Incorrectly allowed to destructure ()

[KeithBauerANZ]

Description

Came across this one by accident:

func returnTuple() -> (Int, String) {
    (1, "hi")
}

func f() {
    // no error on this line, why not?:
    let (a, b) = {
        print("whatever")
        returnTuple() // oops, forgot the `return` keyword
    }()

    // error here instead:
    print(a)
}

swiftc DestructureVoid.swift
DestructureVoid.swift:13:11: error: cannot reference invalid declaration 'a'
    print(a)
          ^
DestructureVoid.swift:7:10: note: 'a' declared here
    let (a, b) = {
         ^

That error is very unhelpful.

Interestingly, something about the indirection is necessary; simpler examples do work as expected:

func f() {
    let (a, b) = ()
}

DestructureVoidSimple.swift:2:18: error: '()' is not convertible to '(_, _)', tuples have a different number of elements
    let (a, b) = ()
                 ^

and

func f() {
    let (a, b) = {}()
}

DestructureVoidSimple.swift:2:18: error: '()' is not convertible to '(_, _)', tuples have a different number of elements
    let (a, b) = {}()
                 ^

Environment
swift-driver version: 1.75.2 Apple Swift version 5.8.1 (swiftlang-5.8.0.124.5 clang-1403.0.22.11.100)
Target: arm64-apple-macosx13.0