Assume the following code:
namespace test
{
namespace detail
{
}
inline namespace v1
{
namespace detail
{
void foo()
{
}
}
}
}
int main()
{
test::detail::foo();
}
As we can see, this code compiles with Clang; not with GCC, however - GCC complains that the reference to namespace detail
is ambiguous:
main.cpp:20:11: error: reference to 'detail' is ambiguous
test::detail::foo();
^
main.cpp:4:5: note: candidates are: namespace test::detail { }
{
^
main.cpp:10:9: note: namespace test::v1::detail { }
{
^
Which compiler does the correct thing here?
Inline namespaces are a library versioning feature akin to symbol versioning, but implemented purely at the C++11 level (ie. cross-platform) instead of being a feature of a specific binary executable format (ie. platform-specific).
Namespaces are used to organize code into logical groups and to prevent name collisions that can occur especially when your code base includes multiple libraries. All identifiers at namespace scope are visible to one another without qualification.
GCC is correct:
Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (3.4.2) whenever one of them is, and a using-directive that names the namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1). Furthermore, each member of the inline namespace can subsequently be explicitly instantiated (14.7.2) or explicitly specialized (14.7.3) as though it were a member of the enclosing namespace. Finally, looking up a name in the enclosing namespace via explicit qualification (3.4.3.2) will include members of the inline namespace brought in by the using-directive even if there are declarations of that name in the enclosing namespace.
(This is at 7.3.1/8 in old n3337 numbering)
I believe you're seeing Clang bug #10361.
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