Support contextual type discrimination based on template literal expressions #62203

Andarist · 2025-08-05T08:11:07Z

Template literal types are allowed as discriminant properties since #46137
but template literal expressions can't be used to discriminate contextual types which manifests in issues like:

type Target =
  | { type: `${string}_FOO`; cb: (arg: number) => void }
  | { type: `${string}_BAR`; cb: (arg: string) => void };

declare const str: string;

const obj1: Target = {
  type: `${str}_FOO`,
  cb: (arg) => {}, // implicit any
};

This PR closes the gap by preventing potential template literal types discriminants from widening to string in the context of context-free types. I don't think this changes all that much the meaning of a context-free type is. Let's consider:

type Target2 =
  | { type: "_FOO"; cb: (arg: number) => void }
  | { type: "_BAR"; cb: (arg: string) => void };

const obj2: Target2 = {
  type: "_FOO",
  cb: (arg) => {},
};

The context-free type of that "_FOO" expression is a fresh "_FOO" string literal type. In the same fashion, a context-free type of a template literal expression can be that unwidened template literal type.

fixes #57231

…le discriminants

…`isPatternLiteralPlaceholderType`

Copilot

Pull Request Overview

This PR adds support for contextual type discrimination based on template literal types. It allows template literal expressions to be used to discriminate contextual types, fixing cases where template literal expressions couldn't properly narrow union types during object literal type checking.

Key changes:

Introduces a new contextFreeType intrinsic type to handle context-free template literal type inference
Modifies template literal type checking to prevent widening to string when template literals can serve as discriminants
Updates getContextFreeType to use the new context-free type instead of anyType

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
`src/compiler/checker.ts`	Adds `contextFreeType` intrinsic and updates template literal type checking logic to preserve discriminant information
`tests/cases/conformance/types/union/discriminatedUnionTypesOverlappingDiscriminants1.ts`	New comprehensive test cases for template literal discriminants in various scenarios
`tests/cases/compiler/templateLiteralConstantEvaluation.ts`	Adds test case for empty string template literal evaluation
`tests/cases/compiler/evolvingArrayTemplateLiterals1.ts`	New test for template literal behavior in evolving arrays
`tests/cases/compiler/discriminantUsingEvaluatableTemplateExpression.ts`	Adds test for evaluatable template expression discrimination
Various baseline files	Expected type checking results and error outputs for the new test cases

Comments suppressed due to low confidence (2)

jakebailey · 2025-08-05T14:35:56Z

@typescript-bot test it

typescript-bot · 2025-08-05T14:36:02Z

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-bot · 2025-08-05T14:59:11Z

Hey @jakebailey, the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

typescript-bot · 2025-08-05T15:01:23Z

@jakebailey Here are the results of running the user tests with tsc comparing main and refs/pull/62203/merge:

Everything looks good!

