Preserve reverse mapped types in declaration emit by jakebailey · Pull Request #64558 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-09-30T17:03:47Z

Gets rid of more silently-produced elided anys. See the second commit for the actual change.

Reverse-mapped property and index signature types are lost in emitted declarations even when source consumers retain them. Cover finite nested shapes, recursive inputs, and consumers of the generated declarations.

Display placeholders discard real property and index signature types when reused for declaration emit. Preserve those types in declarations while leaving editor display unchanged. Reverse-mapped types without symbols can still recurse. Track their type identities and bound expansion by mapped declaration when changing type arguments prevent repetition. Report cycles and truncation rather than silently replacing these types with any.

Copilot

Copilot review overview

🟢 Approval recommended

The implementation preserves declaration accuracy, bounds recursive expansion, and includes comprehensive regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Preserves inferred reverse-mapped types during declaration emit while retaining concise Quick Info display behavior.

Changes:

Expands reverse-mapped properties and index signatures for declarations.
Adds recursion and growth safeguards.
Adds compiler, incremental-build, and Quick Info coverage.

File	Description
`tsc/internal/checker/nodebuilderimpl.go`	Implements declaration serialization and recursion limits.
`tsc/testdata/tests/cases/compiler/reverseMappedDeclarationSerialization.ts`	Covers finite and recursive mappings.
`tsc/internal/fourslash/tests/reverseMappedDeclarationQuickInfo_test.go`	Tests Quick Info behavior.
`tsc/internal/execute/tsctests/tsc_test.go`	Tests incremental declaration consumption.
`tsc/testdata/baselines/reference/compiler/reverseMappedDeclarationSerialization.js`	Records emit results.
`tsc/testdata/baselines/reference/compiler/reverseMappedDeclarationSerialization.errors.txt`	Records recursion diagnostics.
`tsc/testdata/baselines/reference/compiler/reverseMappedDeclarationSerialization.symbols`	Records symbol baselines.
`tsc/testdata/baselines/reference/compiler/reverseMappedDeclarationSerialization.types`	Records inferred-type baselines.
`tsc/testdata/baselines/reference/fourslash/quickInfo/reverseMappedDeclarationQuickInfo.baseline`	Records hover output.
`tsc/testdata/baselines/reference/tsbuild/incremental/reverse-mapped-declaration-consumption.js`	Records incremental-build behavior.

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Jake Bailey (jakebailey) · 2026-09-30T22:29:24Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-09-30T22:29:30Z

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

typescript-automation · 2026-09-30T22:52:44Z

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 · 2026-09-30T22:58:01Z

Jake Bailey (@jakebailey) Here are the results of running the user tests with tsc comparing 792ffccb90f548cd3bdd7fc0b44f0ca9e1b169a7 and refs/pull/64558/merge:

Everything looks good!

