Instantiate original target type in substituteIndexedMappedType #49205
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typescript-bot
added
Author: Team
For Milestone Bug
| @@ -15798,7 +15798,7 @@ namespace ts { | |||
| function substituteIndexedMappedType(objectType: MappedType, index: Type) { | |||
| const mapper = createTypeMapper([getTypeParameterFromMappedType(objectType)], [index]); | |||
| const templateMapper = combineTypeMappers(objectType.mapper, mapper); | |||
| return instantiateType(getTemplateTypeFromMappedType(objectType), templateMapper); | |||
| return instantiateType(getTemplateTypeFromMappedType(objectType.target as MappedType || objectType), templateMapper); | |||
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Is .target like a "link" to the original type if you end up creating a new instantiated type from something? 🤔 Would in such a case first "call" to this lack the .target but all future calls would be able to use it?
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Yes, an instantiated type has a target property that links to the original mapped type and a mapper property that provides a mapping from type parameters to corresponding type arguments. The original mapped type doesn't have target and mapper properties. It's target is itself and it's type parameters are also its type arguments, so to speak.
It could, but it would just add another code path and not save anything. |
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I just bring it up because only about half the callers do this target following right now, and I'm unsure if the other half should be, as a rule. |
No tests in this PR as it only affects performance. Have manually verified that issue in #49136 is gone (we go from >4M instantiations to just 172 when compiling the original example).
Fixes #49136.