struct S<T> : Codable {
  var s: [T]!
}

Assertion failed: (!type->hasArchetype() && "already have a contextual type"), function mapTypeIntoContext, file /Users/varun/foss-swift/swift/lib/AST/GenericEnvironment.cpp, line 87.
Stack dump:
0.      Program arguments: /Users/varun/foss-swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift-frontend -frontend -c -primary-file tmp.swift -target x86_64-apple-darwin19.5.0 -enable-objc-interop -color-diagnostics -module-name tmp -o /var/folders/9t/dn6p4v392nb8c8cwxfyxc84c0000gn/T/tmp-3621f0.o
1.      Swift version 5.3-dev (LLVM f9957f081870012, Swift 43fd786ae08cf97)
2.      While evaluating request TypeCheckSourceFileRequest(source_file "tmp.swift")
3.      While type-checking 'S' (at tmp.swift:1:1)
4.      While evaluating request ResolveImplicitMemberRequest(tmp.(file).S@tmp.swift:1:8, resolve CodingKeys)
5.      While type-checking protocol conformance to 'Decodable' (in module 'Swift') for type 'S<T>' (declared at [tmp.swift:1:1 - line:3:1] RangeText="struct S<T>: Codable {
    var x: [T]!
")
0  swift-frontend           0x000000010cf003a5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift-frontend           0x000000010ceff378 llvm::sys::RunSignalHandlers() + 248
2  swift-frontend           0x000000010cf00986 SignalHandler(int) + 262
3  libsystem_platform.dylib 0x00007fff6d8b55fd _sigtramp + 29
4  libsystem_platform.dylib 000000000000000000 _sigtramp + 18446603338678315552
5  libsystem_c.dylib        0x00007fff6d78b808 abort + 120
6  libsystem_c.dylib        0x00007fff6d78aac6 err + 0
7  swift-frontend           0x000000010d3f3a91 swift::GenericEnvironment::mapTypeIntoContext(swift::GenericEnvironment*, swift::Type) (.cold.2) + 33
8  swift-frontend           0x0000000109b2a6f9 swift::GenericEnvironment::mapTypeIntoContext(swift::GenericEnvironment*, swift::Type) + 73
9  swift-frontend           0x00000001096da5ba varConformsToCodable(swift::DeclContext*, swift::VarDecl*, swift::ProtocolDecl*) + 74
10 swift-frontend           0x00000001096d9493 canSynthesize(swift::DerivedConformance&, swift::ValueDecl*) + 3507
11 swift-frontend           0x00000001096d9dfb swift::DerivedConformance::deriveDecodable(swift::ValueDecl*) + 379
12 swift-frontend           0x00000001097bd38b swift::TypeChecker::deriveProtocolRequirement(swift::DeclContext*, swift::NominalTypeDecl*, swift::ValueDecl*) + 443
13 swift-frontend           0x00000001097bd0d5 swift::ConformanceChecker::resolveWitnessViaDerivation(swift::ValueDecl*) + 149
14 swift-frontend           0x00000001097be223 swift::ConformanceChecker::resolveValueWitnesses() + 371
15 swift-frontend           0x00000001097b8cb5 swift::ConformanceChecker::checkConformance(swift::MissingWitnessDiagnosisKind) + 245