GSB: Replace RequirementSource::Derived with ::Layout

'Derived' was not a great name, since we already use the term
'derived requirement source' to mean something else.

A Derived source was only added in one place, when recording
a superclass constraint -- the idea is that this source
supercedes any explicit layout constraint, eg

    class SomeClass {}
    func foo<T>(_: T) where T : SomeClass, T : AnyObject {}

Here we have two sources for the 'T : AnyObject' layout constraint:

    Explicit: T
    Explicit: T -> Derived

Note that the 'Derived' requirement source does not store
a 'proof' -- we can't figure out _how_ we determined that
the explicit 'T : AnyObject' constraint is redundant here.

In the case where a superclass requirement makes a protocol
conformance redundant, we do have a 'proof', because the
'Superclass' requirement source stores a conformance:

    class SomeClass : SomeProto {}
    func foo<T>(_: T) where T : SomeClass, T : SomeProto {}

    Explicit: T
    Explicit: T -> Superclass: [SomeClass : P]

From looking at the second requirement source, we can
determine that the requirement was imposed by the explicit
constraint 'T : SomeClass'.

For the 'Layout' requirement source, there's not really a
"conformance", so we can just store the superclass type:

    Explicit: T
    Explicit: T -> Layout: SomeClass

Author: slavapestov

include/swift/AST/GenericSignatureBuilder.h

@@ -886,9 +886,12 @@ class GenericSignatureBuilder::RequirementSource final
     /// requirement.
     Concrete,
 
-    /// A requirement that was resolved based on structural derivation from
-    /// another requirement.
-    Derived,
+    /// A requirement that was resolved based on a layout requirement
+    /// imposed by a superclass constraint.
+    ///
+    /// This stores the \c LayoutConstraint used to resolve the
+    /// requirement.
+    Layout,
 
     /// A requirement that was provided for another type in the
     /// same equivalence class, but which we want to "re-root" on a new
@@ -944,7 +947,7 @@ class GenericSignatureBuilder::RequirementSource final
     case Superclass:
     case Parent:
     case Concrete:
-    case Derived:
+    case Layout:
     case EquivalentType:
       return 0;
     }
@@ -989,7 +992,7 @@ class GenericSignatureBuilder::RequirementSource final
     case Superclass:
     case Parent:
     case Concrete:
-    case Derived:
+    case Layout:
     case EquivalentType:
       return false;
     }
@@ -1141,17 +1144,18 @@ class GenericSignatureBuilder::RequirementSource final
                                      GenericSignatureBuilder &builder,
                                      ProtocolConformanceRef conformance) const;
 
+  /// A constraint source that describes a layout constraint that was implied
+  /// by a superclass requirement.
+  const RequirementSource *viaLayout(GenericSignatureBuilder &builder,
+                                     Type superclass) const;
+
   /// A constraint source that describes that a constraint that is resolved
   /// for a nested type via a constraint on its parent.
   ///
   /// \param assocType the associated type that
   const RequirementSource *viaParent(GenericSignatureBuilder &builder,
                                      AssociatedTypeDecl *assocType) const;
 
-  /// A constraint source that describes a constraint that is structurally
-  /// derived from another constraint but does not require further information.
-  const RequirementSource *viaDerived(GenericSignatureBuilder &builder) const;
-
   /// A constraint source that describes a constraint that is structurally
   /// derived from another constraint but does not require further information.
   const RequirementSource *viaEquivalentType(GenericSignatureBuilder &builder,

lib/AST/GenericSignatureBuilder.cpp

@@ -555,6 +555,7 @@ bool RequirementSource::isAcceptableStorageKind(Kind kind,
   case NestedTypeNameMatch:
   case ConcreteTypeBinding:
   case EquivalentType:
+  case Layout:
     switch (storageKind) {
     case StorageKind::StoredType:
       return true;
@@ -599,17 +600,6 @@ bool RequirementSource::isAcceptableStorageKind(Kind kind,
     case StorageKind::None:
       return false;
     }
-
-  case Derived:
-    switch (storageKind) {
-    case StorageKind::None:
-      return true;
-
-    case StorageKind::StoredType:
-    case StorageKind::ProtocolConformance:
-    case StorageKind::AssociatedTypeDecl:
-      return false;
-    }
   }
 
   llvm_unreachable("Unhandled RequirementSourceKind in switch.");
@@ -669,7 +659,7 @@ bool RequirementSource::isInferredRequirement() const {
     case ProtocolRequirement:
     case RequirementSignatureSelf:
     case Superclass:
-    case Derived:
+    case Layout:
       break;
     }
   }
@@ -695,7 +685,7 @@ bool RequirementSource::isDerivedRequirement() const {
   case Superclass:
   case Concrete:
   case RequirementSignatureSelf:
-  case Derived:
+  case Layout:
   case EquivalentType:
     return true;
 
