Issue originally discovered by@eyelidlessness on Gitter.

TypeScript Version: 3.8.0-dev.20191123

Search Terms:genericreturntype

Code

declareconstfoo:<R1,F1extends()=>R1,R2,F2extends(input:ReturnType<F1>)=>R2>(f1:F1,f2:F2)=>anyconstf1=()=>({a:'a'})constf2=(input:{a:string})=>({b:'b'})foo(f1,f2)// GoodconstmakeF1=()=>f1;foo(makeF1(),f2)// GoodconstmakeF1p=<T>(t:T)=>f1;foo(makeF1p(''),f2// Error WHY :();constmade=makeF1p('');foo(made,// This is finef2)

Expected behavior:

Bothfoo() calls should compile as the only difference is whether the result of a function call is assigned to a temporary first.

Actual behavior:

The first call tofoo(), wheremakeF1p() is called inline, produces a type error at compile time. The second, where the return value ofmakeF1p() is assigned to temp variable first, compiles fine. The error is:

Argument of type '(input: { a: string; }) => { b: string; }' is not assignable to parameter of type '(input: unknown) => unknown'.  Types of parameters 'input' and 'input' are incompatible.    Type 'unknown' is not assignable to type '{ a: string; }'.

It almost seems like the type parameter inference machinery is leaking outside the scope of the call expression or something? It's weird, at any rate.

Playground Link:

https://www.typescriptlang.org/play/?ts=3.8.0-dev.20191123&ssl=1&ssc=1&pln=33&pc=1#code/CYUwxgNghgTiAEYD2A7AzgF3gMyUgXPADwBQ858ASgIwA0ZFAYtfCAB4YgrBrwAUASngBeAHxU6DcpQBM9CvEYzWHLj34BLFAAcArhkKUQGXTBQAVAJ7aQRZqKFiqMkqL5Sc1Qs3kVsM7xdHcSgUSxISZHQsbBZhfmD+AG94KEIAcih0+ABfAUjUTBxleL4tPQN4FLT4TBgtAHNcxL4UgCMMtuy8iNwkPljaYqEAehH4AHE8YAioooBbKABrEGYRBJFxWIBuXrw+RZXmQSH-UfGppBmC6PhD1eptdaJzN0rzRJ29-o975m0+Ol0gJfOR-PAxvAAKIwGBIGDwADqAAkAJrwfDuAS7G4LKCgdZ-R6A4E4vruBSLUBDSHmAAWGl4jJwWhAHn8JHyQA

Related Issues: I didn't find any, but@jack-williams says#30215 might be the root cause.