Compiler fails to detect illegal use of undeclared variable

[MojtabaHs]

Description

When using the left hand side of an assignment, inside a closure, on the right hand side, compiler fails to detect this illegal use and compiles the code.

It seems it unexpectedly uses unsafeBitCast(0.0, to: TheType.self) to default the value and it has different behavior on different types. For example:

String -> returns nil -> Crashes at runtime -> EXC-BAD-ACCESS
Date -> returns reference date -> 2001-01-01 00:00:00 +0000
Int -> returns 0
Double -> returns 0.0
Custom enum without rawValue -> returns the first case of the enum

Also discussed Here

Reproduction

func foo(date: Date, action: () -> Void) -> Date {
    action()
    return date
}

var actionDate: Date!

let resultDate = foo(date: Date.now) {
    actionDate = resultDate
    print("Assigned")
}

print(actionDate == resultDate) // 👈 false
print(actionDate) // 👈 Optional(2001-01-01 00:00:00 +0000)

Expected behavior

It should prevent the use of variable in the first place or at least throws some warnings or errors like one of these:

Environment

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx26.0

Tested in DEBUG and RELEASE mode
Also tested on an online compiler

Additional information

It seems like if we put the variable declaration in a do { } block, the compiler starts to throws a meaningful error and refuses compiling:

enum Bar {
    case a
    case b
}

func foo(bar: Bar, action: () -> Void) -> Bar {
    action()
    return bar
}

do {
    var actionBar: Bar!

    let resultBar = foo(bar: Bar.b) {
        actionBar = resultBar // 👈 🛑 Closure captures 'resultBar' before it is declared
        print("Assigned")
    }

    print(actionBar == resultBar)
    print(actionBar)
}

Some folks suggested that it is because of the single file top-level code or main.swift file but you can reproduce it in a iOS project using a SPM without these conditions. Explained here

[jamieQ]

slightly reduced example demonstrating a similar (or possibly the same) issue (edit: now even more reduced):

func fwd(_ i: () -> Any) -> Any { i() }

func repro() {
    let useBeforeInit = fwd { useBeforeInit } // ✅ compiles without error
    _ = useBeforeInit // 💥 bad pointer dereference at runtime
}

this compiles without error on the 6.2 release compiler (OSS & Xcode 26), but does trigger a SIL verification failure when passing the frontend flag -sil-verify-all (and also crashes at runtime with a bad pointer dereference):

SIL memory lifetime failure in @$s6output5reproyyF: memory is not initialized, but should be
memory location:   %0 = alloc_stack [lexical] [var_decl] $Any, let, name "useBeforeInit" // users: %12, %11, %7, %3
at instruction:   copy_addr %0 to [init] %2 : $*Any               // id: %3

in function:
// repro()
// Isolation: unspecified
sil hidden [ossa] @$s6output5reproyyF : $@convention(thin) () -> () {
bb0:
  %0 = alloc_stack [lexical] [var_decl] $Any, let, name "useBeforeInit" // users: %12, %11, %7, %3
  // function_ref closure #1 in repro()
  %1 = function_ref @$s6output5reproyyFypyXEfU_ : $@convention(thin) (@in_guaranteed Any) -> @out Any // user: %4
  %2 = alloc_stack $Any                           // users: %10, %4, %3
  copy_addr %0 to [init] %2                       // id: %3
  %4 = partial_apply [callee_guaranteed] %1(%2) : $@convention(thin) (@in_guaranteed Any) -> @out Any // users: %9, %5
  %5 = convert_escape_to_noescape [not_guaranteed] %4 to $@noescape @callee_guaranteed () -> @out Any // users: %8, %7
  // function_ref fwd(_:)
  %6 = function_ref @$s6output3fwdyypypyXEF : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> @out Any) -> @out Any // user: %7
  %7 = apply %6(%0, %5) : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> @out Any) -> @out Any
  destroy_value %5                                // id: %8
  destroy_value %4                                // id: %9
  dealloc_stack %2                                // id: %10
  destroy_addr %0                                 // id: %11
  dealloc_stack %0                                // id: %12
  %13 = tuple ()                                  // user: %14
  return %13                                      // id: %14
} // end sil function '$s6output5reproyyF'