is duplicated by:

• [SR-514] Generic type cannot construct itself with a transformed type #43131
• [SR-4155] Generic argument deduction disabled in generic context #46738
• [SR-8401] Generic class, type aliases and type inference at compile for a static function on a generic class #50926
• [SR-10765] Dictionary(uniqueKeysWithValues:) in Dictionary extension causes error #53155
• [SR-11472] Generic parameter inference problem #53872

relates to:

• [SR-4247] Generic Factory Method on Struct Reports Unhelpful Type Error #46830

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.