typescript-automation · 2026-09-30T22:58:38Z

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	_115,276	_~	_~	_~	_{p=1.000 n=12}
_Types	_79,613	_79,613	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{141,231k (± 0.44%)}	_{141,832k (± 0.47%)}	_~	_140,043k	_143,716k	_{p=0.053 n=12}
_{Memory Allocs}	_{2,194,734 (± 0.01%)}	_{2,194,570 (± 0.00%)}	_{-165 (- 0.01%)}	_2,194,354	_2,194,770	_{p=0.046 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 4.05%)}	_{0.041s (± 3.25%)}	_~	_0.038s	_0.045s	_{p=0.893 n=12}
_{Bind Time}	_{0.014s (±10.03%)}	_{0.013s (±10.11%)}	_~	_0.010s	_0.016s	_{p=0.576 n=12}
_{Check Time}	_{0.500s (± 0.86%)}	_{0.510s (± 1.12%)}	_{+0.010s (+ 1.98%)}	_0.498s	_0.524s	_{p=0.016 n=12}
_{Emit Time}	_{0.267s (± 1.99%)}	_{0.268s (± 1.77%)}	_~	_0.258s	_0.283s	_{p=0.988 n=12}
_{Total Time}	_{0.828s (± 0.82%)}	_{0.838s (± 1.00%)}	_{+0.010s (+ 1.22%)}	_0.816s	_0.859s	_{p=0.037 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,623 (± 0.09%)}	_{846,264 (± 0.13%)}	_~	_843,478	_848,415	_{p=0.755 n=12}
_Types	_{249,940 (± 0.00%)}	_{249,940 (± 0.00%)}	_~	_249,938	_249,949	_{p=0.078 n=12}
_{Memory Used}	_{789,198k (± 0.06%)}	_{789,106k (± 0.05%)}	_~	_788,146k	_790,334k	_{p=0.977 n=12}
_{Memory Allocs}	_{12,938,276 (± 0.07%)}	_{12,933,709 (± 0.02%)}	_~	_12,930,716	_12,940,995	_{p=1.000 n=12}
_{Config Time}	_0.016s	_0.016s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.265s (± 2.89%)}	_{0.269s (± 2.51%)}	_~	_0.246s	_0.284s	_{p=0.640 n=12}
_{Bind Time}	_{0.071s (±19.13%)}	_{0.064s (±13.10%)}	_~	_0.057s	_0.103s	_{p=0.322 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.612s (± 1.43%)}	_{1.613s (± 1.74%)}	_~	_1.570s	_1.706s	_{p=0.989 n=12}
_{Total Time}	_{1.982s (± 1.64%)}	_{1.979s (± 1.69%)}	_~	_1.932s	_2.078s	_{p=0.965 n=12}
_{mui-docs - native}
_Errors	_{11,400 (± 0.04%)}	_{11,396 (± 0.06%)}	_~	_11,374	_11,405	_{p=0.615 n=12}
_Symbols	_4,450,780	_4,450,780	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,441,847	_1,441,847	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{3,190,939k (± 0.05%)}	_{3,190,663k (± 0.05%)}	_~	_3,183,874k	_3,193,553k	_{p=0.671 n=12}
_{Memory Allocs}	_{34,634,841 (± 0.05%)}	_{34,636,172 (± 0.04%)}	_~	_34,609,953	_34,680,625	_{p=0.932 n=12}
_{Config Time}	_{0.016s (± 1.98%)}	_{0.015s (± 2.02%)}	_~	_0.015s	_0.016s	_{p=0.220 n=12}
_{Parse Time}	_{0.580s (± 1.42%)}	_{0.587s (± 2.07%)}	_~	_0.535s	_0.605s	_{p=0.075 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{8.679s (± 1.19%)}	_{8.670s (± 1.26%)}	_~	_8.500s	_8.967s	_{p=0.921 n=12}
_{Emit Time}	_{0.577s (±15.18%)}	_{0.580s (±14.60%)}	_~	_0.428s	_0.716s	_{p=0.966 n=12}
_{Total Time}	_{10.613s (± 0.46%)}	_{10.627s (± 0.55%)}	_~	_10.497s	_10.819s	_{p=0.702 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,390,229	_1,390,229	_~	_~	_~	_{p=1.000 n=12}
_Types	_441,498	_441,498	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,686,148k (± 0.89%)}	_{1,687,916k (± 0.81%)}	_~	_1,648,674k	_1,709,606k	_{p=0.977 n=12}
_{Memory Allocs}	_{92,697,543 (± 0.13%)}	_{92,808,499 (± 0.09%)}	_~	_92,567,578	_92,977,749	_{p=0.089 n=12}
_{Config Time}	_{0.004s (±13.41%)}	_{0.004s (±19.92%)}	_~	_0.002s	_0.007s	_{p=0.738 n=12}
_{Parse Time}	_{0.211s (± 3.53%)}	_{0.211s (± 2.95%)}	_~	_0.194s	_0.230s	_{p=0.702 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.810s (± 0.54%)}	_{1.822s (± 0.57%)}	_~	_1.792s	_1.847s	_{p=0.147 n=12}
_{Emit Time}	_{0.309s (± 4.77%)}	_{0.305s (± 2.91%)}	_~	_0.273s	_0.323s	_{p=0.401 n=12}
_{Total Time}	_{23.534s (± 0.49%)}	_{23.652s (± 0.63%)}	_~	_23.218s	_24.000s	_{p=0.319 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_334,953	_334,953	_~	_~	_~	_{p=1.000 n=12}
_Types	_197,616	_197,616	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{313,347k (± 0.09%)}	_{313,210k (± 0.03%)}	_~	_312,951k	_313,420k	_{p=0.590 n=12}
