[Bug c++/106993] New: GCC accepts invalid program involving templated conversion function

[Bug c++/106993] New: GCC accepts invalid program involving templated conversion function

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106993

            Bug ID: 106993
           Summary: GCC accepts invalid program involving templated
                    conversion function
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following invalid(afaik) program is accepted by gcc and msvc. Demo link:
https://godbolt.org/z/E5crEqqfx

```
#include <string>
#include <string_view>

struct object{
  operator std::string(){return "";}
};

struct foo{
  foo operator[](std::string_view s){
    return foo{};
  }

  template <typename T>
  operator T(){
    return object{};
  }
};

int main(){
  foo f;
  std::string s = f["a"]; 
}
```

[Bug c++/106993] GCC accepts invalid program involving templated conversion function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106993

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

struct string_view { string_view(const char*) { } };

struct object{};

struct foo{
  foo operator[](string_view s){
    return foo{};
  }

  template <typename T>
  operator T(){
    return object{};
  }
};

int main(){
  foo f;
  f["a"]; 
}

Clang and EDG both say this is ambiguous, because f["a"] could be either:

   "a"[(ptrdiff_t) f]

or:

   f.operator[](string_view("a"));

GCC and MSVC select the second one.


If the conversion operator isn't a template, then all compilers agree it's
ambiguous:

struct string_view { string_view(const char*) { } };

struct foo{
  foo operator[](string_view s){
    return foo{};
  }

  operator long(){
    return 0;
  }
};

int main(){
  foo f;
  f["a"]; 
}