[cxx-abi-dev] Mangling ambiguity with argument packs in template argument lists

[Doug Gregor]

On Mon, Dec 20, 2010 at 9:41 AM, Mike Herrick <mjh at edg.com> wrote:
> We ran into a mangling ambiguity with argument packs in template argument lists (where both use the letter "I"):
>
>  <template-args> ::= I <template-arg>+ E
>  <template-arg> ::= <type>                         # type or template
>                 ::= X <expression> E               # expression
>                 ::= <expr-primary>                 # simple expressions
>                 ::= I <template-arg>* E            # argument pack
>                 ::= sp <expression>                # pack expansion (C++0x)
>
> Here's an example where two different source constructs generate the same mangled name (_Z1fI1BI1AIiEEEDTcmfp_fp_ET_):
>
> $ cat foo.cpp
>  template <class T> auto f(T p1) -> decltype(p1, p1);
>  template <class T> struct A {};
> #if FOO
>  template <class T> struct B {};
>  B<A<int>> b;
> #else
>  template <template <class TT> class T, class ... U> struct B {};
>  B<A,int> b;
> #endif
>  int main() {
>    f(b);
>  }
> $ g++451 -DFOO -std=c++0x foo.cpp
> /tmp/ccOqFYBI.o(.text+0xc): In function `main':
> : undefined reference to `decltype ({parm#1},{parm#1}) f<B<A<int> > >(B<A<int> >)'
> collect2: ld returned 1 exit status
> $ g++451 -UFOO -std=c++0x foo.cpp
> /tmp/cc448n5U.o(.text+0xc): In function `main':
> : undefined reference to `decltype ({parm#1},{parm#1}) f<B<A<int> > >(B<A<int> >)'
> collect2: ld returned 1 exit status
> $
>
> When the demangler gets to the "I" after "1A", it's ambiguous whether the "I" represents an argument pack or a template argument (as noted by g++ 4.5.1 issuing the same demangled name for both cases).
>
> To address this, we propose changing the mangling for argument packs to use "J" rather than "I":
>
>  <template-arg> ::= J <template-arg>* E              # argument pack
>
> This change is not backward compatible.
>
> I'm attaching a patch that includes this change with the others that I've proposed.

These changes seem perfectly reasonable to me.

  - Doug