duplicates:

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

Issue Description:

Possibly a dupe of SR-8984 but the trace is quite different

To reproduce:

$ git clone https://github.com/vapor/fluent
$ cd fluent
$ git checkout -f 270b6fa372f03809b9795e8f8b9d1c31267a0ff3
$ swift package generate-xcodeproj
$ open Fluent.xcodeproj

Then in Xcode:

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

2. Navigate to SQL+JoinSupporting.swift

3. Replace the file contents with the below:

   extension JoinSupporting where
       QueryJoin: SQLJoin,
       QueryJoinMethod == QueryJoin.Method,
       QueryJoin.Expression.ColumnIdentifier.TableIdentifier == QueryJoin.TableIdentifier
   {
   
   extension JoinSupporting where QueryJoinMethod: SQLJoinMethod {
       /// See `JoinSupporting`.
       public static var queryJoinMethodDefault: QueryJoinMethod {
           return .default
       }
   }
   
   extension JoinSupporting where Query: FluentSQLQuery, QueryJoin == Query.Join {
       /// See `JoinSupporting`.
       public static func queryJoinApply(_ join: QueryJoin, to query

4. Build to get the trace 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 ... 
   1.  While type-checking extension of JoinSupporting at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/fluent/Sources/FluentSQL/SQL+JoinSupporting.swift:1:1
   2.  While type-checking extension of JoinSupporting at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/fluent/Sources/FluentSQL/SQL+JoinSupporting.swift:7:3
   3.  While type-checking declaration 0x7fdf7719bf00 at /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/fluent/Sources/FluentSQL/SQL+JoinSupporting.swift:9:12
   4.  While resolving type QueryJoinMethod at [/Users/nathanhawes/workspace/open-swift-source-compat-suite-master/project_cache/fluent/Sources/FluentSQL/SQL+JoinSupporting.swift:9:47 - line:9:47] RangeText=""
    
   
   Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
   0   swift                           0x000000010ae3a9ac swift::GenericEnvironment::mapTypeIntoContext(swift::GenericEnvironment*, swift::Type) + 12
   1   swift                           0x000000010ab7a4fc swift::TypeChecker::resolveTypeInContext(swift::TypeDecl*, swift::DeclContext*, swift::TypeResolution, swift::TypeResolutionOptions, bool) + 444
   2   swift                           0x000000010ab7e922 resolveTypeDecl(swift::TypeDecl*, swift::SourceLoc, swift::DeclContext*, swift::TypeResolution, swift::GenericIdentTypeRepr*, swift::TypeResolutionOptions) + 210
   3   swift                           0x000000010ab7c669 resolveIdentTypeComponent(swift::TypeResolution, llvm::ArrayRef<swift::ComponentIdentTypeRepr*>, swift::TypeResolutionOptions) + 1065
   4   swift                           0x000000010ab7be4b swift::TypeChecker::resolveIdentifierType(swift::TypeResolution, swift::IdentTypeRepr*, swift::TypeResolutionOptions) + 155
   5   swift                           0x000000010ab7d25a (anonymous namespace)::TypeResolver::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 282
   6   swift                           0x000000010ab7b4a1 swift::TypeResolution::resolveType(swift::TypeRepr*, swift::TypeResolutionOptions) + 225
   7   swift                           0x000000010ab7d0b2 swift::TypeChecker::validateType(swift::TypeLoc&, swift::TypeResolution, swift::TypeResolutionOptions) + 98
   8   swift                           0x000000010ab27700 validateTypedPattern(swift::TypeChecker&, swift::TypeResolution, swift::TypedPattern*, swift::TypeResolutionOptions) + 112
   9   swift                           0x000000010ab2723e swift::TypeChecker::typeCheckPattern(swift::Pattern*, swift::DeclContext*, swift::TypeResolutionOptions) + 350
   10  swift                           0x000000010aaf6780 validatePatternBindingEntries(swift::TypeChecker&, swift::PatternBindingDecl*) + 656
   11  swift                           0x000000010aaeecd4 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 884
   12  swift                           0x000000010aafd2bb (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) + 443
   13  swift                           0x000000010aaeea44 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 228
   14  swift                           0x000000010aafd2bb (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) + 443
   15  swift                           0x000000010aaeea44 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 228
   16  swift                           0x000000010aaee956 swift::TypeChecker::typeCheckDecl(swift::Decl*) + 38
   17  swift                           0x000000010ab88218 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 1944
   18  swift                           0x000000010a829a35 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 629
   19  swift                           0x000000010a829038 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 616
   20  swift                           0x0000000109b63067 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1335
   21  swift                           0x0000000109b61a9d swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3021
   22  swift                           0x0000000109b13f0e main + 686
   23  libdyld.dylib                   0x00007fff742fd085 start + 1