_{Memory Allocs}	_{4,628,249 (± 0.01%)}	_{4,628,053 (± 0.01%)}	_~	_4,627,200	_4,629,035	_{p=0.541 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.114s (± 4.99%)}	_{0.116s (± 5.88%)}	_~	_0.095s	_0.131s	_{p=0.580 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.050s (± 0.53%)}	_{1.058s (± 0.60%)}	_{+0.008s (+ 0.78%)}	_1.038s	_1.073s	_{p=0.040 n=12}
_{Emit Time}	_{0.132s (± 9.27%)}	_{0.133s (±12.74%)}	_~	_0.096s	_0.174s	_{p=0.560 n=12}
_{Total Time}	_{1.349s (± 0.55%)}	_{1.361s (± 1.04%)}	_~	_1.323s	_1.396s	_{p=0.147 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_303,834	_303,834	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,663	_181,663	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{273,926k (± 0.02%)}	_{273,851k (± 0.02%)}	_~	_273,736k	_274,032k	_{p=0.068 n=12}
_{Memory Allocs}	_{1,617,992 (± 0.01%)}	_{1,618,015 (± 0.02%)}	_~	_1,617,493	_1,618,630	_{p=0.977 n=12}
_{Config Time}	_{0.000s (±146.91%)}	_0.000s	_~	_~	_~	_{p=0.478 n=12}
_{Parse Time}	_{0.097s (± 3.02%)}	_{0.101s (± 4.53%)}	_~	_0.089s	_0.112s	_{p=0.070 n=12}
_{Bind Time}	_{0.037s (±10.49%)}	_{0.036s (± 6.93%)}	_~	_0.028s	_0.045s	_{p=0.854 n=12}
_{Check Time}	_{0.825s (± 0.75%)}	_{0.835s (± 0.59%)}	_{+0.010s (+ 1.19%)}	_0.821s	_0.851s	_{p=0.004 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.974s (± 0.70%)}	_{0.987s (± 0.65%)}	_{+0.013s (+ 1.33%)}	_0.969s	_1.003s	_{p=0.010 n=12}
_{vscode - native}
_Errors	₃₈₂	₃₈₂	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_10,141,897	_10,141,897	_~	_~	_~	_{p=1.000 n=12}
_Types	_3,417,826	_3,417,826	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{6,621,023k (± 0.01%)}	_{6,620,556k (± 0.02%)}	_~	_6,617,839k	_6,623,757k	_{p=0.713 n=12}
_{Memory Allocs}	_{51,016,950 (± 0.01%)}	_{51,018,703 (± 0.02%)}	_~	_51,006,565	_51,058,824	_{p=0.671 n=12}
_{Config Time}	_{0.067s (± 0.42%)}	_{0.067s (± 0.42%)}	_~	_0.067s	_0.068s	_{p=1.000 n=12}
_{Parse Time}	_{1.885s (± 2.79%)}	_{1.874s (± 3.51%)}	_~	_1.722s	_2.025s	_{p=0.875 n=12}
_{Bind Time}	_{0.467s (±15.87%)}	_{0.516s (±22.15%)}	_~	_0.412s	_0.839s	_{p=0.680 n=12}
_{Check Time}	_{13.610s (± 1.08%)}	_{13.586s (± 1.31%)}	_{-0.024s (- 0.18%)}	_13.104s	_13.763s	_{p=0.043 n=12}
_{Emit Time}	_{4.126s (±11.01%)}	_{4.281s (±11.97%)}	_~	_3.731s	_5.689s	_{p=0.541 n=12}
_{Total Time}	_{20.263s (± 1.74%)}	_{20.434s (± 1.98%)}	_~	_19.933s	_21.564s	_{p=0.193 n=12}
_{webpack - native}
_Errors	₅₈₁	₅₈₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,274,916	_1,274,916	_~	_~	_~	_{p=1.000 n=12}
_Types	_595,481	_595,481	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{972,378k (± 0.01%)}	_{972,471k (± 0.01%)}	_~	_972,280k	_972,636k	_{p=0.266 n=12}
_{Memory Allocs}	_{6,277,155 (± 0.03%)}	_{6,278,650 (± 0.05%)}	_~	_6,275,217	_6,292,337	_{p=0.242 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.291s (± 2.34%)}	_{0.284s (± 2.33%)}	_~	_0.265s	_0.297s	_{p=0.147 n=12}
_{Bind Time}	_{0.075s (±18.61%)}	_{0.083s (±18.12%)}	_~	_0.060s	_0.127s	_{p=0.766 n=12}
_{Check Time}	_{1.827s (± 0.62%)}	_{1.827s (± 0.73%)}	_~	_1.788s	_1.865s	_{p=0.702 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.238s (± 0.64%)}	_{2.240s (± 0.75%)}	_~	_2.218s	_2.298s	_{p=0.921 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,030,692	_1,030,692	_~	_~	_~	_{p=1.000 n=12}
_Types	_376,396	_376,396	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{598,426k (± 0.02%)}	_{598,432k (± 0.01%)}	_~	_598,279k	_598,669k	_{p=0.944 n=12}
_{Memory Allocs}	_{4,762,103 (± 0.04%)}	_{4,761,220 (± 0.03%)}	_~	_4,757,621	_4,764,416	_{p=0.590 n=12}
_{Config Time}	_{0.003s (±13.14%)}	_{0.003s (±10.34%)}	_~	_0.002s	_0.003s	_{p=0.400 n=12}
_{Parse Time}	_{0.144s (± 3.57%)}	_{0.146s (± 3.91%)}	_~	_0.131s	_0.159s	_{p=0.384 n=12}
_{Bind Time}	_{0.044s (±19.10%)}	_{0.043s (±18.91%)}	_~	_0.033s	_0.071s	_{p=0.697 n=12}
_{Check Time}	_{1.071s (± 0.86%)}	_{1.070s (± 0.60%)}	_~	_1.060s	_1.095s	_{p=0.876 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.272s (± 0.92%)}	_{1.272s (± 0.63%)}	_~	_1.251s	_1.291s	_{p=0.944 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:

