if constexpr appears to not work correctly in some nested templates

[HolyBlackCat]

I think I've seen similar tickets at some point, so this could be a duplicate.

The following code doesn't compile, but it should: https://gcc.godbolt.org/z/648eY8eh5

template <typename T>
struct FalsePredicate
{
    static constexpr bool value = false;
};

template <typename T, template <typename> typename Cond>
void foo()
{
    []<template <typename> typename Cond2 = Cond>
    {
        if constexpr (Cond2<T>::value)
            T::foo();
    }();
}

int main()
{
    foo<int, FalsePredicate>();
}

It says that T::foo() is invalid for T=int, despite there being if constexpr.

Tested on Clang 20 and trunk.

[frederick-vs-ja]

IIUC this is P0588R1. Closing as duplicate of its tracking issue.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Egor (HolyBlackCat)

I think I've seen similar tickets at some point, so this could be a duplicate.

The following code doesn't compile, but it should: https://gcc.godbolt.org/z/648eY8eh5

template <typename T>
struct FalsePredicate
{
    static constexpr bool value = false;
};

template <typename T, template <typename> typename Cond>
void foo()
{
    []<template <typename> typename Cond2 = Cond>
    {
        if constexpr (Cond2<T>::value)
            T::foo();
    }();
}

int main()
{
    foo<int, FalsePredicate>();
}

It says that T::foo() is invalid for T=int, despite there being if constexpr.

Tested on Clang 20 and trunk.

[HolyBlackCat]

@frederick-vs-ja From the first glance that seems unrelated, as I don't use captures in this example. I think I get it. 🙂