Fix TS2783 false positive for union types in object spread expressions by Copilot · Pull Request #62656 · microsoft/TypeScript

Copilot · 2025-10-22T05:30:25Z

Problem

TypeScript incorrectly reported error TS2783 ("'x' is specified more than once, so this usage will be overwritten") when spreading union types in object literals, even when the property wasn't guaranteed to exist in all constituents of the union.

For example, this code incorrectly produced an error:

type Thing = {
    id: string;
    label: string;
};

function find(id: string): undefined | Thing {
    return things.find(thing => thing.id === id);
}

fun({
    id: 'foo',
    ...find('foo') ?? { label: 'Foo' },  // ❌ False positive: 'id' is specified more than once
});

The error claimed id would "always" be overwritten, but this was incorrect. The spread expression evaluates to Thing | { label: string }, and the fallback object { label: 'Foo' } doesn't have an id property at all, so id is only overwritten when find() returns a Thing.

Root Cause

The checkSpreadPropOverrides function only checked if a property had SymbolFlags.Optional before reporting the error. It didn't account for properties that exist in some constituents of a union type but not others. These properties are marked with CheckFlags.Partial (specifically ReadPartial or WritePartial), indicating they're only present in a subset of the union's constituents.

Solution

Added an additional check for CheckFlags.Partial in checkSpreadPropOverrides. Properties marked as partial in union types should not trigger the "always overwrites" error since they don't exist in all code paths.

// Only report error if property is both non-optional AND not partial
if (!(right.flags & SymbolFlags.Optional) && !(getCheckFlags(right) & CheckFlags.Partial)) {
    // Report error...
}

Test Coverage

Added comprehensive test file spreadUnionPropOverride.ts that verifies:

The original issue is fixed: spreading Thing | { label: string } no longer errors on id
Properties present in only some union constituents don't trigger false positives
Properties present in all union constituents still correctly trigger the error
Optional properties in unions are handled correctly
Required properties that exist in all union constituents still error as expected

The fix preserves all existing correct behavior while eliminating the false positive for partial properties in union types.

Original prompt

This section details on the original issue you should resolve

<issue_title>TS2783 False Positive</issue_title>
<issue_description>### 🔎 Search Terms

TS2783: "... is specified more than once, so this usage will be overwritten", "This spread always overwrites this property"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about 5.9.3

⏯ Playground Link

https://www.typescriptlang.org/play/?esModuleInterop=false&target=11&jsx=0&verbatimModuleSyntax=true&strictBuiltinIteratorReturn=true&exactOptionalPropertyTypes=true&noImplicitOverride=true&noPropertyAccessFromIndexSignature=true&ts=5.8.3#code/C4TwDgpgBAKgFgSwHYHMoF4oG8BQV9QIAmAXFAM7ABOyKA3HgQDYCGARhE2ZTagwL4McAYwD2SSlGCJU5MvFoBtALoYoKoQDMArkmHAE4qJuREAFMW7VaASjK6iEE0ghEoAH1gy0uAlCoQwNpUSFLe5AB0zubStBgAfGG0EcQY6JjENgI4OI7CrAHGuvqGoTpIZrGo8t52UABuosRauma+BJZQAOSaoqJdADSM+BGj0WY9fV02UAD8s9jDflCsHFzdAGJTQ378Q-xZOEA

💻 Code
type Thing = {
    id: string;
    label: string;
};

const things: Thing[] = [];

function find(id: string): undefined | Thing {
    return things.find(thing => thing.id === id);
}

declare function fun(thing: Thing): void;

fun({
    id: 'foo',
    ...find('foo') ?? {
        label: 'Foo',
    },
});
🙁 Actual behavior

TS2783 on line 15: 'id' is specified more than once is a false positive.

🙂 Expected behavior

Expect no errors. The return type of find('foo') includes undefined, and the right-hand side of the nullish coalescing operation does not override the 'id' property.

Additional information about the issue

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #62655

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

RyanCavanaugh · 2025-10-22T16:29:08Z

@typescript-bot test it

typescript-bot · 2025-10-22T16:29:13Z

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 · 2025-10-22T16:52:56Z

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

Everything looks the same!

