Compute dervied generic-ness for substitutions and create lower priority inferences for substitutes #43765
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…ity inferences for substitutes
typescript-bot
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Author: Team
For Milestone Bug
andrewbranch
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Apr 21, 2021
RyanCavanaugh
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Apr 23, 2021
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Fixes #43591
We're been making non-generic substitutions for quite awhile now, but haven't handled them appropriately in
isGenericObjectTypeorisGenericIndexType(they behave as though they're generic, hence the unexecuted conditional in the linked issue). With that in place, the linked issue further makes it obvious that these substitutions can create unwanted inference results -Array<number> extends Array<infer T>should clearly infernumberforT, but naively does not in this context, becauseArray<number>actually substitutes toArray<number> & Array<any>, which in turn taints the result. By making an inference from the base type of the substitute first, and then a lower priority inference from the full substitute, we can preferentially pull the result from the original, un-substituted type where possible (thus allowing the result from the linked issue to be the expectednumber).