❯ swift test.swift
 test.swift:1:16: warning: coercion from '[String : Int?]' to '[String : String?]' may fail; use 'as?' or 'as!' instead
 ["v": Int?(1)] as [String: String?]
 ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
 test.swift:1:16: warning: expression of type '[String : String?]' is unused
 ["v": Int?(1)] as [String: String?]
 ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
 PHI node operands are not the same type as the result!
 %36 = phi i64 [ 0, %33 ], [ %29, %31 ], !dbg !48
 in function main
 <unknown>:0: error: fatal error encountered during compilation; please submit a bug report ([https://swift.org/contributing/#reporting-bugs]) and include the project
 <unknown>:0: note: Broken function found, compilation aborted!
 Please submit a bug report ([https://swift.org/contributing/#reporting-bugs]) and include the project and the crash backtrace.
 Stack dump:
 0. Program arguments: /Applications/Xcode_12.5.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -interpret test.swift -enable-objc-interop -sdk /Applications/Xcode_12.5.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk -color-diagnostics -target-sdk-version 11.3 -module-name test 
 1. Apple Swift version 5.4 (swiftlang-1205.0.26.9 clang-1205.0.19.55)
 2. While running user code "test.swift"
 3. Running pass 'Module Verifier' on function '@main'
 0 swift-frontend 0x0000000113b04e85 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
 1 swift-frontend 0x0000000113b03e78 llvm::sys::RunSignalHandlers() + 248
 2 swift-frontend 0x0000000113b05446 SignalHandler(int) + 262
 3 libsystem_platform.dylib 0x00007fff203dbd7d _sigtramp + 29
 4 libsystem_platform.dylib 0x00007ffee0f78108 _sigtramp + 18446744072647984040
 5 libsystem_c.dylib 0x00007fff202ea720 abort + 120
 6 swift-frontend 0x000000010ed27a31 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*)::$_1::__invoke(void*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool) + 1169
 7 swift-frontend 0x0000000113a5a2d0 llvm::report_fatal_error(llvm::Twine const&, bool) + 288
 8 swift-frontend 0x0000000113a5a1ab llvm::report_fatal_error(char const*, bool) + 43
 9 swift-frontend 0x00000001139fa37f (anonymous namespace)::VerifierLegacyPass::runOnFunction(llvm::Function&) + 111
 10 swift-frontend 0x00000001139940b9 llvm::FPPassManager::runOnFunction(llvm::Function&) + 1353
 11 swift-frontend 0x00000001139933a0 llvm::legacy::FunctionPassManagerImpl::run(llvm::Function&) + 112
 12 swift-frontend 0x000000011399a835 llvm::legacy::FunctionPassManager::run(llvm::Function&) + 341
 13 swift-frontend 0x000000010f0d33e8 swift::performLLVMOptimizations(swift::IRGenOptions const&, llvm::Module*, llvm::TargetMachine*) + 1688
 14 swift-frontend 0x000000010f0d4486 swift::performLLVM(swift::IRGenOptions const&, swift::DiagnosticEngine&, llvm::sys::SmartMutex<false>*, llvm::GlobalVariable*, llvm::Module*, llvm::TargetMachine*, llvm::StringRef, swift::UnifiedStatsReporter*) + 2582
 15 swift-frontend 0x000000010ed6938d swift::RunImmediately(swift::CompilerInstance&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, swift::IRGenOptions const&, swift::SILOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >&&) + 461
 16 swift-frontend 0x000000010ed33375 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*) + 2421
 17 swift-frontend 0x000000010ed250ab swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7275
 18 swift-frontend 0x000000010eca6e82 main + 1266
 19 libdyld.dylib 0x00007fff203b2621 start + 1
 20 libdyld.dylib 0x000000000000000c start + 18446603339975416300
 [1] 5197 abort swift test.swift

 ❯ swift test2.swift
 test2.swift:1:10: warning: coercion from '[String : Int]' to '[String : String]' may fail; use 'as?' or 'as!' instead
 ["v": 1] as [String: String]
 ~~~~~~~~~^~~~~~~~~~~~~~~~~~~
 test2.swift:1:10: warning: expression of type '[String : String]' is unused
 ["v": 1] as [String: String]
 ~~~~~~~~~^~~~~~~~~~~~~~~~~~~
 Please submit a bug report ([https://swift.org/contributing/#reporting-bugs]) and include the project and the crash backtrace.
 Stack dump:
 0. Program arguments: /Applications/Xcode_12.5.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -interpret test2.swift -enable-objc-interop -sdk /Applications/Xcode_12.5.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk -color-diagnostics -target-sdk-version 11.3 -module-name test2 
 1. Apple Swift version 5.4 (swiftlang-1205.0.26.9 clang-1205.0.19.55)
 2. While running user code "test2.swift"
 3. While evaluating request IRGenRequest(IR Generation for module test2)
 4. While emitting IR SIL function "@main".
 0 swift-frontend 0x000000010cec9e85 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
 1 swift-frontend 0x000000010cec8e78 llvm::sys::RunSignalHandlers() + 248
 2 swift-frontend 0x000000010ceca446 SignalHandler(int) + 262
 3 libsystem_platform.dylib 0x00007fff203dbd7d _sigtramp + 29
 4 libsystem_platform.dylib 0x00007f81fd627d40 _sigtramp + 18446743536253845472
 5 swift-frontend 0x00000001084d1096 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 10198
 6 swift-frontend 0x0000000108358dc7 swift::irgen::IRGenerator::emitGlobalTopLevel(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) + 1623
 7 swift-frontend 0x000000010849dd11 swift::IRGenRequest::evaluate(swift::Evaluator&, swift::IRGenDescriptor) const + 10801
 8 swift-frontend 0x00000001084ce5ed swift::SimpleRequest<swift::IRGenRequest, swift::GeneratedModule (swift::IRGenDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::IRGenRequest const&, swift::Evaluator&) + 157
 9 swift-frontend 0x00000001084a94dc llvm::Expected<swift::IRGenRequest::OutputType> swift::Evaluator::getResultUncached<swift::IRGenRequest>(swift::IRGenRequest const&) + 1404
 10 swift-frontend 0x00000001084a0143 swift::performIRGeneration(swift::ModuleDecl*, swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, llvm::GlobalVariable**) + 1523
 11 swift-frontend 0x000000010812e2ea swift::RunImmediately(swift::CompilerInstance&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, swift::IRGenOptions const&, swift::SILOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >&&) + 298
 12 swift-frontend 0x00000001080f8375 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*) + 2421
 13 swift-frontend 0x00000001080ea0ab swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7275
 14 swift-frontend 0x000000010806be82 main + 1266
 15 libdyld.dylib 0x00007fff203b2621 start + 1
 [1] 5409 segmentation fault swift test2.swift