typescript-bot · 2025-08-05T15:22:15Z

@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 - node (v18.15.0, x64)}
_Errors	₃₄	₃₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,370	_62,370	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,386	_50,387	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{192,929k (± 0.01%)}	_{194,229k (± 0.94%)}	_{+1,299k (+ 0.67%)}	_192,973k	_196,591k	_{p=0.005 n=6}
_{Parse Time}	_{1.30s (± 0.84%)}	_1.31s	_~	_~	_~	_{p=0.071 n=6}
_{Bind Time}	_0.73s	_0.73s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{9.74s (± 0.37%)}	_{9.77s (± 0.29%)}	_~	_9.72s	_9.79s	_{p=0.195 n=6}
_{Emit Time}	_{2.74s (± 0.38%)}	_{2.73s (± 0.73%)}	_~	_2.71s	_2.76s	_{p=0.222 n=6}
_{Total Time}	_{14.51s (± 0.24%)}	_{14.53s (± 0.23%)}	_~	_14.49s	_14.58s	_{p=0.422 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₅₆	₅₆	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_948,914	_948,914	_~	_~	_~	_{p=1.000 n=6}
_Types	_410,884	_410,886	_{+2 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{1,226,430k (± 0.01%)}	_{1,226,446k (± 0.01%)}	_~	_1,226,347k	_1,226,546k	_{p=0.689 n=6}
_{Parse Time}	_{6.49s (± 0.53%)}	_{6.54s (± 0.71%)}	_~	_6.49s	_6.62s	_{p=0.064 n=6}
_{Bind Time}	_{1.88s (± 0.29%)}	_{1.88s (± 0.55%)}	_~	_1.86s	_1.89s	_{p=0.663 n=6}
_{Check Time}	_{31.98s (± 0.38%)}	_{32.01s (± 0.35%)}	_~	_31.79s	_32.11s	_{p=0.334 n=6}
_{Emit Time}	_{14.73s (± 0.59%)}	_{14.78s (± 0.41%)}	_~	_14.72s	_14.89s	_{p=0.747 n=6}
_{Total Time}	_{55.08s (± 0.35%)}	_{55.20s (± 0.25%)}	_~	_54.94s	_55.31s	_{p=0.229 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,548,614	_2,548,614	_~	_~	_~	_{p=1.000 n=6}
_Types	_903,207	_903,230	_{+23 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,833,394k (± 0.01%)}	_{2,833,374k (± 0.00%)}	_~	_2,833,272k	_2,833,636k	_{p=0.575 n=6}
_{Parse Time}	_{8.78s (± 0.27%)}	_{8.77s (± 0.41%)}	_~	_8.72s	_8.83s	_{p=0.627 n=6}
_{Bind Time}	_{2.25s (± 0.81%)}	_{2.25s (± 0.36%)}	_~	_2.24s	_2.26s	_{p=0.801 n=6}
_{Check Time}	_{86.00s (± 0.49%)}	_{85.63s (± 0.66%)}	_~	_84.96s	_86.56s	_{p=0.173 n=6}
_{Emit Time}	_{2.02s (±41.97%)}	_{2.02s (±42.00%)}	_~	_0.30s	_2.49s	_{p=1.000 n=6}
_{Total Time}	_{99.05s (± 0.92%)}	_{98.67s (± 0.50%)}	_~	_97.89s	_99.23s	_{p=0.170 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,227,066	_1,227,071	_{+5 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_267,480	_267,496	_{+16 (+ 0.01%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,363,395k (± 0.02%)}	_{2,607,004k (±14.46%)}	_~	_2,363,251k	_3,094,497k	_{p=0.230 n=6}
_{Parse Time}	_{5.20s (± 0.37%)}	_{5.27s (± 1.15%)}	_~	_5.19s	_5.34s	_{p=0.077 n=6}
_{Bind Time}	_{1.78s (± 1.21%)}	_{1.77s (± 1.36%)}	_~	_1.74s	_1.81s	_{p=0.511 n=6}
_{Check Time}	_{35.36s (± 0.19%)}	_{35.44s (± 0.52%)}	_~	_35.25s	_35.73s	_{p=0.575 n=6}
_{Emit Time}	_{2.97s (± 1.81%)}	_{2.99s (± 0.75%)}	_~	_2.97s	_3.03s	_{p=0.575 n=6}
_{Total Time}	_{45.32s (± 0.26%)}	_{45.47s (± 0.49%)}	_~	_45.27s	_45.82s	_{p=0.378 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,227,066	_1,227,071	_{+5 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_267,480	_267,496	_{+16 (+ 0.01%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,797,762k (±14.24%)}	_{3,040,566k (± 9.77%)}	_~	_2,433,507k	_3,162,728k	_{p=0.230 n=6}
_{Parse Time}	_{6.85s (± 1.87%)}	_{6.89s (± 0.83%)}	_~	_6.78s	_6.93s	_{p=1.000 n=6}
_{Bind Time}	_{2.16s (± 1.21%)}	_{2.19s (± 1.34%)}	_~	_2.15s	_2.22s	_{p=0.127 n=6}
_{Check Time}	_{42.81s (± 0.49%)}	_{43.02s (± 0.45%)}	_~	_42.68s	_43.19s	_{p=0.128 n=6}
_{Emit Time}	_{3.55s (± 1.99%)}	_{3.53s (± 1.62%)}	_~	_3.46s	_3.61s	_{p=0.630 n=6}
_{Total Time}	_{55.38s (± 0.75%)}	_{55.62s (± 0.44%)}	_~	_55.17s	_55.88s	_{p=0.298 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,555	_262,560	_{+5 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_107,165	_107,169	_{+4 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{441,955k (± 0.01%)}	_{441,988k (± 0.01%)}	_~	_441,927k	_442,053k	_{p=0.078 n=6}
_{Parse Time}	_{4.36s (± 0.55%)}	_{4.39s (± 1.07%)}	_~	_4.35s	_4.48s	_{p=0.253 n=6}
_{Bind Time}	_{1.63s (± 1.05%)}	_{1.62s (± 0.72%)}	_~	_1.61s	_1.64s	_{p=0.323 n=6}
_{Check Time}	_{23.41s (± 0.40%)}	_{23.46s (± 0.25%)}	_~	_23.38s	_23.55s	_{p=0.198 n=6}
_{Emit Time}	_{1.90s (± 0.86%)}	_{1.91s (± 0.98%)}	_~	_1.88s	_1.93s	_{p=0.373 n=6}
