Improve narrowing of generic types in control flow analysis #43183
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@ahejlsberg Here they are:Comparison Report - master..43183
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The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master. |
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@ahejlsberg Here they are:Comparison Report - master..43183
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src/compiler/checker.ts
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| function getConstraintForReference(type: Type, reference: Identifier | ElementAccessExpression | PropertyAccessExpression | QualifiedName, checkMode: CheckMode | undefined) { | ||
| // When the type of a reference is or contains an instantiable type with a union type constraint, and | ||
| // when the reference is in a constraint position (where it is known we'll obtain the apparent type) or | ||
| // has a contextual type containing no instantiables (meaning constraints will determine assignability), |
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Suggested change
| // has a contextual type containing no instantiables (meaning constraints will determine assignability), | |
| // has a contextual type containing no top-level instantiables (meaning constraints will determine assignability), |
or
Suggested change
| // has a contextual type containing no instantiables (meaning constraints will determine assignability), | |
| // has a contextual type containing no immediate instantiables (meaning constraints will determine assignability), |
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Hey @DanielRosenwasser, 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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@ahejlsberg might be worth adding in the following test cases - interface Box<T> {
item: T;
}
declare function isBox(x: any): x is Box<unknown>;
declare function isUndefined(x: unknown): x is undefined;
declare function unbox<T>(x: Box<T>): T;
function f1<T extends Box<T> | undefined>(x: T) {
if (isBox(x)) {
unbox(x);
}
}
function f2<T extends Box<T> | undefined>(x: T) {
if (!isUndefined(x)) {
unbox(x);
}
}
function f3<T extends Box<T> | undefined>(x: T) {
if (!isBox(x)) {
// error
unbox(x);
}
}
function f4<T extends Box<T> | undefined>(x: T) {
if (isUndefined(x)) {
// error
unbox(x);
}
} |
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@DanielRosenwasser Will do. |
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Performance looks to be unaffected. RWC test suite looks good too, only change is a couple of error messages that now show constraint types instead of generic types (as expected). |
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TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. Fixes #330
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TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. To resolve this, *loosen* the constraints on what is acceptable as input to `toString`, by making the implementation itself more robust. Doing so also fixes a bug with the output for `toString` when working with strings: previously if you had `Maybe("a string")`, the `toString` output would be `'Maybe(a string)'`; it is now `'Maybe("a string")'`. Fixes #330
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Apr 22, 2022
TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. To resolve this, *loosen* the constraints on what is acceptable as input to `toString`, by making the implementation itself more robust. Doing so also fixes a bug with the output for `toString` when working with strings: previously if you had `Maybe("a string")`, the `toString` output would be `'Maybe(a string)'`; it is now `'Maybe("a string")'`. Fixes #330
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(cherry picked from commit 25adb1a) --- Original message: Add support for TypeScript 4.7 TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. To resolve this, *loosen* the constraints on what is acceptable as input to `toString`, by making the implementation itself more robust. Doing so also fixes a bug with the output for `toString` when working with strings: previously if you had `Maybe("a string")`, the `toString` output would be `'Maybe(a string)'`; it is now `'Maybe("a string")'`. Fixes #330
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In #15576 we improved control flow analysis for references of generic types with nullable constraints when those references occur in constraint positions (locations where it is known we'll fetch the constraint).
This PR expands the concept further. Specifically, when the type of a reference is or contains a generic type with a union type constraint, and when the reference is in a constraint position or has a contextual type containing no generic types (meaning constraints will determine assignability), we substitute constraints for all generic types in the type of the reference to give control flow analysis an opportunity to narrow it further. For example:
Above, in
x.lengthwe substitutestring | undefinedforTin the type ofxbecausexoccurs in a constraint position. Similarly inreturn xwe substitutestring | undefinedforTin the type ofxbecausexhas the contextual typestring(due to the return type annotation on the containing function) which contains no generic types. Following substitution, control flow analysis is then able to narrowstring | undefinedtostringbased on the truthiness check in the containingifstatement.Some additional examples:
Fixes #13995.
Fixes #20375.
Fixes #42939.