`template <auto>` doesn't work with partial specialization

[llvmbot]

Bugzilla Link	43076
Version	trunk
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@Quuxplusone,@kamleshbhalui,@ldalessa,@zygoloid

Extended Description

Example:

template <auto> struct foo { static constexpr int value = 0; };
template <int I> struct foo<I> { static constexpr int value = 1; };

static_assert(foo<42L>::value == 0);

According to clang, foo<42L>::value is 1, but the partial specialization should not match here (we fail [temp.deduct.type]/19 because the type of 42L differs from the type of the parameter).

[llvmbot]

assigned to @kamleshbhalui

[Quuxplusone]

Here's another example of the bad interplay between partial specializations and template. (And only partial specializations! Notice that Clang correctly ignores the irrelevant explicit specialization; and Clang would correctly pick a relevant explicit specialization.)

// https://godbolt.org/z/ek2a6b
template<auto I, class T> struct S {
static constexpr int value = 0;
};
template struct S<42, T> {
static constexpr int value = 1;
};
template<> struct S<42, int> {
static constexpr int value = 2;
};

int i = S<42u, int>::value; // Clang says "1"; the correct answer is "0"

[cor3ntin]

@mizvekov

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [43076](https://llvm.org/bz43076) | | Version | trunk | | OS | Windows NT | | Reporter | LLVM Bugzilla Contributor | | CC | @Quuxplusone,@kamleshbhalui,@ldalessa,@zygoloid |

Extended Description

Example:

template <auto> struct foo { static constexpr int value = 0; };
template <int I> struct foo<I> { static constexpr int value = 1; };

static_assert(foo<42L>::value == 0);

According to clang, foo<42L>::value is 1, but the partial specialization should not match here (we fail [temp.deduct.type]/19 because the type of 42L differs from the type of the parameter).