is duplicated by:

• SR-514 Generic type cannot construct itself with a transformed type
• SR-4155 Generic argument deduction disabled in generic context
• SR-8401 Generic class, type aliases and type inference at compile for a static function on a generic class
• SR-10765 Dictionary(uniqueKeysWithValues: ) in Dictionary extension causes error
• SR-11472 Generic parameter inference problem

relates to:

• SR-4247 Generic Factory Method on Struct Reports Unhelpful Type Error

Issue Description:

The following code fails because Swift can't infer the types:

struct Product<A, B> { 
    let left: A 
    let right: B 
     
    func map<C>(f: (A) -> C) -> Product<C, B> { 
        return Product(left: f(left), right: right) 
    } 
}

Running this results in the following type error:

error: repl.swift:6:23: error: '(left: C, right: B)' is not convertible to '(left: A, right: B)'
        return Product(left: f(left), right: right)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

What I Expected

Given that the return value for the generic type says `Product<C, B>`, that's what I would have expected Swift to infer the type as, in which case the code should type check. But instead Swift is inferring it as `Product<A, B>`.

A different❓ type inference error occurs when you have a tuple for the values there instead of the values in separated fields. In this case, Swift infers the tuple to be `(_, _)`.

struct Product<A, B> { 
    let values: (A, B)
     
    func map<C>(f: (A) -> C) -> Product<C, B> { 
        let (a, b) = values
        return Product(values: (f(a), b))
    } 
} 
error: repl.swift:6:32: error: cannot convert value of type '(C, B)' to expected argument type '(_, _)'
        return Product(values: (f(a), b))
                               ^~~~~~~~~

In both cases I expected the code to type check correctly.