[cfe-dev] custom attribute crashed due to "Arg access out of range"

[Yafei Liu via cfe-dev]

I'm adding a custom attribute followed by this
http://clang.llvm.org/docs/InternalsManual.html#how-to-add-an-attribute,
here's my def:

def MEItem : InheritableParamAttr {
  let Spellings = [CXX11<"me", "type">, C2x<"me", "type">];
  let Args = [StringArgument<"Type", 1>, StringArgument<"Getter", 1>,
StringArgument<"Setter", 1>];
  let Subjects = SubjectList<[Field]>;
  let Documentation = [Undocumented];
}

I just copied what ExternalSourceSymbol does, so the handling code is like
this:

static void handleMEItemAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
  if (!checkAttributeAtLeastNumArgs(S, AL, 1))
    return;
  assert(checkAttributeAtMostNumArgs(S, AL, 3) &&
         "Invalid number of arguments in an item attribute");

  StringRef Type;
  if (const auto *SE = dyn_cast_or_null<StringLiteral>(AL.getArgAsExpr(0)))
    Type = SE->getString();

  StringRef Getter;
  if (const auto *SE = dyn_cast_or_null<StringLiteral>(AL.getArgAsExpr(1)))
    Getter = SE->getString();

  StringRef Setter;
  if (const auto *SE = dyn_cast_or_null<StringLiteral>(AL.getArgAsExpr(2)))
    Setter = SE->getString();

  D->addAttr(::new (S.Context) MEItemAttr(S.Context, AL, Type, Getter,
Setter));
}

but if I use this attribute in my code, for example [[me::type("int")]],
the code crashed at AL.getArgAsExpr(1):

clang::ArgsUnion clang::ParsedAttr::getArg(unsigned int) const: Assertion
`Arg < NumArgs && "Arg access out of range!"' failed.

then I tried this: [[me::type(Type="int", Getter="xxx", Setter="xxx")]]
error: use of undeclared identifier 'Type' [[me::type(Type="int",
Getter="xxx", Setter="xxx")]]

So what did I miss?
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