False positive warning `warning: instantiation of variable 'declvar<A>' required here, but no definition is available`, when the definition is not required

[HolyBlackCat]

Consider the following code: https://gcc.godbolt.org/z/acd5W9YMn

struct A {explicit constexpr operator bool() const {return true;}};
struct B {int x = 1; explicit constexpr operator bool() const {return x;}};

template <typename T>
extern T declvar;

template <auto T>
constexpr int dummy = 0;

template <typename T>
concept C = requires{dummy<declvar<T> ? true : false>;};

static_assert(C<A>);
static_assert(!C<B>);

This compiles correctly on all 3 bug compilers including Clang (and seems to be the only good way of doing this).

But Clang emits a useless warning:

<source>:11:28: warning: instantiation of variable 'declvar<A>' required here, but no definition is available [-Wundefined-var-template]
   11 | concept C = requires{dummy<declvar<T> ? true : false>;};
      |                            ^
<source>:5:10: note: forward declaration of template entity is here
    5 | extern T declvar;
      |          ^
<source>:11:28: note: add an explicit instantiation declaration to suppress this warning if 'declvar<A>' is explicitly instantiated in another translation unit
   11 | concept C = requires{dummy<declvar<T> ? true : false>;};
      |                            ^

The variable definition isn't needed here, so it shouldn't warn.

[frederick-vs-ja]

I think declvar<A> & declvar<B> are still odr-used per [basic.def.odr]/7, even though the odr-uses are constant-evaluation-only.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Egor (HolyBlackCat)

Consider the following code: https://gcc.godbolt.org/z/acd5W9YMn ```cpp struct A {explicit constexpr operator bool() const {return true;}}; struct B {int x = 1; explicit constexpr operator bool() const {return x;}};

template <typename T>
extern T declvar;

template <auto T>
constexpr int dummy = 0;

template <typename T>
concept C = requires{dummy<declvar<T> ? true : false>;};

static_assert(C<A>);
static_assert(!C<B>);

This compiles correctly on all 3 bug compilers including Clang (and seems to be the only good way of doing this).

But Clang emits a useless warning:
```none
<source>:11:28: warning: instantiation of variable 'declvar<A>' required here, but no definition is available [-Wundefined-var-template]
   11 | concept C = requires{dummy<declvar<T> ? true : false>;};
      |                            ^
<source>:5:10: note: forward declaration of template entity is here
    5 | extern T declvar;
      |          ^
<source>:11:28: note: add an explicit instantiation declaration to suppress this warning if 'declvar<A>' is explicitly instantiated in another translation unit
   11 | concept C = requires{dummy<declvar<T> ? true : false>;};
      |                            ^

The variable definition isn't needed here, so it shouldn't warn.

[HolyBlackCat]

I think declvar<A> & declvar<B> are still odr-used per [basic.def.odr]/7, even though the odr-uses are constant-evaluation-only.

Even if they technically are, I think Clang shouldn't warn if there's no linker error.