_{Total Time}	_{31.31s (± 0.27%)}	_{31.38s (± 0.19%)}	_~	_31.29s	_31.43s	_{p=0.170 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₁	₇₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,367	_225,367	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,290	_94,291	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{371,089k (± 0.01%)}	_{371,123k (± 0.02%)}	_~	_371,060k	_371,245k	_{p=0.471 n=6}
_{Parse Time}	_{2.86s (± 1.41%)}	_{2.89s (± 0.80%)}	_~	_2.86s	_2.92s	_{p=0.295 n=6}
_{Bind Time}	_{1.60s (± 1.10%)}	_{1.60s (± 0.79%)}	_~	_1.58s	_1.61s	_{p=0.805 n=6}
_{Check Time}	_{16.40s (± 0.55%)}	_{16.46s (± 0.27%)}	_~	_16.41s	_16.53s	_{p=0.196 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{20.87s (± 0.55%)}	_{20.95s (± 0.31%)}	_~	_20.86s	_21.05s	_{p=0.199 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,562,109	_3,562,109	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,201,176	_1,201,211	_{+35 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{3,606,481k (± 0.01%)}	_{3,606,803k (± 0.01%)}	_~	_3,606,521k	_3,607,204k	_{p=0.128 n=6}
_{Parse Time}	_{15.38s (± 3.71%)}	_{15.15s (± 0.16%)}	_~	_15.12s	_15.18s	_{p=0.518 n=6}
_{Bind Time}	_{4.84s (± 1.26%)}	_{4.86s (± 0.53%)}	_~	_4.83s	_4.90s	_{p=0.807 n=6}
_{Check Time}	_{99.51s (± 0.94%)}	_{99.52s (± 1.33%)}	_~	_98.63s	_102.11s	_{p=0.936 n=6}
_{Emit Time}	_{31.50s (± 7.87%)}	_{31.66s (± 9.12%)}	_~	_30.22s	_37.55s	_{p=1.000 n=6}
_{Total Time}	_{151.23s (± 2.12%)}	_{151.19s (± 2.15%)}	_~	_149.03s	_157.23s	_{p=0.936 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_321,124	_321,124	_~	_~	_~	_{p=1.000 n=6}
_Types	_140,106	_140,124	_{+18 (+ 0.01%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{477,577k (± 0.01%)}	_{477,556k (± 0.03%)}	_~	_477,326k	_477,766k	_{p=0.936 n=6}
_{Parse Time}	_{4.32s (± 0.77%)}	_{4.34s (± 0.52%)}	_~	_4.30s	_4.36s	_{p=0.295 n=6}
_{Bind Time}	_{1.86s (± 0.56%)}	_{1.84s (± 1.42%)}	_~	_1.82s	_1.89s	_{p=0.064 n=6}
_{Check Time}	_{21.15s (± 0.39%)}	_{21.16s (± 0.37%)}	_~	_21.05s	_21.26s	_{p=1.000 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_0.00s	_~	_~	_~	_{p=0.405 n=6}
_{Total Time}	_{27.34s (± 0.29%)}	_{27.34s (± 0.34%)}	_~	_27.18s	_27.45s	_{p=0.936 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_663,258	_663,258	_~	_~	_~	_{p=1.000 n=6}
_Types	_198,184	_198,185	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{570,255k (± 0.03%)}	_{570,253k (± 0.03%)}	_~	_570,085k	_570,433k	_{p=0.575 n=6}
_{Parse Time}	_{4.27s (± 0.36%)}	_{4.28s (± 0.61%)}	_~	_4.24s	_4.31s	_{p=0.255 n=6}
_{Bind Time}	_{1.33s (± 1.13%)}	_{1.33s (± 0.61%)}	_~	_1.32s	_1.34s	_{p=0.498 n=6}
_{Check Time}	_{20.12s (± 1.71%)}	_{20.26s (± 1.87%)}	_~	_19.89s	_20.66s	_{p=0.471 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=1.000 n=6}
_{Total Time}	_{25.71s (± 1.32%)}	_{25.88s (± 1.49%)}	_~	_25.48s	_26.31s	_{p=0.378 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Compiler-Unions - node (v18.15.0, x64)
angular-1 - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-build-src-public-api - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
ts-pre-modules - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate-main - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

typescript-bot · 2025-08-05T16:18:05Z

@jakebailey Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62203/merge:

Everything looks good!

Andarist added 5 commits August 5, 2025 00:30

Don't widen template literal types when considering them to be possib…

877c5a3

…le discriminants

add error tests

9b7c526

rename the new type

9567b6f

only produce context free template literal types when all types pass …

685aa60

…`isPatternLiteralPlaceholderType`

add extra test case

808bb3a

Copilot AI review requested due to automatic review settings August 5, 2025 08:11

github-project-automation bot added this to PR Backlog Aug 5, 2025

github-project-automation bot moved this to Not started in PR Backlog Aug 5, 2025

Copilot AI reviewed Aug 5, 2025

View reviewed changes

Andarist mentioned this pull request Aug 5, 2025

Evaluate expressions with annotated constant variables #57236

Closed

Andarist changed the title ~~Support contextual type discrimination based on template literal types~~ Support contextual type discrimination based on template literal expressions Aug 5, 2025

fixup

f9d0fbd

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