GSB: PotentialArchetypes now store their canonical type

Instead of recomputing it on every call to getDependentType(),
we can just store it in there instead of the GenericParamKey
or AssociatedTypeDecl.

We still need to 're-sugar' user-visible dependent types to
get the right generic parameter name; a new getSugaredDependentType()
utility method is called in the right places for that.

Author: slavapestov

include/swift/AST/GenericSignatureBuilder.h

@@ -1477,31 +1477,17 @@ struct GenericSignatureBuilder::Constraint {
 };
 
 class GenericSignatureBuilder::PotentialArchetype {
-  /// The parent of this potential archetype (for a nested type) or the
-  /// ASTContext in which the potential archetype resides.
-  llvm::PointerUnion<PotentialArchetype*, ASTContext*> parentOrContext;
-
-  /// The identifier describing this particular archetype.
-  ///
-  /// \c parentOrBuilder determines whether we have a nested type vs. a root.
-  union PAIdentifier {
-    /// The associated type for a resolved nested type.
-    AssociatedTypeDecl *assocType;
-
-    /// The generic parameter key for a root.
-    GenericParamKey genericParam;
-
-    PAIdentifier(AssociatedTypeDecl *assocType) : assocType(assocType) {}
-
-    PAIdentifier(GenericParamKey genericParam) : genericParam(genericParam) { }
-  } identifier;
+  /// The parent of this potential archetype, for a nested type.
+  PotentialArchetype* parent;
 
   /// The representative of the equivalence class of potential archetypes
   /// to which this potential archetype belongs, or (for the representative)
   /// the equivalence class itself.
   mutable llvm::PointerUnion<PotentialArchetype *, EquivalenceClass *>
     representativeOrEquivClass;
 
+  mutable CanType depType;
+
   /// A stored nested type.
   struct StoredNestedType {
     /// The potential archetypes describing this nested type, all of which
@@ -1539,7 +1525,7 @@ class GenericSignatureBuilder::PotentialArchetype {
   PotentialArchetype(PotentialArchetype *parent, AssociatedTypeDecl *assocType);
 
   /// Construct a new potential archetype for a generic parameter.
-  PotentialArchetype(ASTContext &ctx, GenericParamKey genericParam);
+  explicit PotentialArchetype(GenericTypeParamType *genericParam);
 
 public:
   /// Retrieve the representative for this archetype, performing
@@ -1558,42 +1544,17 @@ class GenericSignatureBuilder::PotentialArchetype {
   /// Retrieve the parent of this potential archetype, which will be non-null
   /// when this potential archetype is an associated type.
   PotentialArchetype *getParent() const { 
-    return parentOrContext.dyn_cast<PotentialArchetype *>();
+    return parent;
   }
 
   /// Retrieve the type declaration to which this nested type was resolved.
   AssociatedTypeDecl *getResolvedType() const {
-    assert(getParent() && "Not an associated type");
-    return identifier.assocType;
+    return cast<DependentMemberType>(depType)->getAssocType();
   }
 
   /// Determine whether this is a generic parameter.
   bool isGenericParam() const {
-    return parentOrContext.is<ASTContext *>();
-  }
-
-  /// Retrieve the generic parameter key for a potential archetype that
-  /// represents this potential archetype.
-  ///
-  /// \pre \c isGenericParam()
-  GenericParamKey getGenericParamKey() const {
-    assert(isGenericParam() && "Not a generic parameter");
-    return identifier.genericParam;
-  }
-
-  /// Retrieve the generic parameter key for the generic parameter at the
-  /// root of this potential archetype.
-  GenericParamKey getRootGenericParamKey() const {
-    if (auto parent = getParent())
-      return parent->getRootGenericParamKey();
-
-    return getGenericParamKey();
-  }
-
-  /// Retrieve the name of a nested potential archetype.
-  Identifier getNestedName() const {
-    assert(getParent() && "Not a nested type");
-    return identifier.assocType->getName();
+    return parent == nullptr;
   }
 
   /// Retrieve the set of nested types.
@@ -1643,12 +1604,18 @@ class GenericSignatureBuilder::PotentialArchetype {
                                  AssociatedTypeDecl *assocType,
                                  ArchetypeResolutionKind kind);
 
+  /// Retrieve the dependent type that describes this potential
+  /// archetype.
+  CanType getDependentType() const {
+    return depType;
+  }
+
   /// Retrieve the dependent type that describes this potential
   /// archetype.
   ///
   /// \param genericParams The set of generic parameters to use in the resulting
   /// dependent type.
-  Type getDependentType(TypeArrayView<GenericTypeParamType> genericParams)const;
+  Type getDependentType(TypeArrayView<GenericTypeParamType> genericParams) const;
 
   /// True if the potential archetype has been bound by a concrete type
   /// constraint.
@@ -1659,9 +1626,6 @@ class GenericSignatureBuilder::PotentialArchetype {
     return false;
   }
 
-  /// Retrieve the AST context in which this potential archetype resides.
-  ASTContext &getASTContext() const;
-
   SWIFT_DEBUG_DUMP;
 
   void dump(llvm::raw_ostream &Out, SourceManager *SrcMgr,