enum Foo: Equatable { case x(()) }

$ swift /tmp/test.swift
/tmp/test.swift:1:6: error: type 'Foo' does not conform to protocol 'Equatable'
enum Foo: Equatable { case x(()) }
     ^
Swift.==:1:24: note: candidate would match if 'Foo' conformed to 'RawRepresentable'
@inlinable public func == <T>(lhs: T, rhs: T) -> Bool where T : RawRepresentable, T.RawValue : Equatable
                       ^
Swift.FloatingPoint:2:24: note: candidate would match if 'Foo' conformed to 'FloatingPoint'
    public static func == (lhs: Self, rhs: Self) -> Bool
                       ^
Swift.BinaryInteger:2:24: note: candidate would match if 'Foo' conformed to 'BinaryInteger'
    public static func == <Other>(lhs: Self, rhs: Other) -> Bool where Other : BinaryInteger
                       ^
Swift._Pointer:2:24: note: candidate would match if 'Foo' conformed to '_Pointer'
    public static func == (lhs: Self, rhs: Self) -> Bool
                       ^
Swift.Strideable:3:35: note: candidate would match if 'Foo' conformed to 'Strideable'
    @inlinable public static func == (x: Self, y: Self) -> Bool
                                  ^
Swift.StringProtocol:2:35: note: candidate would match if 'Foo' conformed to 'StringProtocol'
    @inlinable public static func == <RHS>(lhs: Self, rhs: RHS) -> Bool where RHS : StringProtocol
                                  ^
Swift.SIMD:4:24: note: candidate would match if 'Foo' conformed to 'SIMD'
    public static func == (lhs: Self, rhs: Self) -> Bool
                       ^
Swift.Equatable:2:17: note: protocol requires function '==' with type '(Foo, Foo) -> Bool'; do you want to add a stub?
    static func == (lhs: Self, rhs: Self) -> Bool
                ^

  1> func f<E: Equatable>(_ x: E) {}
  2> f(())
error: repl.swift:2:3: error: argument type '()' does not conform to expected type 'Equatable'
f(())
  ^