[NFC] AST: Define SubstitutionMap::getInnermostReplacementTypes

include/swift/AST/SubstitutionMap.h

@@ -150,6 +150,10 @@ public:
   /// generic parameters.
   ArrayRef<Type> getReplacementTypes() const;
 
+  /// Retrieve the array of replacement types for the innermost generic
+  /// parameters.
+  ArrayRef<Type> getInnermostReplacementTypes() const;
+
   /// Query whether any replacement types in the map contain archetypes.
   bool hasArchetypes() const;
 

include/swift/AST/Types.h

@@ -1806,7 +1806,7 @@ public:
   /// Get the direct generic arguments, which correspond to the generic
   /// arguments that are directly applied to the typealias declaration
   /// this type references.
-  SmallVector<Type, 2> getDirectGenericArgs() const;
+  ArrayRef<Type> getDirectGenericArgs() const;
 
   // Support for FoldingSet.
   void Profile(llvm::FoldingSetNodeID &id) const;

lib/AST/SubstitutionMap.cpp

@@ -97,6 +97,13 @@ ArrayRef<Type> SubstitutionMap::getReplacementTypes() const {
   return getReplacementTypesBuffer();
 }
 
+ArrayRef<Type> SubstitutionMap::getInnermostReplacementTypes() const {
+  if (empty()) return { };
+
+  return getReplacementTypes().take_back(
+      getGenericSignature()->getInnermostGenericParams().size());
+}
+
 GenericSignature SubstitutionMap::getGenericSignature() const {
   return storage ? storage->getGenericSignature() : nullptr;
 }

lib/AST/Type.cpp

@@ -1385,26 +1385,12 @@ Type SugarType::getSinglyDesugaredTypeSlow() {
   return UnderlyingType;
 }
 
-SmallVector<Type, 2> TypeAliasType::getDirectGenericArgs() const {
-  SmallVector<Type, 2> result;
+ArrayRef<Type> TypeAliasType::getDirectGenericArgs() const {
+  if (!typealias->isGeneric()) return { };
 
-  // If the typealias is not generic, there are no generic arguments
-  if (!typealias->isGeneric()) return result;
-
-  // If the substitution map is empty, bail out.
-  auto subMap = getSubstitutionMap();
-  if (subMap.empty()) return result;
-
-  // Retrieve the substitutions for the generic parameters (only).
-  auto genericSig = subMap.getGenericSignature();
-  unsigned numAllGenericParams = genericSig->getGenericParams().size();
-  unsigned numMyGenericParams = typealias->getGenericParams()->size();
-  result.reserve(numMyGenericParams);
-  unsigned startIndex = numAllGenericParams - numMyGenericParams;
-  for (auto gp : genericSig->getGenericParams().slice(startIndex)) {
-    result.push_back(Type(gp).subst(subMap));
-  }
-  return result;
+  // Otherwise, the innermost replacement types are the direct
+  // generic arguments.
+  return getSubstitutionMap().getInnermostReplacementTypes();
 }
 
 unsigned GenericTypeParamType::getDepth() const {