Infer method name for parameter decorator

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
typescript
Domain
compilers

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.

Description

Needs Investigation

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;
    };
}
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