SIL verifier crash: _read yielding ~Escapable value with @_lifetime(borrow)

[coenttb]

Description

The compiler crashes (signal 6) during SIL verification when a _read coroutine yields a ~Copyable & ~Escapable value whose initializer has a @_lifetime(borrow) lifetime dependency. The SIL verifier detects a double-consume: mark_dependence [nonescaping] and destroy_value both consume the yielded value.

This crash only occurs with the swift.org open-source toolchain (+assertions build). The Xcode-bundled toolchain (same version, no assertions) compiles successfully.

Environment

• Swift version (crashes): Apple Swift version 6.2.3 (swift-6.2.3-RELEASE), Build config: +assertions
• Swift version (also crashes): Apple Swift version 6.3-dev (main-snapshot-2026-02-05), Build config: +assertions
• Swift version (works): Apple Swift version 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
• Target: arm64-apple-macosx26.0
• Crash location: SIL verifier, function @$s24SILVerifierReadEscapable1PPAARi_zrlE7wrapperAA7WrapperVyxGvr

Minimal Reproduction (20 lines)

public struct Wrapper<Base: ~Copyable>: ~Copyable, ~Escapable {
    @usableFromInline
    var _base: UnsafeMutablePointer<Base>

    @inlinable
    @_lifetime(borrow base)
    public init(_ base: UnsafeMutablePointer<Base>) {
        unsafe _base = base
    }
}

public protocol P: ~Copyable {}

extension P where Self: ~Copyable {
    public var wrapper: Wrapper<Self> {
        mutating _read {
            yield unsafe Wrapper<Self>(&self)  // CRASHES
        }
    }
}

Reproduction repo: https://github.com/coenttb/swift-issue-sil-verifier-read-escapable-lifetime

git clone https://github.com/coenttb/swift-issue-sil-verifier-read-escapable-lifetime
cd swift-issue-sil-verifier-read-escapable-lifetime
swift build  # Using swift.org 6.2.3-RELEASE toolchain (+assertions)

Crash Output

Begin Error in Function: '$s24SILVerifierReadEscapable1PPAARi_zrlE7wrapperAA7WrapperVyxGvr'
Found over consume?!
Value:   %7 = apply ... Wrapper<Self>
User:   %8 = mark_dependence [nonescaping] %7 : $Wrapper<Self> on %5 : $UnsafeMutablePointer<Self>
Block: bb0
Consuming Users:
  destroy_value %7 : $Wrapper<Self>
  %8 = mark_dependence [nonescaping] %7 : $Wrapper<Self> on %5 : $UnsafeMutablePointer<Self>

End Error in Function: '$s24SILVerifierReadEscapable1PPAARi_zrlE7wrapperAA7WrapperVyxGvr'
Found ownership error?!
<unknown>:0: error: fatal error encountered during compilation; please submit a bug report
<unknown>:0: note: triggering standard assertion failure routine

Conditions Required

All 5 conditions must be present to trigger the crash:

#	Condition	Description
1	~Copyable & ~Escapable struct	The yielded type must be both non-copyable and non-escapable
2	@_lifetime(borrow) init	The struct's initializer must have a lifetime dependency
3	_read coroutine accessor	The property must use _read to yield the value
4	where Self: ~Copyable	The protocol extension must constrain Self as ~Copyable
5	+assertions toolchain	Only swift.org open-source builds with assertions enabled

Workaround

Use the Xcode-bundled Swift 6.2.3 toolchain instead of the swift.org open-source toolchain:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift build