[SR-1789] Swift can't infer types in generic methods that reuse part of the struct/class' parameters

[swift-ci]

Previous ID	SR-1789
Radar	None
Original Reporter	robotlolita (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, LanguageFeatureRequest, TypeChecker
Assignee	None
Priority	Medium

md5: 0101359cc12f70fff9f02db7121d39ee

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.

[belkadan]

Just to confirm, this does work when you explicitly say Product<C, B>, so it seems to be a case of us eagerly assuming the parameters are the same as for self. (That's usually a feature, but not when it gets in the way of other inference.)

[belkadan]

Some minor discussion on #46738 about this determines that we'd be open to changing this, but we'd want to put it through the Swift Evolution Process.

[mdiep]

This seems like a duplicate of #43131?