Create an inherited conformance in SubstitutionMap::lookupConformance when needed.

If the substituted type for a conformance found via a superclass constraint is a subclass of that
superclass, then we should represent that with an InheritedProtocolConformance rather than with
the original root conformance that applies to the superclass. If we don't do this, then we end up
with spurious inequalities in generic signatures that ought to be equivalent, because some
paths use the inherited conformance and some don't, as in SR-12330 | rdar://problem/60174186.

Author: jckarter

lib/AST/SubstitutionMap.cpp

@@ -355,8 +355,10 @@ SubstitutionMap::lookupConformance(CanType type, ProtocolDecl *proto) const {
   // Check whether the superclass conforms.
   if (auto superclass = genericSig->getSuperclassBound(type)) {
     LookUpConformanceInSignature lookup(getGenericSignature().getPointer());
-    if (auto conformance = lookup(type->getCanonicalType(), superclass, proto))
+    auto substType = type.subst(*this);
+    if (auto conformance = lookup(type->getCanonicalType(), substType, proto)){
       return conformance;
+    }
   }
 
   // If the type doesn't conform to this protocol, the result isn't formed

lib/AST/Type.cpp

@@ -2400,7 +2400,7 @@ getForeignRepresentable(Type type, ForeignLanguage language,
 
   if (wasOptional && !result.isRepresentableAsOptional())
     return failure();
-
+  
   // If our nominal type has type arguments, make sure they are
   // representable as well. Because type arguments are not actually
   // translated separately, whether they are trivially representable
@@ -2447,6 +2447,16 @@ getForeignRepresentable(Type type, ForeignLanguage language,
         break;
       }
     }
+    
+    // Specialize the conformance we were given for the type we're testing.
+    if (result.getKind() == ForeignRepresentableKind::Bridged
+        && !result.getConformance()->getType()->isEqual(type)) {
+      auto specialized = type->getASTContext()
+        .getSpecializedConformance(type, result.getConformance(),
+             boundGenericType->getContextSubstitutionMap(dc->getParentModule(),
+                                                 boundGenericType->getDecl()));
+      result = ForeignRepresentationInfo::forBridged(specialized);
+    }
   }
 
   return { result.getKind(), result.getConformance() };

test/SILGen/function_type_lowering_inherited_conformance.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+protocol P {}
+
+class C: P {}
+class D: C {}
+
+struct Butt<T: P> {}
+
+func foo<T: P>(_: (Butt<T>) -> ()) {}
+
+// CHECK-LABEL: sil{{.*}}3bar
+func bar(_ f: (Butt<D>) -> ()) {
+  // CHECK: convert_function {{.*}} $@noescape @callee_guaranteed (Butt<D>) -> () to $@noescape @callee_guaranteed @substituted <τ_0_0 where τ_0_0 : P> (Butt<τ_0_0>) -> () for <D>
+  foo(f)
+}