[Bug c++/68049] New: template instantiation involving may_alias defines symbol twice

["kretz at kde dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68049

            Bug ID: 68049
           Summary: template instantiation involving may_alias defines
                    symbol twice
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kretz at kde dot org
  Target Milestone: ---

The following testcase fails to compile at -O0, but works at -O1 and higher
(-std=c++11 is the only required compiler flag):

template <typename T> struct Bar
{
    using type = T;
};
template <typename T> struct Foo
{
    typedef typename Bar<T>::type alias_type [[gnu::may_alias]];

    alias_type operator()() { return {}; }
};

template <typename T> void print(T) {}

int main()
{
    print(Foo<int>()());
    print(0);
    return 0;
}

The template instantiation of print(T) is done twice:
/tmp/ccDEkoML.s: Assembler messages:
/tmp/ccDEkoML.s:69: Error: symbol `_Z5printIiEvT_' is already defined

With a recent GCC 6 snapshot I get:
failure.cpp:19:1: error: Two symbols with same comdat_group are not linked by
the same_comdat_group list.
 }
 ^
_Z5printIiEvT_/3 (void print(T) [with T = int]) @0x7faa000d75c0
  Type: function definition analyzed
  Visibility: public weak comdat comdat_group:_Z5printIiEvT_ one_only
  previous sharing asm name: 2
  References: 
  Referring: 
  First run: 0
  Function flags: body
  Called by: main/0 (1.00 per call) 
  Calls: 
_Z5printIiEvT_/2 (void print(T) [with T = Bar<int>::type]) @0x7faa000d7450
  Type: function definition analyzed
  Visibility: public weak comdat comdat_group:_Z5printIiEvT_ one_only
  next sharing asm name: 3
  References: 
  Referring: 
  First run: 0
  Function flags: body
  Called by: main/0 (1.00 per call) 
  Calls: 
failure.cpp:19:1: internal compiler error: symtab_node::verify failed
0x90a6e9 symtab_node::verify_symtab_nodes()
        ../.././gcc/symtab.c:1126
0x91d3d5 symbol_table::compile()
        ../.././gcc/cgraphunit.c:2377
0x920162 symbol_table::compile()
        ../.././gcc/cgraphunit.c:2536
0x920162 symbol_table::finalize_compilation_unit()
        ../.././gcc/cgraphunit.c:2562