duplicates:

• SR-8984 Compiler crash: swift::GenericEnvironment::mapTypeIntoContext(swift::GenericEnvironment*, swift::Type)

Issue Description:

To reproduce:

$ git clone https://github.com/bignerdranch/Deferred.git
$ cd Deferred
$ git checkout -f c374a6e7072ed88ed0c79299098ca2ba7c27e720
$ open Deferred.xcodeproj 

Then in Xcode:

1. Select the October 3 swift.org dev snapshot toolchain

2. Navigate to Either.swift

3. Replace the file contents with the below:

   @available(swift, deprecated: 100000)
   public protocol Either {
   
   extension Either where Left == Error {
       /// Derive a success value by calling a failable function `body`.
       @_inlineable
       public init(from

1. Build to get the crash below:

   Application Specific Information:
   Stack dump:
   0.  Program arguments: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2018-10-03-a.xctoolchain/usr/bin/swift -frontend -c ...
   1.  While type-checking 'Either' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Deferred/Sources/Task/Either.swift:2:8
   2.  While type-checking extension of Either at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Deferred/Sources/Task/Either.swift:4:3
   3.  While type-checking 'init()' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Deferred/Sources/Task/Either.swift:7:12
   4.  While validating 'init()' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Deferred/Sources/Task/Either.swift:7:12
    
   
   Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
   0   swift                           0x000000011061c9e2 swift::FunctionType::get(llvm::ArrayRef<swift::AnyFunctionType::Param>, swift::Type, swift::AnyFunctionType::ExtInfo) + 98
   1   swift                           0x00000001106bc264 swift::AbstractFunctionDecl::computeType(swift::AnyFunctionType::ExtInfo) + 436
   2   swift                           0x00000001103da357 swift::TypeChecker::validateGenericFuncSignature(swift::AbstractFunctionDecl*) + 1559
   3   swift                           0x00000001103ae95b swift::TypeChecker::validateDecl(swift::ValueDecl*) + 7067
   4   swift                           0x00000001103a9a62 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 258
   5   swift                           0x00000001103b82bb (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) + 443
   6   swift                           0x00000001103a9a44 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 228
   7   swift                           0x00000001103b7a0b (anonymous namespace)::DeclChecker::visitProtocolDecl(swift::ProtocolDecl*) + 587
   8   swift                           0x00000001103a9f9d (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 1597
   9   swift                           0x00000001103a9956 swift::TypeChecker::typeCheckDecl(swift::Decl*) + 38
   10  swift                           0x0000000110443218 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 1944
   11  swift                           0x00000001100e4a35 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 629
   12  swift                           0x00000001100e4038 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 616
   13  swift                           0x000000010f41e067 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1335
   14  swift                           0x000000010f41ca9d swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3021
   15  swift                           0x000000010f3cef0e main + 686
   16  libdyld.dylib                   0x00007fff742fd085 start + 1