What does the standard say about this pointer-to-member-function type template parameter? Is my code wrong, or is MSVS 16.6 buggy?

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Here's some code that works in GCC, Clang and MSVS (at least, those versions currently available on Compiler Explorer):

template <typename T, auto T::* MemberPtr>
struct Foo
{
    Foo(const T& e) : _e(e) {}

    void operator()() const
    {
        (_e.*MemberPtr)();
    }

private:
    const T& _e;
};

struct Bar
{
    void baz() const {}

    auto bind()
    {
        using BindingType = Foo<Bar, &Bar::baz>;
        return BindingType(*this);
    }
};

int main()
{
    Bar i;
    i.bind();
}

Starting from v16.6.1, however, MSVS rejects it:

Severity  Code   Description                                                                      Line
Error     C2973  'Foo': invalid template argument 'int'                                           23
Error     E2886  cannot deduce 'auto' template parameter type "auto T::*" from "void (Bar::*)()"  21
Error     C2440  'specialization': cannot convert from 'overloaded-function' to 'auto Bar::* '    22
Error     C3535  cannot deduce type for 'auto Bar::* ' from 'int'                                 23
Error     C2440  'specialization': cannot convert from 'int' to 'int Bar::* '                     23

The code can be "fixed" in that version by taking out the T::* qualifier for MemberPtr; so:

template <typename T, auto MemberPtr>

What does the standard say about this? Is VS v16.6.1 introducing a new regression, or is it now diagnosing code that was always subtly broken?

c++17 language-lawyer pointer-to-member templates c++
1 Answers

The parameter/argument combo is valid as is

[temp.param]

4 A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

  • ...
  • a type that contains a placeholder type.

[temp.arg.nontype]

1 If the type of a template-parameter contains a placeholder type, the deduced parameter type is determined from the type of the template-argument by placeholder type deduction. If a deduced parameter type is not permitted for a template-parameter declaration ([temp.param]), the program is ill-formed.

Now, auto T::* is a type the contains a placeholder type. And placholder type deduction works just fine in a declaration of a variable in the form

auto Bar::* foo = &Bar::baz;

So VS v16.6.1 has no business rejecting such a non-type template parameter.

user_8176430
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