SILGen: Strip off DynamicSelfType when tranforming values

We lower away a top-level DynamicSelfType, so let's just strip them off
before trying the other conversions. For now this is NFC, but we can end
up here after the next patch.

Even then we shouldn't end up here except when emitting protocol witnesses.
The relevant case is when a base class method (return @dynamic_self Base)
is used by a derived class to witness a requirement on a protocol
(which returns @dynamic_self Derived since its a derived class conformance).

In this case, we know both values have the same type, but the lowered
types are $Base and $Derived, respectively, so we must emit an unchecked
downcast.

Author: slavapestov

lib/SILGen/SILGenPoly.cpp

@@ -466,6 +466,15 @@ ManagedValue Transform::transform(ManagedValue v,
 
   // Subtype conversions:
 
+  // A base class method returning Self can be used in place of a derived
+  // class method returning Self.
+  if (auto outputSelfType = dyn_cast<DynamicSelfType>(outputSubstType)) {
+    if (auto inputSelfType = dyn_cast<DynamicSelfType>(inputSubstType)) {
+      inputSubstType = inputSelfType.getSelfType();
+      outputSubstType = outputSelfType.getSelfType();
+    }
+  }
+
   //  - upcasts for classes
   if (outputSubstType->getClassOrBoundGenericClass() &&
       inputSubstType->getClassOrBoundGenericClass()) {
@@ -482,7 +491,9 @@ ManagedValue Transform::transform(ManagedValue v,
       // Upcast to a superclass.
       return SGF.B.createUpcast(Loc, v, loweredResultTy);
     } else {
-      // Unchecked-downcast to a covariant return type.
+      // FIXME: Should only happen with the DynamicSelfType case above,
+      // except that convenience inits return the static self and not
+      // DynamicSelfType.
       assert(inputSubstType->isExactSuperclassOf(outputSubstType)
              && "should be inheritance relationship between input and output");
       return SGF.B.createUncheckedRefCast(Loc, v, loweredResultTy);