proposal: infer parameter type from arguments in call to result function

[pat42smith]

Suppose we have a generic function F whose result type is a function. In an expression that calls F and immediately calls the result function, it would be nice to use the arguments to the result function to help determine the type parameters for F.

For example, consider:

func check[X any](t *testing.T) func(X, error) X {
	return func(x X, e error) X {
		if e != nil {
			t.Helper()
			t.Fatal(e)
		}
		return x
	}
}

f := check(t)(os.Open("path"))

In Go 1.21.4, this does not compile; the error is "cannot infer X". But there is only one reasonable choice for X: *os.File. In principle, the compiler could figure this out; I don't know how difficult it would be in practice.

In this specific example, the problem can be worked around by changing the order of parameter lists:

func vet[X any](x X, e error) func(t *testing.T) X {
	// obvious content omitted
)

f := vet(os.Open("path"))(t)

However, I find this order much less desirable, for various reasons not relevant here. And there are undoubtedly cases where such a workaround is not possible. In particular, if both parameter lists use type parameters:

func foo[X, Y any](X) func(Y) { ... }

then there seems to be no way to write a call foo(x)(y) without explicitly specifying at least one type parameter.

Like #50285 (comment), this seems to be a request to infer types from the context of a function call, although in a different manner.

There would be some open questions about how far this might go. Could we have

f(1)(2)(3)(4)(5)(6)("seven")

where "seven" is used to infer a type parameter of f? What if the result type is not a function, but the expression calls a method on the result? Should this work?

type Box[T any] struct{}

func (b Box[T]) hold(T) {}

func crate[T any]() Box[T] {
	return Box[T]{}
}

func oops() {
	crate().hold("some string")
}

[earthboundkid]

See https://github.com/carlmjohnson/be/pull/9/files where I made a test helper like check, but had to reverse the arguments to make it work with inference. I never merged the PR because the API is awkward with the arguments reversed.