Infer type arguments when type parameter is used as return type

[turbolent]

Issue to be solved

Generic functions which have a type parameter that is not used in an argument, but is used in a return type, should have their type argument inferred when an expected type is given.

For example, given a generic function

fun foo<T>() -> T { /* ... */ }

Then an invocation with an expected type should succeed, inferring the type parameter T to Bool:

let x: Bool = foo()

However, in an invocation without an expected type, the type argument cannot be inferred and an explicit type argument is necessary:

let x = foo<Bool>()

Suggested Solution

If an expected type is provided for an invocation, infer the type arguments based on the return type (if any).