AST: Add TypeBase::getRootGenericParam() utility method

Author: slavapestov

include/swift/AST/Types.h

@@ -516,6 +516,9 @@ class alignas(1 << TypeAlignInBits) TypeBase {
     return getRecursiveProperties().hasTypeParameter();
   }
 
+  /// Return the root generic parameter of this type parameter type.
+  GenericTypeParamType *getRootGenericParam();
+
   /// Determines whether this type is an lvalue. This includes both straight
   /// lvalue types as well as tuples or optionals of lvalues.
   bool isLValueType() {
@@ -4370,6 +4373,15 @@ inline bool TypeBase::isTypeParameter() {
   return false;
 }
 
+inline GenericTypeParamType *TypeBase::getRootGenericParam() {
+  Type t(this);
+
+  while (auto *memberTy = t->getAs<DependentMemberType>())
+    t = memberTy->getBase();
+
+  return t->castTo<GenericTypeParamType>();
+}
+
 inline bool TypeBase::isExistentialType() {
   return getCanonicalType().isExistentialType();
 }

lib/AST/ASTMangler.cpp

@@ -1468,12 +1468,10 @@ static bool isMethodDecl(const Decl *decl) {
 }
 
 static bool genericParamIsBelowDepth(Type type, unsigned methodDepth) {
-  if (auto gp = type->getAs<GenericTypeParamType>()) {
+  if (type->isTypeParameter()) {
+    auto gp = type->getRootGenericParam();
     return gp->getDepth() >= methodDepth;
   }
-  if (auto dm = type->getAs<DependentMemberType>()) {
-    return genericParamIsBelowDepth(dm->getBase(), methodDepth);
-  }
   // Non-dependent types in a same-type requirement don't affect whether we
   // mangle the requirement.
   return false;

lib/AST/Decl.cpp

@@ -2914,10 +2914,7 @@ findProtocolSelfReferences(const ProtocolDecl *proto, Type type,
 
   // Special handling for associated types.
   if (!skipAssocTypes && type->is<DependentMemberType>()) {
-    while (auto depMemTy = type->getAs<DependentMemberType>()) {
-      type = depMemTy->getBase();
-    }
-
+    type = type->getRootGenericParam();
     if (proto->getProtocolSelfType()->isEqual(type))
       return SelfReferenceKind::Other();
   }

lib/AST/Mangle.cpp

@@ -470,12 +470,10 @@ static bool isMethodDecl(const Decl *decl) {
 }
 
 static bool genericParamIsBelowDepth(Type type, unsigned methodDepth) {
-  if (auto gp = type->getAs<GenericTypeParamType>()) {
+  if (type->isTypeParameter()) {
+    auto gp = type->getRootGenericParam();
     return gp->getDepth() >= methodDepth;
   }
-  if (auto dm = type->getAs<DependentMemberType>()) {
-    return genericParamIsBelowDepth(dm->getBase(), methodDepth);
-  }
   // Non-dependent types in a same-type requirement don't affect whether we
   // mangle the requirement.
   return false;

lib/Sema/TypeCheckGeneric.cpp

@@ -558,13 +558,9 @@ static bool isSelfDerivedOrConcrete(Type protoSelf, Type type) {
   if (!type->hasTypeParameter())
     return true;
 
-  // Unwrap dependent member types.
-  while (auto depMem = type->getAs<DependentMemberType>()) {
-    type = depMem->getBase();
-  }
-
-  if (type->is<GenericTypeParamType>())
-    return type->isEqual(protoSelf);
+  if (type->isTypeParameter() &&
+      type->getRootGenericParam()->isEqual(protoSelf))
+    return true;
 
   return false;
 }