Contextual generic function types

[ahejlsberg]

With this PR we do not erase the type parameters of contextual generic function types. Instead, the type parameters are allowed to propagate into the contextually typed expression. For example:

const f: <A>(a: A) => A = a => a;  // Type of a is A

Previously, contextual generic function types were ignored and a would therefore have been of type any.

This PR also expands upon #16072 to permit the contextual type of a generic function to itself be a generic function type. For example:

const arrayMap = <T, U>(f: (x: T) => U) => (a: T[]) => a.map(f);
const arrayFilter = <T>(f: (x: T) => boolean) => (a: T[]) => a.filter(f);

const f1: (a: string[]) => number[] = arrayMap(x => x.length);  // x: string
const f2: <A>(a: A[]) => A[][] = arrayMap(x => [x]);  // x: A
const f3: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));  // value: A
const f4: (a: string[]) => string[] = arrayFilter(x => x.length > 10);  // x: string
const f5: <T extends { value: number }>(a: T[]) => T[] = arrayFilter(x => x.value > 10);  // x: T

Fixes #16293.

[zpdDG4gta8XKpMCd]

will miss this shirt #13039 (comment)

[DanielRosenwasser]

type 'any'

[DanielRosenwasser]

What is meant by "generic pure function type"?

[ahejlsberg]

A type that has nothing but a single call signature.