@dynamicMemberLookup
public protocol Entity {    
    associatedtype Interface: Entity = Never
}

extension Entity {
    
    public var interface: Interface {
        fatalError("")
    }
    
    public subscript<Value>(dynamicMember keyPath: KeyPath<Interface, Value>) -> Value {
        interface[keyPath: keyPath]
    }
}

extension Never: Entity {}

swift Entity.swift

swift: /home/buildnode/jenkins/workspace/oss-swift-5.3-package-linux-ubuntu-18_04/swift/lib/Sema/CSSimplify.cpp:9928: void swift::constraints::ConstraintSystem::addKeyPathApplicationRootConstraint(swift::Type, swift::constraints::ConstraintLocatorBuilder): Assertion `(path.size() == 1 && path[0].getKind() == ConstraintLocator::SubscriptMember) || (path.size() == 2 && path[1].getKind() == ConstraintLocator::KeyPathDynamicMember)' failed.
Stack dump:
0.  Program arguments: /usr/bin/swift -frontend -interpret Entity.swift -disable-objc-interop -color-diagnostics -module-name Entity 
1.  Swift version 5.3.1 (swift-5.3.1-RELEASE)
2.  While evaluating request TypeCheckSourceFileRequest(source_file "Entity.swift")
3.  While evaluating request TypeCheckFunctionBodyUntilRequest(Entity.(file).Entity extension._@Entity.swift:14:88, )
4.  While type-checking statement at [Entity.swift:14:88 - line:16:5] RangeText="{
        interface[keyPath: keyPath]
    "
5.  While type-checking statement at [Entity.swift:15:9 - line:15:35] RangeText="interface[keyPath: keyPath"
6.  While type-checking expression at [Entity.swift:15:9 - line:15:35] RangeText="interface[keyPath: keyPath"
/usr/bin/swift[0x51fa1b4]
/usr/bin/swift[0x51f7dae]
/usr/bin/swift[0x51fa48c]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x12980)[0x7f5b5c0bd980]
/lib/x86_64-linux-gnu/libc.so.6(gsignal+0xc7)[0x7f5b5a728fb7]
/lib/x86_64-linux-gnu/libc.so.6(abort+0x141)[0x7f5b5a72a921]
/lib/x86_64-linux-gnu/libc.so.6(+0x3048a)[0x7f5b5a71a48a]
/lib/x86_64-linux-gnu/libc.so.6(+0x30502)[0x7f5b5a71a502]
/usr/bin/swift[0x12c6a56]
/usr/bin/swift[0x12c6b6f]
/usr/bin/swift[0x1351b38]
/usr/bin/swift[0x12c7a82]
/usr/bin/swift[0x134ff50]
/usr/bin/swift[0x12ba44c]
/usr/bin/swift[0x128a36f]
/usr/bin/swift[0x1350c85]
/usr/bin/swift[0x1352c2c]
/usr/bin/swift[0x12c7a82]
/usr/bin/swift[0x12c8290]
/usr/bin/swift[0x12dce76]
/usr/bin/swift[0x12f039d]
/usr/bin/swift[0x12efafa]
/usr/bin/swift[0x12d6402]
/usr/bin/swift[0x12d86ec]
/usr/bin/swift[0x12d81ff]
/usr/bin/swift[0x12d7c30]
/usr/bin/swift[0x13c54d1]
/usr/bin/swift[0x13c526e]
/usr/bin/swift[0x14804e1]
/usr/bin/swift[0x1480c6f]
/usr/bin/swift[0x147f6fa]
/usr/bin/swift[0x147c73f]
/usr/bin/swift[0x147b872]
/usr/bin/swift[0x1483b7e]
/usr/bin/swift[0x1483690]
/usr/bin/swift[0x148359e]
/usr/bin/swift[0x147c1b0]
/usr/bin/swift[0x147b35f]
/usr/bin/swift[0x14affd6]
/usr/bin/swift[0x14b1f40]
/usr/bin/swift[0x14b1ca7]
/usr/bin/swift[0x14b1bee]
/usr/bin/swift[0x14afc0e]
/usr/bin/swift[0x14afbe4]
/usr/bin/swift[0x6b6ac8]
/usr/bin/swift[0x6b5b2b]
/usr/bin/swift[0x6b52d0]
/usr/bin/swift[0x5605f7]
/usr/bin/swift[0x55f069]
/usr/bin/swift[0x4e8ce8]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f5b5a70bbf7]
/usr/bin/swift[0x4e888a]
Aborted