[Bug c++/61556] New: [c++11][regression] â*(const ValueType*)thisâ is not a constant expression with valid code

["l.jirkovsky at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61556

            Bug ID: 61556
           Summary: [c++11][regression] â*(const ValueType*)thisâ is not a
                    constant expression with valid code
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: l.jirkovsky at gmail dot com

When used in a certain way, a template class cannot be instantiated in another
template class using a static constexpr. I tracked the regression down to the
following commit:

commit 18619da58c77461642c36cee9f0463c968877f75
Author: jason <jason@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Mon Apr 1 19:05:12 2013 +0000

        * call.c (add_function_candidate): Take the address of 'this' here.
        (build_over_call): And here.
        (build_new_method_call_1, build_op_call_1): Not here.
        (build_user_type_conversion_1): Or here.
        (add_candidates): Adjust.

Reverting this commit fixes the problem on the current gcc.

The problematic code that should be compilable is attached after the "CODE"
section.

When compiled with gcc 4.9.0, the
ValueTypeInfo<ValueTypeEnum::doubleval> v; in the FillFunctor
constructor gives an error:

    bug2.cpp: In constructor âFillFunctor<Format>::FillFunctor()â:
    bug2.cpp:24:41:   in constexpr expansion of âconst
ValueType{1}->ValueType::operator int()â
    bug2.cpp:24:41: error: â*(const ValueType*)thisâ is not a constant
expression
       ValueTypeInfo<ValueTypeEnum::doubleval> v;
            ^
    bug2.cpp:24:41: note: in template argument for type âintâ
    bug2.cpp:24:44: error: invalid type in declaration before â;â token
       ValueTypeInfo<ValueTypeEnum::doubleval> v;


However, the same expression compiles just fine if its used in a
function, such as main(). Removing m_ID makes the code compile, too.
Also, the compilation works after replacing:

  ValueTypeInfo<ValueTypeEnum::doubleval> v;

with a:

  constexpr ValueType doubleval = ValueType(1);
  ValueTypeInfo<doubleval> v;

CODE
#####

class ValueType {
public:
    constexpr operator int() const {return m_ID;};
    constexpr ValueType(const int v)
        : m_ID(v) {}
private:
    int m_ID;
};

class ValueTypeEnum {
public:
    static constexpr ValueType doubleval = ValueType(1);
};

template <int format>
class ValueTypeInfo {
};

template <typename Format>
class FillFunctor {
public:
    FillFunctor() {
        ValueTypeInfo<ValueTypeEnum::doubleval> v;
    }
};

int main() {
    ValueTypeInfo<ValueTypeEnum::doubleval> v;
}