Issue Description:

To reproduce:

$ git clone https://github.com/kickstarter/Kickstarter-Prelude
$ cd Kickstarter-Prelude
$ git checkout -f 9b2cc23ce8acf29b4b666a80a665c67cba800aac
$ open Prelude.xcodeproj

The in Xcode:

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

2. Select the "Prelude-iOS > Generic iOS Device" scheme

3. Navigate to Array.swift

4. Replace its contents with the below:

   public extension Array where Element: OptionalType {
     public func compact() -> [Element.Wrapped
   
   public extension Array where Element: Semigroup {
     public func sconcat(_ initial: Element) 
   
   public extension Array {
     public var randomElement: Element? {
       guard !self.isEmpty else 
   
   public extension Array where Element: Equatable {
     public func distincts() -> Array {
       return 
   
   extension Array: Semigroup {
     public func op(_ other: Array) -> Array {
   
   extension Array where Element: EitherType {
     func lefts() -> [Element.A] 

5. 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 -typecheck -primary-file ...
   1.  While type-checking 'op(_:)' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Kickstarter-Prelude/Prelude/Array.swift:38:22
   2.  While type-checking statement at [/Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Kickstarter-Prelude/Prelude/Array.swift:38:55 - line:47:45] RangeText="{
    
                   extension Array where Element: EitherType {
    
                     /**
                      Extracts the left values from the array.
    
                      - returns: A new array of left values.
                      */
                     func lefts() -> [Element.A"
   3.  While type-checking extension of <<error type>> at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Kickstarter-Prelude/Prelude/Array.swift:40:17
   4.  While type-checking 'lefts()' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Kickstarter-Prelude/Prelude/Array.swift:47:19
   5.  While validating 'lefts()' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Kickstarter-Prelude/Prelude/Array.swift:47:19
   6.  While resolving type [Element.A] at [/Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/Kickstarter-Prelude/Prelude/Array.swift:47:35 - line:47:45] RangeText="[Element.A"
    
   
   Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
   0   swift                           0x00000001053f2104 swift::GenericSignature::getASTContext() const + 4
   1   swift                           0x000000010512e9a8 swift::TypeResolution::resolveDependentMemberType(swift::Type, swift::DeclContext*, swift::SourceRange, swift::ComponentIdentTypeRepr*) const + 136
   2   swift                           0x0000000105132735 resolveIdentTypeComponent(swift::TypeResolution, llvm::ArrayRef<swift::ComponentIdentTypeRepr*>, swift::TypeResolutionOptions) + 1269
   3   swift                           0x0000000105131e4b swift::TypeChecker::resolveIdentifierType(swift::TypeResolution, swift::IdentTypeRepr*, swift::TypeResolutionOptions) + 155
   4   swift                           0x000000010513325a (anonymous namespace)::TypeResolver::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 282
   5   swift                           0x0000000105133a64 (anonymous namespace)::TypeResolver::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 2340
   6   swift                           0x00000001051314a1 swift::TypeResolution::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 225
   7   swift                           0x00000001051330b2 swift::TypeChecker::validateType(swift::TypeLoc&, swift::TypeResolution, swift::TypeResolutionOptions) + 98
   8   swift                           0x00000001050d5565 checkGenericFuncSignature(swift::TypeChecker&, swift::GenericSignatureBuilder*, swift::AbstractFunctionDecl*, swift::TypeResolution) + 325
   9   swift                           0x00000001050d5349 swift::TypeChecker::validateGenericFuncSignature(swift::AbstractFunctionDecl*) + 1545
   10  swift                           0x00000001050a94e3 swift::TypeChecker::validateDecl(swift::ValueDecl*) + 5923
   11  swift                           0x00000001050b2f70 (anonymous namespace)::DeclChecker::visitFuncDecl(swift::FuncDecl*) + 48
   12  swift                           0x00000001050a4a18 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 184
   13  swift                           0x00000001050b32bb (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) + 443
   14  swift                           0x00000001050a4a44 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 228
   15  swift                           0x00000001050a4956 swift::TypeChecker::typeCheckDecl(swift::Decl*) + 38
   16  swift                           0x000000010511e243 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 451
   17  swift                           0x000000010511d578 bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) + 136
   18  swift                           0x000000010511c114 swift::TypeChecker::typeCheckFunctionBodyUntil(swift::FuncDecl*, swift::SourceLoc) + 260
   19  swift                           0x000000010511cc4f swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 271
   20  swift                           0x000000010513d116 typeCheckFunctionsAndExternalDecls(swift::SourceFile&, swift::TypeChecker&) + 310
   21  swift                           0x000000010513e28d swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 2061
   22  swift                           0x0000000104ddfa35 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 629
   23  swift                           0x0000000104ddf038 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 616
   24  swift                           0x0000000104119067 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1335
   25  swift                           0x0000000104117a9d swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3021
   26  swift                           0x00000001040c9f0e main + 686
   27  libdyld.dylib                   0x00007fff742fd085 start + 1