[clang] ASTContext: make DecltypeType creation more well-behaved

This makes sure a canonical DecltypeType round-trips back to a
canonical type when rebuilt from its components.

Author: mizvekov

clang/include/clang/AST/Type.h

@@ -5918,7 +5918,7 @@ class DecltypeType : public Type {
 /// of this class via DecltypeType nodes.
 class DependentDecltypeType : public DecltypeType, public llvm::FoldingSetNode {
 public:
-  DependentDecltypeType(Expr *E, QualType UnderlyingTpe);
+  DependentDecltypeType(Expr *E);
 
   void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context) {
     Profile(ID, Context, getUnderlyingExpr());

clang/lib/AST/ASTContext.cpp

@@ -6355,34 +6355,36 @@ QualType ASTContext::getReferenceQualifiedType(const Expr *E) const {
 /// nodes. This would never be helpful, since each such type has its own
 /// expression, and would not give a significant memory saving, since there
 /// is an Expr tree under each such type.
-QualType ASTContext::getDecltypeType(Expr *e, QualType UnderlyingType) const {
-  DecltypeType *dt;
-
+QualType ASTContext::getDecltypeType(Expr *E, QualType UnderlyingType) const {
   // C++11 [temp.type]p2:
   //   If an expression e involves a template parameter, decltype(e) denotes a
   //   unique dependent type. Two such decltype-specifiers refer to the same
   //   type only if their expressions are equivalent (14.5.6.1).
-  if (e->isInstantiationDependent()) {
+  QualType CanonType;
+  if (!E->isInstantiationDependent()) {
+    CanonType = getCanonicalType(UnderlyingType);
+  } else if (!UnderlyingType.isNull()) {
+    CanonType = getDecltypeType(E, QualType());
+  } else {
     llvm::FoldingSetNodeID ID;
-    DependentDecltypeType::Profile(ID, *this, e);
+    DependentDecltypeType::Profile(ID, *this, E);
 
     void *InsertPos = nullptr;
-    DependentDecltypeType *Canon
-      = DependentDecltypeTypes.FindNodeOrInsertPos(ID, InsertPos);
-    if (!Canon) {
-      // Build a new, canonical decltype(expr) type.
-      Canon = new (*this, alignof(DependentDecltypeType))
-          DependentDecltypeType(e, DependentTy);
-      DependentDecltypeTypes.InsertNode(Canon, InsertPos);
-    }
-    dt = new (*this, alignof(DecltypeType))
-        DecltypeType(e, UnderlyingType, QualType((DecltypeType *)Canon, 0));
-  } else {
-    dt = new (*this, alignof(DecltypeType))
-        DecltypeType(e, UnderlyingType, getCanonicalType(UnderlyingType));
-  }
-  Types.push_back(dt);
-  return QualType(dt, 0);
+    if (DependentDecltypeType *Canon =
+            DependentDecltypeTypes.FindNodeOrInsertPos(ID, InsertPos))
+      return QualType(Canon, 0);
+
+    // Build a new, canonical decltype(expr) type.
+    auto *DT =
+        new (*this, alignof(DependentDecltypeType)) DependentDecltypeType(E);
+    DependentDecltypeTypes.InsertNode(DT, InsertPos);
+    Types.push_back(DT);
+    return QualType(DT, 0);
+  }
+  auto *DT = new (*this, alignof(DecltypeType))
+      DecltypeType(E, UnderlyingType, CanonType);
+  Types.push_back(DT);
+  return QualType(DT, 0);
 }
 
 QualType ASTContext::getPackIndexingType(QualType Pattern, Expr *IndexExpr,

clang/lib/AST/Type.cpp

@@ -4055,8 +4055,8 @@ QualType DecltypeType::desugar() const {
   return QualType(this, 0);
 }
 
-DependentDecltypeType::DependentDecltypeType(Expr *E, QualType UnderlyingType)
-    : DecltypeType(E, UnderlyingType) {}
+DependentDecltypeType::DependentDecltypeType(Expr *E)
+    : DecltypeType(E, QualType()) {}
 
 void DependentDecltypeType::Profile(llvm::FoldingSetNodeID &ID,
                                     const ASTContext &Context, Expr *E) {