Fix type parameter comparability to consistently allow comparisons on unconstrained type parameters #48861
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… unconstrained type parameters
weswigham
requested review from
DanielRosenwasser,
ahejlsberg and
RyanCavanaugh
typescript-bot
added
Author: Team
For Milestone Bug
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@typescript-bot pack this |
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Heya @weswigham, I've started to run the tarball bundle task on this PR at 57f17bb. You can monitor the build here. |
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Heya @weswigham, I've started to run the diff-based community code test suite on this PR at 57f17bb. You can monitor the build here. Update: The results are in! |
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Heya @weswigham, I've started to run the extended test suite on this PR at 57f17bb. You can monitor the build here. |
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Heya @weswigham, I've started to run the parallelized Definitely Typed test suite on this PR at 57f17bb. You can monitor the build here. |
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Heya @weswigham, I've started to run the perf test suite on this PR at 57f17bb. You can monitor the build here. Update: The results are in! |
weswigham
commented
Apr 27, 2022
src/compiler/checker.ts
Outdated
| @@ -19549,7 +19563,7 @@ namespace ts { | |||
| if (sourceFlags & TypeFlags.TypeVariable) { | |||
| // IndexedAccess comparisons are handled above in the `targetFlags & TypeFlage.IndexedAccess` branch | |||
| if (!(sourceFlags & TypeFlags.IndexedAccess && targetFlags & TypeFlags.IndexedAccess)) { | |||
| const constraint = getConstraintOfType(source as TypeVariable); | |||
| const constraint = getConstraintOfType(source as TypeVariable) || unknownType; | |||
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This is the core of this fix. This is really all we need to make it so T extends unknown and a bare T are actually treated identically.
weswigham
commented
Apr 27, 2022
src/compiler/checker.ts
Outdated
| @@ -19338,6 +19338,20 @@ namespace ts { | |||
| } | |||
| } | |||
| } | |||
| if (relation === comparableRelation && sourceFlags & TypeFlags.TypeParameter) { | |||
| // This is a carve-out in comparability to essentially forbid comparing a type parameter | |||
| // with another type parameter unless one extends the other. (Remember: comparability is mostly bidirectional!) | |||
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This, meanwhile, is a change to make is slightly more strict in all circumstances involving comparing one type parameter to another (regardless if one or both are constrained) so we still forbid comparing two unrelated type parameters, despite the below change.
weswigham
commented
Apr 27, 2022
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| case 2: x; break; // T & 2 | ||
| >2 : 2 | ||
| >x : T & 2 | ||
| >x : (T & 1) | (T & 2) |
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Fallout from permissive comparability: We use comparability in switch-case narrowing (in narrowTypeBySwitchOnDiscriminant) and since T is unconstrained and thus extends unknown, both the 1 and 2 intersections are considered comparable with both 1 and 2 because of the T part. Some extra work could possibly be done to prevent this by changing how comparability works over intersections, probably.
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(It's likely a similar change occurs in narrowing assignments, since we also use bidirectional comparability there, and maybe in other places more subtly since we use unidirectional comparability in narrowTypeByDiscriminant)
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@weswigham |
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Hey @weswigham, I've packed this into an installable tgz. You can install it for testing by referencing it in your and then running There is also a playground for this build and an npm module you can use via |
Andarist
reviewed
Apr 27, 2022
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@weswigham Here they are:Comparison Report - main..48861
System
Hosts
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Developer Information: |
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@typescript-bot pack this |
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Heya @weswigham, I've started to run the diff-based community code test suite on this PR at 4c7e818. You can monitor the build here. Update: The results are in! |
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Heya @weswigham, I've started to run the parallelized Definitely Typed test suite on this PR at 4c7e818. You can monitor the build here. |
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Heya @weswigham, I've started to run the tarball bundle task on this PR at 4c7e818. You can monitor the build here. |
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Heya @weswigham, I've started to run the perf test suite on this PR at 4c7e818. You can monitor the build here. Update: The results are in! |
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Heya @weswigham, I've started to run the extended test suite on this PR at 4c7e818. You can monitor the build here. |
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@weswigham |
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Hey @weswigham, I've packed this into an installable tgz. You can install it for testing by referencing it in your and then running There is also a playground for this build and an npm module you can use via |
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@weswigham Here they are:Comparison Report - main..48861
System
Hosts
Scenarios
Developer Information: |
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DanielRosenwasser
approved these changes
Apr 28, 2022
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@ahejlsberg do you feel comfortable with us bringing this in? |
ahejlsberg
approved these changes
May 2, 2022
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@DanielRosenwasser do you still want to cherry pick this for 4.7? |
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*unless those comparisons to against other unrelated type parameters, since those are a stronger signal that the types are meant to be separate domains.
This is a change in philosophy to the more permissive side in the general case - comparability now consistently succeeds when relating any two types that overlap, unless the two are type parameters that don't extend one another in some way.
Fixes #48680