[Bug c++/79035] New: Rejects valid code due to failure to use user const conversion template operator

[vz at quantitativesystems dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79035

            Bug ID: 79035
           Summary: Rejects valid code due to failure to use user const
                    conversion template operator
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vz at quantitativesystems dot com
  Target Milestone: ---

In the following example, the conversion int value2 = cdc fails, tested in many
versions of g++:

struct DualConverter {
    template<typename T> operator T &();
    template<typename T> operator const T &() const;
};

void instantiate(DualConverter &mdc, const DualConverter &cdc) {
    int &modif1 = mdc;
    const int &const1 = mdc;
    int value1 = mdc;
    const int &const2 = cdc;
    int value2 = cdc; // The compiler can't perform this conversion
}

g++ 6.3.0 gives this error:
src/f5.cpp: In function ‘void instantiate(DualConverter&, const
DualConverter&)’:
src/f5.cpp:11:18: error: cannot convert ‘const DualConverter’ to ‘int’ in
initialization
     int value2 = cdc;
                  ^~~

The older g++ 4.8.3 gives a perhaps more informative error:
src/f5.cpp: In function ‘void instantiate(DualConverter&, const
DualConverter&)’:
src/f5.cpp:11:18: error: invalid user-defined conversion from ‘const
DualConverter’ to ‘int’ [-fpermissive]
     int value2 = cdc;
                  ^
src/f5.cpp:2:26: note: candidate is: DualConverter::operator T&() [with T =
int] <near match>
     template<typename T> operator T &();
                          ^
src/f5.cpp:2:26: note:   no known conversion for implicit ‘this’ parameter from
‘const DualConverter*’ to ‘DualConverter*’
src/f5.cpp:11:18: error: passing ‘const DualConverter’ as ‘this’ argument of
‘DualConverter::operator T&() [with T = int]’ discards qualifiers
[-fpermissive]
     int value2 = cdc;


All versions of clang tested including 3.9 correctly compile this example.

I suspect this other bug is related:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

Because of the multiple conversions that work, it seems the compiler fails to
make the conversion of const references to value when given the option to
convert through the non-const conversion operator template.  I can not
reproduce the bug without the conversions being templates.

Template user conversion operators are unique in the language with respect to
their capabilities.

Thanks.