Compiler crashes if a type has specific `init` overload + `callAsFunction`.

[YOCKOW]

Description

This is probably related to:

• [Typechecker]: Crash involving trailing closures, callAsFunction, and duplicate declarations #85364
• Compiler crash during typechecking involving callAsFunction + trailing closure + explicit init #85376

The compiler crashes when it tries to compile the following code.

Reproduction

struct S {
  init() {
    print("`init` without block is called.")
  }

  init(_ block: () -> Void) {
    print("`init` with block is called.")
    block()
  }

  func callAsFunction(_ block: () -> Void) {
    print("`callAsFunction` is called.")
    block()
  }
}

S {}

Stack dump

Assertion failed: (!empty()), function back, file ArrayRef.h, line 157.
Stack dump:
0.	Program arguments: /Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend -frontend -interpret forums-83061.swift -Xllvm -aarch64-use-tbi -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -color-diagnostics -Xcc -fcolor-diagnostics -empty-abi-descriptor -no-auto-bridging-header-chaining -module-name main -in-process-plugin-server-path /Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/lib/swift/host/libSwiftInProcPluginServer.dylib -plugin-path /Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/local/lib/swift/host/plugins -target-sdk-version 26.0 -target-sdk-name macosx26.0 -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server
1.	Apple Swift version 6.3-dev (LLVM bff1370bd79c983, Swift 57cf4ce563f700b)
2.	Compiling with effective version 5.10
3.	While evaluating request TypeCheckPrimaryFileRequest(source_file "forums-83061.swift")
4.	While type-checking statement at [forums-83061.swift:17:1 - line:17:4] RangeText="S {"
5.	While type-checking expression at [forums-83061.swift:17:1 - line:17:4] RangeText="S {"
6.	While type-checking-target starting at forums-83061.swift:17:1
 #0 0x000000010a2ffb80 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x105c9fb80)
 #1 0x000000010a2fde9c (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x105c9de9c)
 #2 0x000000010a300610 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x105ca0610)
 #3 0x000000019e1ea744 (/usr/lib/system/libsystem_platform.dylib+0x1804d6744)
 #4 0x000000019e1e0888 (/usr/lib/system/libsystem_pthread.dylib+0x1804cc888)
 #5 0x000000019e0e6808 (/usr/lib/system/libsystem_c.dylib+0x1803d2808)
 #6 0x000000019e0e5a3c (/usr/lib/system/libsystem_c.dylib+0x1803d1a3c)
 #7 0x000000010a3102b4 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x105cb02b4)
 #8 0x0000000105987638 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x101327638)
 #9 0x00000001059e1e64 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x101381e64)
#10 0x00000001059defb0 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x10137efb0)
#11 0x00000001059474cc (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1012e74cc)
#12 0x0000000105af4928 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x101494928)
#13 0x0000000105af4768 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x101494768)
#14 0x0000000105af4630 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x101494630)
#15 0x0000000105bcb350 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x10156b350)
#16 0x0000000105bce538 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x10156e538)
#17 0x0000000105bccd64 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x10156cd64)
#18 0x0000000105bccdfc (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x10156cdfc)
#19 0x0000000105c182e8 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1015b82e8)
#20 0x0000000105c1b868 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1015bb868)
#21 0x0000000105c181f4 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1015b81f4)
#22 0x0000000104b9f67c (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x10053f67c)
#23 0x0000000104b97088 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x100537088)
#24 0x0000000104b96fc4 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x100536fc4)
#25 0x000000010493f88c (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1002df88c)
#26 0x0000000104932d30 (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1002d2d30)
#27 0x000000010493048c (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x1002d048c)
#28 0x0000000104698dec (/Users/yockow/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a.xctoolchain/usr/bin/swift-frontend+0x100038dec)
#29 0x000000019de21d54

Expected behavior

The compiler emits an error (or compiles successfully).

Environment

Apple Swift version 6.3-dev (LLVM bff1370bd79c983, Swift 57cf4ce563f700b)
Target: arm64-apple-macosx26.0
Build config: +assertions

Additional information

No response

[YOCKOW]

Sorry, this is not related to, but just a dup of #85364

[jamieQ]

the input source does look slightly different, but it's the same assertion, so it might not be exactly the same – i'll update the other ticket with your example too