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Improve inference to union and intersection types#29847

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ahejlsberg merged 7 commits into
mastermicrosoft/TypeScript:masterfrom
inferToUnionTypesmicrosoft/TypeScript:inferToUnionTypesCopy head branch name to clipboard
Feb 11, 2019
Merged

Improve inference to union and intersection types#29847
ahejlsberg merged 7 commits into
mastermicrosoft/TypeScript:masterfrom
inferToUnionTypesmicrosoft/TypeScript:inferToUnionTypesCopy head branch name to clipboard

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@ahejlsberg

@ahejlsberg ahejlsberg commented Feb 10, 2019 •
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This PR improves inference to union and intersection types containing multiple naked type variables. Previously we would make no inferences to such union and intersection types (for unrelated historical reasons that no longer apply). We now make the the proper inferences:

declare function f1<T, U>(x: T | U): T | U;
declare function f2<T, U>(x: T & U): T & U;

let x1: string = f1('a');
let x2: string = f2('a');

Previously both assignments were errors because we'd infer {} for T and U.

Fixes #29815.

@ahejlsberg ahejlsberg changed the title Improve inference to union types Improve inference to union and intersection types Feb 10, 2019
@weswigham

weswigham commented Feb 11, 2019

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Since I'm curious, what prompted us to only infer to one naked type parameter originally, anyway?

@ahejlsberg

ahejlsberg commented Feb 11, 2019

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@weswigham I think it was a holdover from when we didn't have priorities associated with inference candidates.

@ahejlsberg ahejlsberg merged commit f93f4f3 into master Feb 11, 2019
@ahejlsberg ahejlsberg deleted the inferToUnionTypes branch February 11, 2019 19:34
@weswigham

weswigham commented Feb 11, 2019

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Huh. I wonder why we didn't remove it when we added the NakedTypeParameter priority, then.
🤷

@weswigham weswigham mentioned this pull request Feb 13, 2019
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@ahejlsberg ahejlsberg mentioned this pull request Feb 25, 2019
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@weswigham weswigham weswigham approved these changes

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@ahejlsberg @weswigham
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