(lib/AST/GenericEnvironment) no generic environment provided for type with type parameters!

[tayloraswift]

crashes on DEVELOPMENT-SNAPSHOT-2022-09-29-a:

protocol P
{
    subscript<Value>(x:Value) -> Int
    {
        get
    }
}
struct S:P
{
    subscript<Value>(x:Int) -> Value
    {
    }
}

$ swiftc test9.swift 
error: compile command failed due to signal 6 (use -v to see invocation)
test9.swift:12:5: error: missing return in subscript expected to return 'Value'
    }
    ^
test9.swift:8:8: error: type 'S' does not conform to protocol 'P'
struct S:P
       ^
swift-frontend: /home/build-user/swift/lib/AST/GenericEnvironment.cpp:239: static swift::Type swift::GenericEnvironment::mapTypeIntoContext(swift::GenericEnvironment *, swift::Type): Assertion `(env || !type->hasTypeParameter()) && "no generic environment provided for type with type parameters"' failed.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the project and the crash backtrace.
Stack dump:
0.      Program arguments: /home/ec2-user/.swiftenv/versions/DEVELOPMENT-SNAPSHOT-2022-09-29-a/usr/bin/swift-frontend -frontend -c -primary-file test9.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -color-diagnostics -new-driver-path /home/ec2-user/.swiftenv/versions/DEVELOPMENT-SNAPSHOT-2022-09-29-a/usr/bin/swift-driver -empty-abi-descriptor -resource-dir /home/ec2-user/.swiftenv/versions/DEVELOPMENT-SNAPSHOT-2022-09-29-a/usr/lib/swift -module-name test9 -o /tmp/TemporaryDirectory.ZBcDAU/test9-1.o
1.      Swift version 5.8-dev (LLVM 338153f292479fa, Swift b752d7883c26cea)
2.      Compiling with the current language version
3.      While evaluating request TypeCheckSourceFileRequest(source_file "test9.swift")
4.      While type-checking 'S' (at test9.swift:8:1)
5.      While type-checking protocol conformance to 'P' (at test9.swift:1:1) for type 'S' (declared at [test9.swift:8:1 - line:13:1] RangeText="struct S:P
{
    subscript<Value>(x:Int) -> Value
    {
    }
")

[theblixguy]

I just had a look.... @AnthonyLatsis I see you added this assertion in 1cf9622 but maybe its too strict?

The code is doing subscript->getDeclContext()->mapTypeIntoContext(...) but the subscript's declaration context (struct S) isn't a generic context in this case (and hence we don't have a generic environment), so it trips the assert. Although, if S is generic (i.e. if it was struct S<T>) then it doesn't trip the assert.

In release toolchain, the code doesn't crash and emits the notes:

note: candidate has non-matching type '<Value> (Int) -> Value'
  subscript<Value>(x: Int) -> Value { fatalError() }
  ^
note: protocol requires subscript with type '<Value> (Value) -> Int'; do you want to add a stub?
  subscript<Value>(x: Value) -> Int { get }

[AnthonyLatsis]

I believe the assertion is accurate, i.e., we cannot map an interface type containing type parameters into a contextual type without a generic environment. Rather, it looks like we’re mapping the element type into the wrong context (the parent context, when it should be the subscript itself).