[AST] Add a method to get a call type from an ObjCMessageExpr

[george.karpenkov]

Summary

Due to references, expression type does not always correspond to an expected method return type (e.g. for a method returning int & the expression type of the call would still be int).
We have a helper method for getting the expected type on CallExpr, but not on ObjCMessageExpr

[rjmccall]

clang/include/clang/AST/ExprObjC.h

1185

Maybe something like:

Get the return type of the message being sent.  This is not always the type of the
message expression itself because of references.  It is also not always the declared
return type of the method because of `instancetype`.

[rjmccall]

Alright.

clang/include/clang/AST/ExprObjC.h

   /// generated by the implementation. If false, it was written explicitly
   /// in the source code.
   bool isImplicit() const { return IsImplicit; }
 
   /// Determine the kind of receiver that this message is being
   /// sent to.
   ReceiverKind getReceiverKind() const { return (ReceiverKind)Kind; }
 
+  /// getCallReturnType - Get the return type of the call expr. This is not
+  /// always the type of the expr itself, if the return type is a reference
+  /// type.
+  QualType getCallReturnType(ASTContext &Ctx) const;
+
   /// Source range of the receiver.
   SourceRange getReceiverRange() const;
 
   /// Determine whether this is an instance message to either a
   /// computed object or to super.
   bool isInstanceMessage() const {
     return getReceiverKind() == Instance || getReceiverKind() == SuperInstance;
   }

clang/lib/AST/ExprObjC.cpp

 }
 
 void ObjCMessageExpr::getSelectorLocs(
     SmallVectorImpl<SourceLocation> &SelLocs) const {
   for (unsigned i = 0, e = getNumSelectorLocs(); i != e; ++i)
     SelLocs.push_back(getSelectorLoc(i));
 }
 
+
+QualType ObjCMessageExpr::getCallReturnType(ASTContext &Ctx) const {
+  if (const ObjCMethodDecl *MD = getMethodDecl()) {
+    QualType QT = MD->getReturnType();
+    if (QT == Ctx.getObjCInstanceType()) {
+      // instancetype corresponds to expression types.
+      return getType();
+    }
+    return QT;
+  }
+
+  // Expression type might be different from an expected call return type,
+  // as expression type would never be a reference even if call returns a
+  // reference. Reconstruct the original expression type.
+  QualType QT = getType();
+  switch (getValueKind()) {
+  case VK_LValue:
+    return Ctx.getLValueReferenceType(QT);
+  case VK_XValue:
+    return Ctx.getRValueReferenceType(QT);
+  case VK_RValue:
+    return QT;
+  }
+}
+
 SourceRange ObjCMessageExpr::getReceiverRange() const {
   switch (getReceiverKind()) {
   case Instance:
     return getInstanceReceiver()->getSourceRange();
 
   case Class:
     return getClassReceiverTypeInfo()->getTypeLoc().getSourceRange();