[llvm-bugs] [Bug 44175] New: unused member variable causes code to compile for member to function for undefined function

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44175

            Bug ID: 44175
           Summary: unused member variable causes code to compile for
                    member to function for undefined function
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: marcpawl at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

https://godbolt.org/z/VygQuN

If the class is not instantiated and the variable is not static then the
reference to a non-existing method is not reported as an error.


#include <type_traits>
#include <cstdio>
struct Foo {
  virtual void hello(int) = 0;
};

struct Bar : public Foo {
void hello(int) override {};
};

struct Bad {  };

template <typename T>
struct is_Foo {
    using hello_fn_t = void (T::*)(int);
    void (T::*hello)(int) = &T::hello; 
    constexpr static bool value=true;
};

template <typename T, typename U=is_Foo<T>>
void say(T& t)
{
    static_assert(U::value, "not a Foo");
    puts("hello\n");
    //U u; // Line 22
    //t.hello(4); // Line 23
}



int main()
{
    //Bar b;;
    //Foo& f = b;
    //say(b);
    //say(f);
    Bad bad;
    say(bad);
}


====
Found while exploring a GCC bug
to gcc-bugzilla-account-request

https://godbolt.org/z/SFZmZJ

In clang you get 
no member names 'hello' in 'Bad'
which is the expected result.

In all GCC versions using --std=c++17 from 5.2 onwards the code compiles.

#include <type_traits>
#include <cstdio>
struct Foo {
virtual void hello(int) = 0;
};

struct Bar : public Foo {
void hello(int) override {};
};

struct Bad { };

template <typename T>
struct is_Foo {
using hello_fn_t = void (T::*)(int);
constexpr static void (T::*hello)(int) = &T::hello;
static constexpr bool value=true;
};

template <typename T>
auto say(T& t) -> std::enable_if_t<is_Foo<T>::value, void>{
//U u; // Line 22
//t.hello(4); // Line 23
puts("hello\n");
}

int main()
{
//Bar b;;
//Foo& f = b;
//say(b);
//say(f);
Bad bad;
say(bad);
}

If you change say to
template <typename T, typename U=is_Foo<T>>
void say(T& t)
{
    U u; // Line 22
    //t.hello(4); // Line 23
}
you get the same errors in clang, but no errors in gcc.

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