Type inference from default expression doesn’t compile in free function

[ole]

Description

A function definition that relies on Type inference from default expression (SE-0347) compiles when it's defined inside a type (a method), but does't compile when it's defined as a free function.

Reproduction

As a method, this compiles:

struct MyTask {
    func sleep<C: Clock>(
        until deadline: C.Instant,
        clock: C = ContinuousClock()
    ) async throws {}
}

But: the same function signature as a free function doesn't compile:

func sleep<C: Clock>(
    until deadline: C.Instant,
    // ❌: Cannot use default expression for inference of 'C' 
    // because it is inferrable from parameters #0, #1
    clock: C = ContinuousClock()
) async throws {}

Expected behavior

The free function should compile.

Environment

macOS 14.3, Xcode 15.3 (Swift 5.10)

$ swift --version
swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

Additional information

Forum thread where this is being discussed: https://forums.swift.org/t/type-inference-from-default-expression-doesnt-compile-in-free-function/70521/3

[ph1ps]

I did some digging because I've had this issue in a couple of libraries now. However, I do not have enough knowledge to fix it myself, so I hope someone could help me get up a PR, or someone more knowledgable gets inspired to fix this themselves.

I think there are two problems here, one masking the other one. Both of them are related to the affected parameter counting here: https://github.com/swiftlang/swift/blob/248c0260f064066aa595ffa7b6fffaf407a70ca1/lib/Sema/TypeCheckConstraints.cpp#L635C5-L641C6

Issue 1

The following function has affectedParams.size() == 0. This is because it tries to count the arguments of the function of this shape: Foo.bar(_:)(_:duration:instant) and the parameters are not the clock but rather the instance of Foo. I don't know how you'd call that shape, I guess it is curried?

struct Foo {
    func bar<C: Clock>(_ c: C = ContinuousClock(), duration: C.Duration, instant: C.Instant) {
        fatalError()
    }
}

You can also confirm this, by copying the test case from type_inference_from_default_exprs.swift for this specific error:

func testInferFromOtherPos1<T>(_: T = 42, _: [T]) {}

into a struct, where it does not emit an error anymore.

Issue 2

I think this might be the actual issue. A parameter is counted if it is a GenericTypeParamType. However, DependentMemberTypes are also counted because it de-sugars to GenericTypeParamType. So either, someone would have to exclude DependentMemberType from this counting, or just not count the same parameter multiple times. (and I don't think they should be counted, because eg. the type of clock cannot actually be inferred by the duration or instant, right?)