Mangling ambiguity with argument packs in template argument lists

[Mike Herrick]

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.

Thanks,

Mike.

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