Regression for omitted lambda parameter with tuple args

[ulrichb]

TypeScript Version: 3.4.0-dev.20190309

Search Terms: 3.4 tuple lambda

Code

The following snippet compiles fine in TS 3.3 whereas it emits an error in TS 3.4.

type MyTupleItem = {};
type MyTuple = [MyTupleItem, ...MyTupleItem[]];

type GenericFunction<T extends MyTuple> = (...fromArgs: T) => void;

class GenericClass<T extends MyTuple> {
    from: T | undefined;
}

function createClass<T extends MyTuple>(f: GenericFunction<T>): GenericClass<T> {
    return new GenericClass<T>(/* ... use f */);
}

function consumeClass(c: GenericClass<[string, boolean]>) { }

// ERROR:
consumeClass(createClass(str => console.log(str.length)));

// WORKS:
consumeClass(createClass((str, _unused_num) => console.log(str.length)));

Expected behavior:

No compile error.

Actual behavior:

>npx tsc TypeScriptIssueNext.ts
TypeScriptIssueNext.ts:18:14 - error TS2345: Argument of type 'GenericClass<[string]>' is not assignable to parameter of type 'GenericClass<[string, boolean]>'.
  Property '1' is missing in type '[string]' but required in type '[string, boolean]'.

18 consumeClass(createClass(str => console.log(str.length)));
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Playground Link: Link

[RyanCavanaugh]

Anders Hejlsberg (@ahejlsberg) thoughts on the priority of the contextual type vs the lambda?

[ulrichb]

What, 3.5? ... I think it's okay to defer it to a later 3.4.x version, because it's easily workaroundable. But 3.5?

[weswigham]

We haven't triaged anything into a 3.4.2 yet. We'll worry about that once 3.4 actually ships

[ulrichb]

Okay. Thanks for the update.