duplicates:

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

Issue Description:

Assertion failed: (Val && "isa<> used on a null pointer"), function doit, file /Users/buildnode/jenkins/workspace/oss-swift-package-osx/llvm/include/llvm/Support/Casting.h, line 106.

To reproduce:

$ git clone https://github.com/jessesquires/JSQDataSourcesKit.git
$ cd JSQDataSourcesKit
$ git checkout -f 25ee7e2e2ab50d5e2237292ddc74973c68aee89a
$ open JSQDataSourcesKit.xcodeproj

Then in Xcode:

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

2. Select "JSQDataSourcesKit-iOS > Generic iOS Device" for the scheme

3. Navigate to DataSourceProtocol.swift

4. Replace its contents with the below:

   import Foundation
   
   public protocol DataSourceProtocol {
     associatedtype Item
   
     func numberOfSections() -> Int
     func numberOfItems
     extension DataSourceProtocol {
       public func item(atIndexPath indexPath: IndexPath) -> Item

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 -c -primary-file ... 
   1.  While type-checking 'DataSourceProtocol' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/JSQDataSourcesKit/Source/DataSourceProtocol.swift:4:8
   2.  While type-checking extension of DataSourceProtocol at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/JSQDataSourcesKit/Source/DataSourceProtocol.swift:9:3
   3.  While type-checking 'item(atIndexPath:)' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/JSQDataSourcesKit/Source/DataSourceProtocol.swift:10:12
   4.  While validating 'item(atIndexPath:)' at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/JSQDataSourcesKit/Source/DataSourceProtocol.swift:10:12
   5.  While resolving type Item at [/Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/JSQDataSourcesKit/Source/DataSourceProtocol.swift:10:59 - line:10:59] RangeText=""
    
   Assertion failed: (Val && "isa<> used on a null pointer"), function doit, file /Users/buildnode/jenkins/workspace/oss-swift-package-osx/llvm/include/llvm/Support/Casting.h, line 106.
    
   Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
   0   libsystem_kernel.dylib          0x00007fff7443bb86 __pthread_kill + 10
   1   libsystem_pthread.dylib         0x00007fff744f1c50 pthread_kill + 285
   2   libsystem_c.dylib               0x00007fff743a5268 __abort + 144
   3   libsystem_c.dylib               0x00007fff743a51d8 abort + 142
   4   libsystem_c.dylib               0x00007fff7436d868 __assert_rtn + 320
   5   swift                           0x00000001086ce71e swift::TypeChecker::resolveTypeInContext(swift::TypeDecl*, swift::DeclContext*, swift::TypeResolution, swift::TypeResolutionOptions, bool) + 990
   6   swift                           0x00000001086d2922 resolveTypeDecl(swift::TypeDecl*, swift::SourceLoc, swift::DeclContext*, swift::TypeResolution, swift::GenericIdentTypeRepr*, swift::TypeResolutionOptions) + 210
   7   swift                           0x00000001086d0669 resolveIdentTypeComponent(swift::TypeResolution, llvm::ArrayRef<swift::ComponentIdentTypeRepr*>, swift::TypeResolutionOptions) + 1065
   8   swift                           0x00000001086cfe4b swift::TypeChecker::resolveIdentifierType(swift::TypeResolution, swift::IdentTypeRepr*, swift::TypeResolutionOptions) + 155
   9   swift                           0x00000001086d125a (anonymous namespace)::TypeResolver::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 282
   10  swift                           0x00000001086cf4a1 swift::TypeResolution::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 225
   11  swift                           0x00000001086d10b2 swift::TypeChecker::validateType(swift::TypeLoc&, swift::TypeResolution, swift::TypeResolutionOptions) + 98
   12  swift                           0x0000000108673565 checkGenericFuncSignature(swift::TypeChecker&, swift::GenericSignatureBuilder*, swift::AbstractFunctionDecl*, swift::TypeResolution) + 325
   13  swift                           0x0000000108673349 swift::TypeChecker::validateGenericFuncSignature(swift::AbstractFunctionDecl*) + 1545
   14  swift                           0x00000001086474e3 swift::TypeChecker::validateDecl(swift::ValueDecl*) + 5923
   15  swift                           0x0000000108650f70 (anonymous namespace)::DeclChecker::visitFuncDecl(swift::FuncDecl*) + 48
   16  swift                           0x0000000108642a18 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 184
   17  swift                           0x00000001086512bb (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) + 443
   18  swift                           0x0000000108642a44 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 228
   19  swift                           0x0000000108650a0b (anonymous namespace)::DeclChecker::visitProtocolDecl(swift::ProtocolDecl*) + 587
   20  swift                           0x0000000108642f9d (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 1597
   21  swift                           0x0000000108642956 swift::TypeChecker::typeCheckDecl(swift::Decl*) + 38
   22  swift                           0x00000001086dc218 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 1944
   23  swift                           0x000000010837da35 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 629
   24  swift                           0x000000010837d038 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 616
   25  swift                           0x00000001076b7067 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1335
   26  swift                           0x00000001076b5a9d swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3021
   27  swift                           0x0000000107667f0e main + 686
   28  libdyld.dylib                   0x00007fff742fd085 start + 1