const modifier on type parameters
#51865
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Hey @RyanCavanaugh, 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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Heya @ahejlsberg, I've started to run the abridged perf test suite on this PR at f8fd1fd. You can monitor the build here. Update: The results are in! |
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Could this fix #13923 too? |
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@Mlocik97 hard to tell what's going on without seeing the repro case of your problem. I've shared the snippet yesterday after testing it out within the TS codebase (in one of its JS files) and I didn't see any errors caused by this. |
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OK, I tried testing more, and it seems it works fine in CJS, but not ESM. I will try to provide repro soon. |
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hello, I tested it more, and found out why I got the error... TSC correctly handled that type, but VSCode for some reason didn't picked line:
So my issue is now solved. |
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Hi @ahejlsberg , I have a hard time (from your initial example) discovering the rule that makes the inferred type of Thanks for helping here, please. |
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@SalathielGenese notice that both use different functions, It's important to check what is the result of the same declare function bar<T>(obj: [T, T]): T;
const x4 = bar([{ a: 1, b: 'x' }, { a: 2, b: 'y' }]);
// ?^ const x4: { a: number; b: string; }and what is the result of the same thing with a regular declare function bar<T>(obj: [T, T]): T;
const x4 = bar([{ a: 1, b: 'x' } as const, { a: 2, b: 'y' } as const]);
// ^? const x4: { readonly a: 1; readonly b: "x"; } | { readonly a: 2; readonly b: "y"; }We can see here that it's consistent with the existing behavior of |
This was referenced Feb 28, 2023
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dominikjasek
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Jun 16, 2023
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This is a great feature. Would you consider allowing it for type aliases as well? Example use case: Automattic/mongoose#12782 |
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@JavaScriptBach huh? There's no straightforward correspondence from this feature to something that would apply to type aliases. |
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@RyanCavanaugh My point is that I have generic type aliases that expect to take in const type parameters, e.g. something like type Foo<T> = ...It would be great to be able to type |
Again, not understanding. You can't write this, it's syntactically illegal: type X = Foo<{ x: 3 } as const>; |
This was referenced Jan 5, 2024
This was referenced Jun 3, 2024
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With this PR we implement a new
constmodifier for type parameters. In a function, method, or constructor invocation, when a literal expression in an argument is contextually typed by aconsttype parameter, the literal expression is given the most precise type possible, similar to having applied anas constassertion (see #29510). The kinds of literal expressions affected by aconstcontextual type are string, numeric, and boolean literal values, array literals, and object literals.The
constmodifier is permitted on type parameters of functions, methods, and classes. It is an error to apply theconstmodifier to type parameters of interfaces and type aliases.Given the function declarations
the invocations
infer the types
A
consttype parameter has no effect on variables or other kinds of expressions used as function arguments. For example:The example above still requires writing
[1, 2, 3] as constto infer typereadonly [1, 2, 3]fora.When a
consttype parameter is constrained to an array type, that array type should include areadonlymodifier; otherwise, inferences for the type parameter will fail to meet the constraint. For example:Without the
readonlymodifier in the constraint, inference defaults tounknown[]because an inferred readonly tuple type wouldn't be assignable tounknown[].Fixes #30680.
Fixes #41114.