Ambiguity between template-name and other name should be detected

[kariya-mitsuru]

Bugzilla Link	18486
Version	trunk
OS	All
CC	@DougGregor,@zygoloid

Extended Description

I think that sample code below should cause error because TEST is ambiguous.

namespace FOO {
    template<int I>
    struct FOO {};
}
using namespace FOO;
void f() { FOO<0> x; }

However, an error does not actually occur.

If above sample is well-formed, why is next code ill-formed?

template<int I>
struct FOO {};
namespace BAR {
    namespace FOO {
        void f() { FOO<0> x; }
    }
}

[zygoloid]

I think that sample code below should cause error because TEST is ambiguous.

I agree. Name lookup finds both FOO and FOO::FOO, and by 3.4/1 that makes the program ill-formed because name lookup did not find an unambiguous declaration for the name FOO.

I suspect that the problem is that we're getting 14.2/3 wrong ("After name lookup finds that a name is a template-name or that an operator-function-id or a literal-operator-id refers to a set of overloaded functions any member of which is a function template[,] if this is followed by <, the < is always taken as the delimiter of a template-argument-list and never as the less-than operator").

If above sample is well-formed, why is next code ill-formed?

template
struct FOO {};
namespace BAR {
namespace FOO {
void f() { FOO<0> x; }

This is ill-formed for a different reason. Unqualified lookup searches namespace BAR::FOO (finding nothing), then searches namespace BAR (finding namespace FOO), and then stops, because it found something. It never even gets to the global namespace, so never finds ::FOO.

[Endilll]

The first example has been rejected since Clang 9: https://godbolt.org/z/EWKx3EcMP
The second example is ill-formed, and has always been rejected: https://godbolt.org/z/cb4GEjcGP

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (e48d0f12-b0d3-485a-93ce-75b823b0ccac)

| | | | --- | --- | | Bugzilla Link | [18486](https://llvm.org/bz18486) | | Version | trunk | | OS | All | | CC | @DougGregor,@zygoloid |

Extended Description

I think that sample code below should cause error because TEST is ambiguous.

namespace FOO {
    template<int I>
    struct FOO {};
}
using namespace FOO;
void f() { FOO<0> x; }

However, an error does not actually occur.

If above sample is well-formed, why is next code ill-formed?

template<int I>
struct FOO {};
namespace BAR {
    namespace FOO {
        void f() { FOO<0> x; }
    }
}