@@ -879,7 +869,7 @@ const RequirementSource *RequirementSource::getMinimalConformanceSource(
 
     case Concrete:
     case Superclass:
-    case Derived:
+    case Layout:
     case EquivalentType:
       return false;
 
@@ -1077,11 +1067,13 @@ const RequirementSource *RequirementSource::viaParent(
                         0, WrittenRequirementLoc());
 }
 
-const RequirementSource *RequirementSource::viaDerived(
-                           GenericSignatureBuilder &builder) const {
+const RequirementSource *RequirementSource::viaLayout(
+                           GenericSignatureBuilder &builder,
+                           Type superclass) const {
   REQUIREMENT_SOURCE_FACTORY_BODY(
-                        (nodeID, Derived, this, nullptr, nullptr, nullptr),
-                        (Derived, this),
+                        (nodeID, Layout, this, superclass.getPointer(),
+                         nullptr, nullptr),
+                        (Layout, this, superclass),
                         0, WrittenRequirementLoc());
 }
 
@@ -1142,9 +1134,9 @@ const RequirementSource *RequirementSource::withoutRedundantSubpath(
     return parent->withoutRedundantSubpath(builder, start, end)
       ->viaConcrete(builder, getProtocolConformance());
 
-  case Derived:
+  case Layout:
     return parent->withoutRedundantSubpath(builder, start, end)
-      ->viaDerived(builder);
+      ->viaLayout(builder, getStoredType());
 
   case EquivalentType:
     return parent->withoutRedundantSubpath(builder, start, end)
@@ -1211,7 +1203,7 @@ RequirementSource::visitPotentialArchetypesAlongPath(
 
   case RequirementSource::Concrete:
   case RequirementSource::Superclass:
-  case RequirementSource::Derived:
+  case RequirementSource::Layout:
     return parent->visitPotentialArchetypesAlongPath(visitor);
 
   case RequirementSource::EquivalentType: {
@@ -1392,8 +1384,8 @@ void RequirementSource::print(llvm::raw_ostream &out,
     out << "Superclass";
     break;
 
-  case Derived:
-    out << "Derived";
+  case Layout:
+    out << "Layout";
     break;
 
   case EquivalentType:
@@ -1566,7 +1558,7 @@ bool FloatingRequirementSource::isExplicit() const {
     case RequirementSource::ProtocolRequirement:
     case RequirementSource::InferredProtocolRequirement:
     case RequirementSource::Superclass:
-    case RequirementSource::Derived:
+    case RequirementSource::Layout:
     case RequirementSource::EquivalentType:
       return false;
     }
@@ -1588,7 +1580,7 @@ bool FloatingRequirementSource::isExplicit() const {
     case RequirementSource::ConcreteTypeBinding:
     case RequirementSource::Parent:
     case RequirementSource::Superclass:
-    case RequirementSource::Derived:
+    case RequirementSource::Layout:
     case RequirementSource::EquivalentType:
       return false;
     }
@@ -4278,14 +4270,15 @@ bool GenericSignatureBuilder::updateSuperclass(
     // Presence of a superclass constraint implies a _Class layout
     // constraint.
     auto layoutReqSource =
-      source.getSource(*this, type)->viaDerived(*this);
-    addLayoutRequirementDirect(type,
-                         LayoutConstraint::getLayoutConstraint(
-                             superclass->getClassOrBoundGenericClass()->isObjC()
-                                 ? LayoutConstraintKind::Class
-                                 : LayoutConstraintKind::NativeClass,
-                             getASTContext()),
-                         layoutReqSource);
+      source.getSource(*this, type)->viaLayout(*this, superclass);
+
+    auto layout =
+      LayoutConstraint::getLayoutConstraint(
+        superclass->getClassOrBoundGenericClass()->isObjC()
+          ? LayoutConstraintKind::Class
+          : LayoutConstraintKind::NativeClass,
+        getASTContext());
+    addLayoutRequirementDirect(type, layout, layoutReqSource);
     return true;
   }