microsoft / microsoft/TypeScript

Infer method name for parameter decorator

Open
#30,102 7 comments 5 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Needs Investigation
Dominant language
Go
Stars
111k
Forks
14.3k
Avg merge
2d 4h
Merged PRs (30d)
132

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

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. 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

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.