You can check the log here.

typescript-bot · 2025-10-22T16:58:45Z

@RyanCavanaugh Here are the results of running the user tests with tsc comparing main and refs/pull/62656/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

typescript-bot · 2025-10-22T17:19:51Z

@RyanCavanaugh
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	_50,386	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{193,951k (± 0.96%)}	_{193,445k (± 0.79%)}	_~	_192,723k	_196,541k	_{p=1.000 n=6}
_{Parse Time}	_{1.30s (± 0.31%)}	_{1.30s (± 0.40%)}	_~	_1.29s	_1.30s	_{p=0.114 n=6}
_{Bind Time}	_{0.72s (± 0.71%)}	_{0.72s (± 0.75%)}	_~	_0.72s	_0.73s	_{p=0.640 n=6}
_{Check Time}	_{9.75s (± 0.30%)}	_{9.76s (± 0.16%)}	_~	_9.75s	_9.79s	_{p=0.359 n=6}
_{Emit Time}	_{2.75s (± 0.50%)}	_{2.73s (± 1.05%)}	_~	_2.69s	_2.77s	_{p=0.511 n=6}
_{Total Time}	_{14.52s (± 0.23%)}	_{14.52s (± 0.28%)}	_~	_14.48s	_14.59s	_{p=0.627 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_955,117	_955,117	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,619	_415,619	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,254,627k (± 0.01%)}	_{1,254,552k (± 0.00%)}	_{-75k (- 0.01%)}	_1,254,503k	_1,254,595k	_{p=0.045 n=6}
_{Parse Time}	_{6.50s (± 0.68%)}	_{6.54s (± 0.86%)}	_~	_6.47s	_6.61s	_{p=0.197 n=6}
_{Bind Time}	_{1.88s (± 0.43%)}	_{1.88s (± 0.22%)}	_~	_1.88s	_1.89s	_{p=0.584 n=6}
_{Check Time}	_{32.30s (± 0.34%)}	_{32.18s (± 0.29%)}	_~	_32.07s	_32.28s	_{p=0.127 n=6}
_{Emit Time}	_{14.86s (± 0.42%)}	_{14.86s (± 0.46%)}	_~	_14.78s	_14.97s	_{p=0.872 n=6}
_{Total Time}	_{55.54s (± 0.19%)}	_{55.46s (± 0.23%)}	_~	_55.30s	_55.60s	_{p=0.229 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_552,458	_552,458	_~	_~	_~	_{p=1.000 n=6}
_Types	₈₉	₈₉	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{827,395k (± 0.00%)}	_{827,391k (± 0.00%)}	_~	_827,375k	_827,402k	_{p=0.570 n=6}
_{Parse Time}	_{8.60s (± 0.22%)}	_{8.60s (± 0.32%)}	_~	_8.57s	_8.64s	_{p=0.744 n=6}
_{Bind Time}	_{2.19s (± 0.29%)}	_{2.19s (± 0.45%)}	_~	_2.18s	_2.20s	_{p=0.733 n=6}
_{Check Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.27s (± 3.77%)}	_{0.27s (± 1.50%)}	_~	_0.27s	_0.28s	_{p=0.545 n=6}
_{Total Time}	_{11.06s (± 0.22%)}	_{11.06s (± 0.24%)}	_~	_11.03s	_11.10s	_{p=0.871 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,247	_1,237,247	_~	_~	_~	_{p=1.000 n=6}
_Types	_259,866	_259,866	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,364,769k (± 0.01%)}	_{2,425,550k (± 6.10%)}	_~	_2,364,508k	_2,727,750k	_{p=0.128 n=6}
_{Parse Time}	_{5.14s (± 1.14%)}	_{5.19s (± 1.24%)}	_~	_5.09s	_5.28s	_{p=0.228 n=6}
_{Bind Time}	_{1.76s (± 0.36%)}	_{1.77s (± 0.46%)}	_~	_1.76s	_1.78s	_{p=0.177 n=6}
_{Check Time}	_{35.09s (± 0.35%)}	_{35.12s (± 0.64%)}	_~	_34.68s	_35.28s	_{p=0.471 n=6}
_{Emit Time}	_{3.06s (± 2.71%)}	_{3.03s (± 2.53%)}	_~	_2.96s	_3.18s	_{p=0.148 n=6}
_{Total Time}	_{45.08s (± 0.34%)}	_{45.13s (± 0.52%)}	_~	_44.78s	_45.44s	_{p=0.575 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,247	_1,237,247	_~	_~	_~	_{p=1.000 n=6}
_Types	_259,866	_259,866	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,159,182k (± 0.02%)}	_{3,159,070k (± 0.02%)}	_~	_3,157,913k	_3,159,993k	_{p=1.000 n=6}
_{Parse Time}	_{6.86s (± 0.62%)}	_{6.82s (± 1.03%)}	_~	_6.73s	_6.89s	_{p=0.471 n=6}
_{Bind Time}	_{2.15s (± 0.96%)}	_{2.17s (± 1.50%)}	_~	_2.11s	_2.20s	_{p=0.126 n=6}
_{Check Time}	_{42.56s (± 0.39%)}	_{42.66s (± 0.38%)}	_~	_42.46s	_42.95s	_{p=0.298 n=6}
_{Emit Time}	_{3.48s (± 1.74%)}	_{3.52s (± 2.05%)}	_~	_3.40s	_3.59s	_{p=0.229 n=6}
