[LLVMbugs] [Bug 23402] New: Variable template instantiations in template arguments not considered constant expression

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23402

            Bug ID: 23402
           Summary: Variable template instantiations in template arguments
                    not considered constant expression
           Product: clang
           Version: trunk
          Hardware: PC
                OS: FreeBSD
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++14
          Assignee: unassignedclangbugs at nondot.org
          Reporter: daniel at octaforge.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Hello,

I found a bug recently (I believe). This code does not compile with Clang (3.4
as well as 3.7-svn, on FreeBSD x64):

-----
#include <stdio.h>

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

template<> struct foo<int> {
    static constexpr bool value = true;
};

template<typename T> static constexpr bool Foo = foo<T>::value;

template<typename T, bool B = Foo<T>> struct bar {};

int main() {
    bar<int> x;
    return 0;
}
-----

The error is:

const.cpp:13:31: error: non-type template argument is not a constant expression
template<typename T, bool B = Foo<T>> struct bar {};

g++ 5.1 does not exhibit this behavior.

keep in mind that changing to (that is, turning the variable template into
non-template constexpr variable):

-----
static constexpr bool Foo = foo<int>::value;
template<typename T, bool B = Foo> struct bar {};
-----

works in both g++ and clang.

Adding a wrapper also works in both g++ and clang:

-----
template<typename T> static constexpr bool Foo = foo<T>::value;

template<typename T> struct wrap {
    static constexpr bool value = Foo<T>;
};

template<typename T, bool B = wrap<T>::value> struct bar {};
-----

and a static assertion also works in both gcc and clang (does not cause the
error):

-----
template<typename T> static constexpr bool Foo = foo<T>::value;

int main() {
    static_assert(Foo<int>, "hello");
    return 0;
}
-----

So it only happens when the "Foo<T>" happens as a default value for a non-type
template argument. That made me think this is a Clang bug.

I compile with "-std=c++14 -Wall -Wextra".

Please let me know if you need any further information.

Regards,

Daniel

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