microsoft / microsoft/TypeScript
Infer method name for parameter decorator
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Description
TypeScript Version: 3.3.3
Tried with @next (3.4.0-dev.201xxxxx) ? NO
Search Terms: type:issues infer method name parameter decorator
Code
declare function Method():
{
(t: any, m: 'method'): void;
}
declare function Parameter():
{
(t: any, m: 'method', i: number): void;
}
class Play
{
@Method() // err
public another_method ( @Parameter() /* NO ERROR */ test: number ): void
{
throw new Error('Not yet implemented');
}
@Method()
public method ( @Parameter() test: number ): void
{
throw new Error('Not yet implemented');
}
}
Expected behavior: @Parameter() should error just as @Method() when applied to another_method() because the m parameter is typed "method" (which should reduced the set of method). i.e method name should be enforced.
Actual behavior: No error
Playground Link: https://typescript-play.js.org
Use case :
I crafted a type that filters method names with the nth parameters iff that parameters is of the given type T.
See KeysToTypedNthParameter definition here
let hash = {
a( _0: string, _1: string ) {},
b( _0: string, _1: number ) {},
c( _0: number, _1: number ) {},
d( _0: number, _1: string ) {},
}
let string_0: KeysToTypedNthParameter<0, string, typeof hash>; // "a" | "b"
let string_1: KeysToTypedNthParameter<1, string, typeof hash>; // "a" | "d"
let number_0: KeysToTypedNthParameter<0, number, typeof hash>; // "c" | "d"
let number_1: KeysToTypedNthParameter<1, number, typeof hash>; // "c" | "d"
Later, I used that type to infer method name for my parameter decorator - @Inject - in the excerpt below, I marked the ONLY line that should error and why (but both lines actually error)
class Test
{
public blatantly!: boolean;
}
class Play
{
public method(
@Inject({ type: Test })
@Inject({ type: Number }) // err (construtor mismatch instance type)
test: Test,
): void
{
throw new Error( 'Not yet implemented' );
}
}
Play;
Here is @Inject decorator definition and complete code -
const Inject: InjectLike = void 0 as unknown as InjectLike;
interface InjectLike
{
<
C extends ConstructorLike,
>
({}: { type: C }): {
<
// FIXME : not enough to infer the method name - see ./__playground.ts#
M extends ( T extends ConstructorLike ? never : KeysToTypedNthParameter<I, InstanceType<C>, T> ),
I extends number,
T,
>
( target: T, member: M, index: I ): void;
};
}
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the decorator examples in the linked TypeScript Playground, including the InjectLike definition and its __playground.ts reference. Trace how the generic method name, parameter index, and target type are inferred; done means @Parameter() errors on another_method() while the valid @Inject usage is accepted and the marked constructor mismatch is rejected.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 30/100