Poor type inference with typed throws

[karwa]

Description

Consider the following function:

extension StaticString {
    func withCString<R>(_ block: (UnsafePointer<CChar>) -> R) -> R {
        self.utf8Start.withMemoryRebound(to: CChar.self, capacity: utf8CodeUnitCount + 1) {
            block($0)
        }
    }
}

We don't need to write explicit signatures for the closure that we pass to withMemoryRebound, because the compiler can infer it - it knows that block($0) returns an R, so the closure does, so the call to withMemoryRebound does, so the body of the function does.

I would expect this same level of type inference when adopting typed throws, but even simple examples require explicit type annotations.

Reproduction

I take the example above, and add an E generic parameter as the type of error thrown by block:

extension StaticString {
    func withCString<E, R>(_ block: (UnsafePointer<CChar>) throws(E) -> R) throws(E) -> R {
        try self.utf8Start.withMemoryRebound(to: CChar.self, capacity: utf8CodeUnitCount + 1) {
            try block($0)
        }
    }
}

It fails to compile.

<source>:3:13: error: thrown expression type 'any Error' cannot be converted to error type 'E'
1 | extension StaticString {
2 |     func withCString<E, R>(_ block: (UnsafePointer<CChar>) throws(E) -> R) throws(E) -> R {
3 |         try self.utf8Start.withMemoryRebound(to: CChar.self, capacity: utf8CodeUnitCount + 1) {
  |             `- error: thrown expression type 'any Error' cannot be converted to error type 'E'
4 |             try block($0)
5 |         }

To make it work, you need to add an explicit type signature to the closure where block is called:

extension StaticString {
    func withCString<E, R>(_ block: (UnsafePointer<CChar>) throws(E) -> R) throws(E) -> R {
        try self.utf8Start.withMemoryRebound(to: CChar.self, capacity: utf8CodeUnitCount + 1) {
            (ptr: UnsafePointer<CChar>) throws(E) -> R in      // <--------
            try block(ptr)
        }
    }
}

Expected behavior

I expect the first example in the reproduction section (with typed throws, without explicit type annotations in the function body) to compile

Environment

Swift version 6.2-dev (LLVM 81859ac55f8d09a, Swift 8ec8a12)
Target: x86_64-unknown-linux-gnu

Also Xcode 16.0 (16A242d)

Additional information

No response

[xedin]

cc @AnthonyLatsis