Lower priority for speculative variadic tuple inferences #39723
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typescript-bot
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RyanCavanaugh
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Jul 24, 2020
| NoConstraints = 1 << 7, // Don't infer from constraints of instantiable types | ||
| AlwaysStrict = 1 << 8, // Always use strict rules for contravariant inferences | ||
| MaxValue = 1 << 9, // Seed for inference priority tracking | ||
| SpeculativeTuple = 1 << 1, // Speculative tuple inference |
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I think without the explanation, this is unclear.
| @@ -19819,7 +19819,10 @@ namespace ts { | |||
| } | |||
| else if (middleLength === 1 && elementFlags[startLength] & ElementFlags.Variadic) { | |||
| // Middle of target is exactly one variadic element. Infer the slice between the fixed parts in the source. | |||
| inferFromTypes(isTupleType(source) ? sliceTupleType(source, startLength, sourceEndLength) : createArrayType(sourceRestType!), elementTypes[startLength]); | |||
| // If target ends in optional element(s), make a lower priority a speculative inference. | |||
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If the documentation is going to be sparse on SpeculativeTuple, I think showing a concrete example here is worthwhile. Your explanation on the PR would be great.
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This PR reverts #39281 and #39614 and instead implements the originally envisioned solution of giving higher priority to inferences made to
[...T, X]than inferences made to[...T, X?](because the latter speculates that the optional element is present). For example:Fixes #39253.