Recursive conditional types#40002
Recursive conditional types #40002
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typescript-bot added the For Milestone Bug 
ahejlsberg requested review from DanielRosenwasser, RyanCavanaugh and weswigham
ahejlsberg commented Aug 11, 2020
@typescript-bot test this |
Heya @ahejlsberg, I've started to run the parallelized community code test suite on this PR at 7c4d923. You can monitor the build here. |
Heya @ahejlsberg, I've started to run the parallelized Definitely Typed test suite on this PR at 7c4d923. You can monitor the build here. |
Heya @ahejlsberg, I've started to run the extended test suite on this PR at 7c4d923. You can monitor the build here. |
Heya @ahejlsberg, I've started to run the perf test suite on this PR at 7c4d923. You can monitor the build here. Update: The results are in! |
typescript-bot commented Aug 11, 2020
The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master. |
typescript-bot commented Aug 11, 2020
@ahejlsberg Here they are:Comparison Report - master..40002
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src/compiler/checker.ts Outdated
| } | ||
| function invokeOnce(source: Type, target: Type, action: (source: Type, target: Type) => void){ | ||
| function invokeWithDepthLimit(source: Type, target: Type, action: (source: Type, target: Type) => void){ |
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This is pretty much the inference equivalent of recursiveTypeRelatedTo at this point.
| } | ||
| if (type.flags & TypeFlags.Conditional){ | ||
| // The root object represents the origin of the conditional type | ||
| return (type as ConditionalType).root; |
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Huh, and this doesn't affect the user baselines or DT?
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No. Previously conditional types couldn't be recursive, so they weren't included here. But for that same reason we have no tests that could be affected by this.
treybrisbane commented Aug 12, 2020
Awesome to finally see this! 😀 |
DanielRosenwasser commented Aug 12, 2020
@ahejlsberg maybe something worthwhile is to build another PR on top of this to see if changing the definition of |
ahejlsberg commented Aug 13, 2020
The tests revealed OOMs in a few projects due to the switch to use Intuitively, in inference we want to terminate when we encounter a duplicate attempt to infer from source and target types with the same origin, so |
ahejlsberg commented Aug 13, 2020
@typescript-bot test this |
Heya @ahejlsberg, I've started to run the parallelized community code test suite on this PR at fed0e8c. You can monitor the build here. |
AlCalzone commented Aug 14, 2020
@ahejlsberg According to your example in the OP, this also fixes #26223 😊 |
ahejlsberg commented Aug 16, 2020
Test runs all look clean. Slight regression in check time for material-ui, but it's worth it for the added precision in type inference. |
strelga commented Aug 16, 2020
Does this change mean we no longer need hacks like |
tjjfvi commented Sep 4, 2020
Can we get a playground for this PR? I'd like to play around with the new options this gives us. |
levenleven commented Sep 6, 2020
@tjjfvi you can choose "Nightly" version |
why the max number is 43 ? not 48 ? not 98 ? |
With this PR we officially support recursive conditional types. For example:
A bit of context... When conditional types were first introduced, we explicitly restricted them to be non-recursive, i.e. we made it an error for a conditional type to directly or indirectly reference itself. This restriction was put in place primarily as a safeguard against runaway infinite recursion which the compiler didn't handle well at the time. However, it turns out it was still possible to construct recursive conditionally resolved types by combining object types (which have deferred resolution of property types) and conditional types using indexed access types (see #14833 for the core idea). In spite of being cumbersome and non-intuitive, this trick has become commonplace in several libraries. Consequently, over time we have "hardened" the compiler against infinite recursion with depth limiters in relationships, type inference, type instantiation, constraint computation, and so on. We're now at a point where it seems reasonable support an intuitive way of writing recursive conditional types.
Some more examples:
Note that this PR doesn't change the recursion depth limits that are already in place. For example, an error is reported on
T4above because its resolution exceeds the limit of 50 nested type instantiations.The type inference streamlining contained in the PR fixes several issues with inference to recursive types. For example:
Previously, only the
unbox(b1)call produced the expected type inference.Fixes#26223.
Fixes#26980.
Fixes#37801.