_{Total Time}	_{55.04s (± 0.40%)}	_{55.18s (± 0.32%)}	_~	_54.93s	_55.42s	_{p=0.297 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,536	_262,536	_~	_~	_~	_{p=1.000 n=6}
_Types	_104,041	_104,041	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{440,682k (± 0.01%)}	_{440,677k (± 0.01%)}	_~	_440,572k	_440,733k	_{p=0.936 n=6}
_{Parse Time}	_{3.53s (± 1.00%)}	_{3.52s (± 0.88%)}	_~	_3.46s	_3.54s	_{p=0.624 n=6}
_{Bind Time}	_{1.32s (± 1.17%)}	_{1.32s (± 0.62%)}	_~	_1.31s	_1.33s	_{p=1.000 n=6}
_{Check Time}	_{18.95s (± 0.11%)}	_{18.97s (± 0.42%)}	_~	_18.86s	_19.06s	_{p=0.468 n=6}
_{Emit Time}	_{1.52s (± 0.90%)}	_{1.53s (± 1.57%)}	_~	_1.50s	_1.56s	_{p=0.566 n=6}
_{Total Time}	_{25.32s (± 0.18%)}	_{25.34s (± 0.36%)}	_~	_25.23s	_25.45s	_{p=0.520 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,367	_225,367	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,290	_94,290	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{369,984k (± 0.02%)}	_{370,107k (± 0.04%)}	_~	_369,956k	_370,317k	_{p=0.128 n=6}
_{Parse Time}	_{2.82s (± 1.02%)}	_{2.81s (± 0.70%)}	_~	_2.79s	_2.85s	_{p=0.371 n=6}
_{Bind Time}	_{1.60s (± 1.42%)}	_{1.59s (± 0.92%)}	_~	_1.57s	_1.61s	_{p=0.684 n=6}
_{Check Time}	_{16.42s (± 0.27%)}	_{16.45s (± 0.32%)}	_~	_16.40s	_16.54s	_{p=0.463 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{20.83s (± 0.21%)}	_{20.86s (± 0.35%)}	_~	_20.80s	_21.00s	_{p=0.808 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₅	₅	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,961,269	_3,961,269	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,243,017	_1,243,017	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,751,287k (± 0.00%)}	_{3,751,235k (± 0.01%)}	_~	_3,750,863k	_3,751,483k	_{p=0.936 n=6}
_{Parse Time}	_{15.29s (± 0.70%)}	_{15.28s (± 0.76%)}	_~	_15.16s	_15.44s	_{p=0.810 n=6}
_{Bind Time}	_{4.98s (± 0.47%)}	_{4.98s (± 0.61%)}	_~	_4.93s	_5.02s	_{p=1.000 n=6}
_{Check Time}	_{103.06s (± 3.48%)}	_{104.01s (± 3.32%)}	_~	_100.62s	_110.26s	_{p=0.575 n=6}
_{Emit Time}	_{38.95s (±13.52%)}	_{36.91s (± 7.96%)}	_~	_30.99s	_38.52s	_{p=0.689 n=6}
_{Total Time}	_{162.27s (± 2.65%)}	_{161.19s (± 0.85%)}	_~	_159.45s	_162.76s	_{p=1.000 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₀	₄₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_373,614	_373,614	_~	_~	_~	_{p=1.000 n=6}
_Types	_163,495	_163,495	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{530,050k (± 0.01%)}	_{530,032k (± 0.02%)}	_~	_529,899k	_530,184k	_{p=0.688 n=6}
_{Parse Time}	_{4.52s (± 0.79%)}	_{4.48s (± 0.60%)}	_~	_4.45s	_4.51s	_{p=0.105 n=6}
_{Bind Time}	_{1.96s (± 0.77%)}	_{1.95s (± 1.86%)}	_~	_1.91s	_2.00s	_{p=0.332 n=6}
_{Check Time}	_{22.10s (± 0.37%)}	_{22.11s (± 0.31%)}	_~	_22.00s	_22.21s	_{p=1.000 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_0.00s	_~	_~	_~	_{p=0.405 n=6}
_{Total Time}	_{28.57s (± 0.29%)}	_{28.54s (± 0.18%)}	_~	_28.47s	_28.60s	_{p=0.422 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_676,516	_676,516	_~	_~	_~	_{p=1.000 n=6}
_Types	_203,680	_203,680	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{578,449k (± 0.02%)}	_{578,441k (± 0.02%)}	_~	_578,236k	_578,570k	_{p=0.936 n=6}
_{Parse Time}	_{4.19s (± 0.78%)}	_{4.19s (± 0.65%)}	_~	_4.15s	_4.22s	_{p=0.935 n=6}
_{Bind Time}	_{1.34s (± 1.13%)}	_{1.34s (± 0.41%)}	_~	_1.34s	_1.35s	_{p=0.227 n=6}
_{Check Time}	_{20.55s (± 1.95%)}	_{20.52s (± 1.86%)}	_~	_20.21s	_21.04s	_{p=0.521 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{26.08s (± 1.49%)}	_{26.05s (± 1.43%)}	_~	_25.77s	_26.59s	_{p=0.630 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 · 2025-10-22T17:49:42Z

