[Clang18][Concepts] Using static data member as concept argument in partially specialized template fails to compile

[kaimfrai]

The following example compiles with Clang 17 but not in Clang 18 (since approximately 1 week):

template<typename T, typename U, int i>
concept True = true;

template<typename T, int I>
struct Template
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<T, i> auto f)
    {
        return i;
    }
};

template<int I>
struct Template<float, I>
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<float, i> auto f)
    {
        return i;
    }
};

Template<void, 4> f{};

static_assert(+Template<float, 5>{} == 5);

With -std=c++20 on clang 18 I get:

<source>:9:45: error: no member 'i' in 'Template<float, 5>'; it has not yet been instantiated

The error is caused in the static_assert. What's interesting is that it's refering to line 9, which is not the specialization Template<float, 5>.
The error goes away if I remove the variable f, when I don't have a partial specialization, or I use I directly.
This is a problem when I don't want to just use the value of I but the result of a complex expression dependent on I and that result is repeatedly used throughout the specialization, not just 1 function.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Kai Müller (kaimfrai)

The following example compiles with Clang 17 but not in Clang 18 (since approximately 1 week):

template<typename T, typename U, int i>
concept True = true;

template<typename T, int I>
struct Template
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<T, i> auto f)
    {
        return i;
    }
};

template<int I>
struct Template<float, I>
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<float, i> auto f)
    {
        return i;
    }
};

Template<void, 4> f{};

static_assert(+Template<float, 5>{} == 5);

With -std=c++20 on clang 18 I get:

<source>:9:45: error: no member 'i' in 'Template<float, 5>'; it has not yet been instantiated

The error is caused in the static_assert. What's interesting is that it's refering to line 9, which is not the specialization Template<float, 5>.
The error goes away if I remove the variable f, when I don't have a partial specialization, or I use I directly.
This is a problem when I don't want to just use the value of I but the result of a complex expression dependent on I and that result is repeatedly used throughout the specialization, not just 1 function.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Kai Müller (kaimfrai)

The following example compiles with Clang 17 but not in Clang 18 (since approximately 1 week):

template<typename T, typename U, int i>
concept True = true;

template<typename T, int I>
struct Template
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<T, i> auto f)
    {
        return i;
    }
};

template<int I>
struct Template<float, I>
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<float, i> auto f)
    {
        return i;
    }
};

Template<void, 4> f{};

static_assert(+Template<float, 5>{} == 5);

With -std=c++20 on clang 18 I get:

<source>:9:45: error: no member 'i' in 'Template<float, 5>'; it has not yet been instantiated

The error is caused in the static_assert. What's interesting is that it's refering to line 9, which is not the specialization Template<float, 5>.
The error goes away if I remove the variable f, when I don't have a partial specialization, or I use I directly.
This is a problem when I don't want to just use the value of I but the result of a complex expression dependent on I and that result is repeatedly used throughout the specialization, not just 1 function.

[MitalAshok]

This currently crashes on HEAD https://godbolt.org/z/jY5Yrsvoa:

clang++: /root/llvm-project/clang/include/clang/AST/Decl.h:276: llvm::StringRef clang::NamedDecl::getName() const: Assertion `Name.isIdentifier() && "Name is not a simple identifier"' failed.

69066ab is the first commit where this crashes (the previous one compiles this without error or crash)

@cor3ntin

[kaimfrai]

Thanks for the quick fix! Unfortunately, after updating clang the issue I had still persists. The reproducer I provided does not appear to have reflected my problem properly. I changed the example a bit and now it ceased to work on Clang 18 and 17, but still works on Clang 16 and GCC.

template<typename T, int i>
concept True = true;

template<typename T, int I>
struct Template
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<i> auto f)
    {
        return i;
    }
};

template<int I>
struct Template<float, I>
{
    static constexpr int i = I;

    friend constexpr auto operator+(True<i> auto f)
    {
        return i;
    }
};

Template<void, 4> f{};

static_assert(+Template<float, 5>{} == 5);

Note that all I did was remove the type parameter from the concept.

https://godbolt.org/z/156133ETT