Description

I have found a case, where existential type is not implicitly opened upon passing in a generic function. Strangely enough, passing it through an intermediate variable fixes the issue.

Reproduction

protocol Base {} struct A: Base {} struct B: Base {} func foo(_ value: some Base) { } let things: [any Base] = [A(), B()] foo(things.first!) // <-- Type 'any Base' cannot conform to 'Base'

However,

let things: [any Base] = [A(), B()] let thing = things.first! foo(thing) // <-- OK

works.

Expected behavior

Existential should be opened in foo(things.first!) implicitly.

Environment

swift-driver version: 1.109.2 Apple Swift version 6.0 (swiftlang-6.0.0.3.300 clang-1600.0.20.10) Target: arm64-apple-macosx15.0 

Additional information

https://forums.swift.org/t/strange-compiler-behaviour-in-implicitly-opened-existentials/72916

P.S. If you will find this issue easy-looking, I may attempt to try to fix it.