@RyanCavanaugh Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62656/merge:

Everything looks good!

Copilot

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

tests/baselines/reference/spreadUnionPropOverride.errors.txt

tests/baselines/reference/spreadUnionPropOverride.js

tests/baselines/reference/spreadUnionPropOverride.symbols

tests/baselines/reference/spreadUnionPropOverride.types

tests/cases/compiler/spreadUnionPropOverride.ts

Fix issue ref

Port of microsoft/TypeScript#62656. Adds CheckFlags.Partial check in checkSpreadPropOverrides so that properties present in only some union constituents don't trigger the "always overwrites" error. Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Initial plan

13068a7

Copilot AI assigned Copilot and RyanCavanaugh Oct 22, 2025

Copilot started work on behalf of RyanCavanaugh October 22, 2025 05:30 View session

Copilot AI and others added 2 commits October 22, 2025 05:40

Fix TS2783 false positive with union types in spreads

d3293af

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Fix line endings in test file to use CRLF

c7c2295

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix TS2783 false positive issue~~ Fix TS2783 false positive for union types in object spread expressions Oct 22, 2025

Copilot AI requested a review from RyanCavanaugh October 22, 2025 06:05

Copilot finished work on behalf of RyanCavanaugh October 22, 2025 06:05

RyanCavanaugh approved these changes Oct 22, 2025

View reviewed changes

jakebailey marked this pull request as ready for review October 22, 2025 19:04

Copilot AI review requested due to automatic review settings October 22, 2025 19:04

Copilot AI reviewed Oct 22, 2025

View reviewed changes

Copilot AI mentioned this pull request Oct 22, 2025

Port PR 62656: Fix TS2783 false positive for union types in object spread expressions microsoft/typescript-go#1932

Closed