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Add [Constructor]Parameters types to lib.d.ts #26243

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merged 1 commit into from Aug 31, 2018

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sandersn
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@sandersn sandersn commented Aug 6, 2018

Based on #26042 and discussion from the most recent design meeting. Thanks @anurbol for creating the initial PR.

It turns out that our tests have a couple of examples of looser definitions of Parameters. The differences are instructive:

type Parameters<T extends Function> = ((...args: infer U) => any) | (new (...args: infer U) => any) ? U : any[];

This looser definition allows:

  1. Function to create an array of parameters any[].
  2. Either constructors or functions to be passed in.
  3. Tuple-constrained type parameters, for example T extends any[] to be used as the type of rest parameters.

(1) is pretty questionable and is fine to disallow, I think. (2) is now handled by Parameters vs ConstructorParameters. (3) is unsound, as pointed out in #26013, since it is equivalent to allowing the assignability of T extends any[] to any[]. So I think all 3 differences are improvements. I preserved them in the test baselines.

Fixes #26019

@mattmccutchen
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There were three questions involving Parameters on Stack Overflow just today: 1, 2, 3. It would be great if Parameters got into the standard library before more people have to copy it into their own projects.

@sandersn sandersn merged commit c929e74 into master Aug 31, 2018
@sandersn sandersn deleted the add-parameter-to-tuple-type branch August 31, 2018 14:46
@sandersn
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Looks like this broke the build because we now elaborate errors more often. I'll send a follow-up PR that updates the baselines with more elaborations.

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3 participants