[SIL] The Self type of a protocol can conform to multiple protocols.

Through a same-type constraint on an associated type, the Self type of
a protocol can conform to multiple protocols that are not related by
direct inheritance. There were two places that incorrectly assumed
that this didn't happen:

1) The SIL verifier checked that the archetype for Self conformed to
only a single protocol. This only tripped up +Asserts builds, and had
no effect on code generation. Change it to ensure that the archetype
for Self conforms to the expected protocol.
2) SILFunctionType's getDefaultWitnessMethodProtocol() asserted that
the Self type of a protocol only conformed to a single protocol, and
then returned the first protocol in the list. This could end up
returning the wrong protocol, in turn producing an incorrect
substitution list in IRGen. Change it to return the protocol from the
constraint in the generic signature.

Fixes SR-9848 / rdar://problem/47767506.

Author: DougGregor

lib/SIL/SILFunctionType.cpp

@@ -107,9 +107,14 @@ SILFunctionType::getDefaultWitnessMethodProtocol() const {
     auto superclass = GenericSig->getSuperclassBound(paramTy);
     if (superclass)
       return nullptr;
-    auto protos = GenericSig->getConformsTo(paramTy);
-    assert(protos.size() == 1);
-    return protos[0];
+
+    assert(!GenericSig->getRequirements().empty());
+    assert(GenericSig->getRequirements().front().getKind() ==
+             RequirementKind::Conformance);
+    assert(GenericSig->getRequirements().front().getFirstType()
+             ->isEqual(paramTy));
+    return GenericSig->getRequirements().front().getSecondType()
+             ->castTo<ProtocolType>()->getDecl();
   }
 
   return nullptr;

lib/SIL/SILVerifier.cpp

@@ -2542,10 +2542,9 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
             selfRequirement->getKind() == RequirementKind::Conformance,
             "first non-same-typerequirement should be conformance requirement");
     auto conformsTo = genericSig->getConformsTo(selfGenericParam);
-    require(conformsTo.size() == 1,
-            "requirement Self parameter must conform to exactly one protocol");
-    require(conformsTo[0] == protocol,
-            "requirement Self parameter should be constrained by protocol");
+    require(std::find(conformsTo.begin(), conformsTo.end(), protocol)
+              != conformsTo.end(),
+            "requirement Self parameter must conform to called protocol");
 
     auto lookupType = AMI->getLookupType();
     if (getOpenedArchetypeOf(lookupType)) {

test/IRGen/witness_method_default.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend %s -emit-ir | %FileCheck --check-prefix=CHECK %s -DINT=i%target-ptrsize
+
+public protocol DummyProtocol { }
+
+public protocol SIMDStorageStub {
+ associatedtype Scalar : DummyProtocol
+}
+
+public protocol SIMDScalarStub {
+ associatedtype SIMD2Storage : SIMDStorageStub
+   where SIMD2Storage.Scalar == Self
+ 
+ func abs() -> Self
+}
+
+// CHECK: define swiftcc void @"$s22witness_method_default7callAbs1sxx_tAA14SIMDScalarStubRzlF
+public func callAbs<T: SIMDScalarStub>(s: T) -> T {
+  // CHECK: [[ABS_PTR:%[0-9]+]] = getelementptr inbounds i8*, i8** %T.SIMDScalarStub, i32 3
+  // CHECK-NEXT: [[ABS_VALUE:%[0-9]+]] = load i8*, i8** [[ABS_PTR]]
+  // CHECK-NEXT: [[ABS:%[0-9]+]] = bitcast i8* [[ABS_VALUE]]
+  // CHECK-NEXT: call swiftcc void [[ABS]]
+ return s.abs()
+}