (± 2.51%)}	_~	_1.507s	_1.733s	_{p=0.434 n=12}
_{Bind Time}	_{0.381s (±11.77%)}	_{0.367s (±11.28%)}	_~	_0.341s	_0.575s	_{p=0.417 n=12}
_{Check Time}	_{9.957s (± 1.35%)}	_{10.011s (± 1.14%)}	_~	_9.550s	_10.141s	_{p=0.854 n=12}
_{Emit Time}	_{3.481s (±10.68%)}	_{3.300s (± 8.82%)}	_{🟩-0.181s (- 5.20%)}	_3.077s	_4.430s	_{p=0.043 n=12}
_{Total Time}	_{15.624s (± 1.65%)}	_{15.502s (± 1.18%)}	_~	_15.295s	_16.240s	_{p=0.378 n=12}
_{webpack - native}
_Errors	₅₄₀	₅₄₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_854,568	_854,568	_~	_~	_~	_{p=1.000 n=12}
_Types	_392,241	_392,241	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{654,123k (± 0.03%)}	_{653,893k (± 0.02%)}	_~	_653,694k	_654,307k	_{p=0.068 n=12}
_{Memory Allocs}	_{4,372,286 (± 0.04%)}	_{4,372,235 (± 0.03%)}	_~	_4,370,229	_4,376,188	_{p=0.977 n=12}
_{Config Time}	_0.008s	_0.008s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.213s (± 2.29%)}	_{0.213s (± 2.54%)}	_~	_0.200s	_0.227s	_{p=0.831 n=12}
_{Bind Time}	_{0.045s (±11.42%)}	_{0.045s (±18.04%)}	_~	_0.039s	_0.086s	_{p=0.183 n=12}
_{Check Time}	_{1.367s (± 0.67%)}	_{1.382s (± 0.56%)}	_{+0.015s (+ 1.13%)}	_1.356s	_1.395s	_{p=0.009 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.658s (± 0.35%)}	_{1.671s (± 0.49%)}	_{+0.014s (+ 0.81%)}	_1.649s	_1.697s	_{p=0.005 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,022,487	_1,022,487	_~	_~	_~	_{p=1.000 n=12}
_Types	_376,962	_376,962	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{603,457k (± 0.02%)}	_{603,484k (± 0.01%)}	_~	_603,248k	_603,681k	_{p=0.932 n=12}
_{Memory Allocs}	_{4,754,187 (± 0.04%)}	_{4,755,753 (± 0.08%)}	_~	_4,750,883	_4,770,668	_{p=0.887 n=12}
_{Config Time}	_{0.003s (±10.34%)}	_{0.003s (±10.34%)}	_~	_0.002s	_0.003s	_{p=1.000 n=12}
_{Parse Time}	_{0.145s (± 3.24%)}	_{0.147s (± 3.23%)}	_~	_0.138s	_0.159s	_{p=0.921 n=12}
_{Bind Time}	_{0.042s (±18.18%)}	_{0.039s (±13.45%)}	_~	_0.033s	_0.062s	_{p=0.763 n=12}
_{Check Time}	_{1.036s (± 0.75%)}	_{1.039s (± 1.13%)}	_~	_1.011s	_1.083s	_{p=0.989 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.237s (± 1.06%)}	_{1.238s (± 1.03%)}	_~	_1.203s	_1.275s	_{p=0.988 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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Copilot AI balanced review requested due to automatic review settings August 26, 2026 22:59

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Speed up narrowing of literal unions - #64044

Speed up narrowing of literal unions#64044
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