Support capturing the implicit `this` argument to C++ class member functions

[dberris]

Summary

Before this change, we couldn't capture the this pointer that's
implicitly the first argument of class member functions. There are some
interesting things we can do with capturing even just this single
argument for zero-argument member functions.

[dberris]

Fail... it's been a while since I've done this. Will try again.

lib/Sema/SemaDeclAttr.cpp

D->addAttr(::new (S.Context)
                                     Attr.getLayoutCompatible(),
                                     Attr.getMustBeNull(),
                                     Attr.getAttributeSpellingListIndex()));
 }
 
 static void handleXRayLogArgsAttr(Sema &S, Decl *D,
                                   const AttributeList &Attr) {
   uint64_t ArgCount;
+
+  // Treat member functions especially, since we do want to support saving the
+  // implicit this argument for member functions.
+  if (isInstanceMethod(D)) {
+    auto *IdxExpr = Attr.getArgAsExpr(0);
+    llvm::APSInt IdxInt;
+    if (!IdxExpr->isIntegerConstantExpr(IdxInt, S.Context)) {
+      S.Diag(getAttrLoc(Attr), diag::err_attribute_argument_n_type)
+          << getAttrName(Attr) << 1u << AANT_ArgumentIntegerConstant
+          << IdxExpr->getSourceRange();
+      return;
+    }
+    ArgCount = IdxInt.getLimitedValue();
+    bool HP = hasFunctionProto(D);
+    unsigned NumParams = HP ? getFunctionOrMethodNumParams(D) + 1 : 1;
+    if (ArgCount > NumParams) {
+      S.Diag(getAttrLoc(Attr), diag::err_attribute_argument_out_of_bounds)
+          << getAttrName(Attr) << 1 << IdxExpr->getSourceRange();
+      return;
+    }
+    D->addAttr(::new (S.Context)
+                   XRayLogArgsAttr(Attr.getRange(), S.Context, ArgCount,
+                                   Attr.getAttributeSpellingListIndex()));
+    return;
+  }
+
   if (!checkFunctionOrMethodParameterIndex(S, D, Attr, 1, Attr.getArgAsExpr(0),
                                            ArgCount))
     return;
 
   // ArgCount isn't a parameter index [0;n), it's a count [1;n] - hence + 1.
   D->addAttr(::new (S.Context)
                  XRayLogArgsAttr(Attr.getRange(), S.Context, ++ArgCount,
                                  Attr.getAttributeSpellingListIndex()));
 }
 
 //===----------------------------------------------------------------------===//
 // Checker-specific attribute handlers.
 //===----------------------------------------------------------------------===//
 
 static bool isValidSubjectOfNSReturnsRetainedAttribute(QualType type) {
   return type->isDependentType() ||

test/Sema/xray-log-args-class.cpp

+// RUN: %clang_cc1 %s -verify -fsyntax-only -std=c++11 -x c++
+
+class Class {
+  [[clang::xray_always_instrument, clang::xray_log_args(1)]] void Method();
+  [[clang::xray_log_args(-1)]] void Invalid(); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}
+  [[clang::xray_log_args("invalid")]] void InvalidStringArg(); // expected-error {{'xray_log_args'}}
+};