// stdlib's SIMDVector does something like this...
public protocol DummyProtocol {
 // Interestingly this protocol might also define:
 // func abs() -> Self
}
public protocol SIMDStorageStub {
 associatedtype Scalar : DummyProtocol
}
// Another protocol in SIMDVector.swift introduces a recursive constraint.
// That seems ok in itself, but the extra conformance on DummyProtocol breaks
// witness method invocation at the SIL level.
public protocol SIMDScalarStub {
 associatedtype SIMD2Storage : SIMDStorageStub where SIMD2Storage.Scalar == Self
}
// Now we have an app that wants to conform to this protocol...
// Everything up to here could be replaced by:
// import simd
// /s/SIMDScalarStub/SIMDScalar
public protocol FPScalar: SIMDScalarStub {
 func abs() -> Self
}
public func callAbs<T: FPScalar>(s: T) -> T {
 return s.abs() // Are we supposed to call a method on FPScalar or DummyProtocol and does it matter?
}

auto conformsTo = genericSig->getConformsTo(selfGenericParam);
 require(conformsTo.size() == 1,
 "requirement Self parameter must conform to exactly one protocol"); 

SIL verification failed: requirement Self parameter must conform to exactly one protocol: conformsTo.size() == 1
Verifying instruction:
-> %3 = witness_method $T, #FPScalar.abs!1 : <Self where Self : FPScalar> (Self) -> () -> @dynamic_self Self : $@convention(witness_method: FPScalar) <τ_0_0 where τ_0_0 : FPScalar> (@in_guaranteed τ_0_0) -> @out τ_0_0 // user: %4
 %4 = apply %3<T>(%0, %1) : $@convention(witness_method: FPScalar) <τ_0_0 where τ_0_0 : FPScalar> (@in_guaranteed τ_0_0) -> @out τ_0_0
In function:
// callAbs<A>(s:)
sil [ossa] @$s11testconform7callAbs1sxx_tAA8FPScalarRzlF : $@convention(thin) <T where T : FPScalar> (@in_guaranteed T) -> @out T {

τ_0_0: FPScalar *[τ_0_0: Explicit]
 & SIMDScalarStub [τ_0_0: Explicit -> Protocol requirement (via Self in FPScalar)]
 & DummyProtocol [τ_0_0: Explicit -> Protocol requirement (via Self in FPScalar)
 -> Protocol requirement (via Self.SIMD2Storage in SIMDScalarStub)
 -> Protocol requirement (via Self.Scalar in SIMDStorageStub)]
 [equivalence class τ_0_0[.SIMDScalarStub].SIMD2Storage[.SIMDStorageStub].Scalar]
 SIMD2Storage: SIMDStorageStub [τ_0_0: Explicit -> Protocol requirement (via Self in FPScalar) -> Protocol requirement (via Self.SIMD2Storage in SIMDScalarStub)]
 Scalar [represented by τ_0_0] [equivalence class τ_0_0]