Download Benchmarks

typescript-automation · 2026-09-30T23:44:24Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing 792ffccb90f548cd3bdd7fc0b44f0ca9e1b169a7 and refs/pull/64558/merge:

Everything looks good!

Jake Bailey (jakebailey) added 2 commits September 30, 2026 09:05

Reproduce type loss in reverse mapped declaration emit

f0b3d0d

Reverse-mapped property and index signature types are lost in emitted declarations even when source consumers retain them. Cover finite nested shapes, recursive inputs, and consumers of the generated declarations.

Jake Bailey (jakebailey) requested review from Wesley Wigham (weswigham) and a balanced review from Copilot September 30, 2026 17:03

typescript-automation Bot assigned Jake Bailey (jakebailey) Sep 30, 2026

typescript-automation Bot added Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Sep 30, 2026

Copilot started reviewing on behalf of Jake Bailey (jakebailey) September 30, 2026 17:04 View session

Copilot AI reviewed Sep 30, 2026

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Wesley Wigham (weswigham) approved these changes Sep 30, 2026

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Jake Bailey (jakebailey) added this pull request to the merge queue Sep 30, 2026

Merged via the queue into microsoft:main with commit edf7da4 Oct 1, 2026
29 checks passed

Jake Bailey (jakebailey) deleted the fix-reverse-mapped-declarations branch October 1, 2026 00:02

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Preserve reverse mapped types in declaration emit - #64558

Preserve reverse mapped types in declaration emit#64558
Jake Bailey (jakebailey) merged 2 commits into
microsoft:mainfrom
jakebailey:fix-reverse-mapped-declarations

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