[llvm-bugs] [Bug 44615] New: -fconcepts-ts causes incorrect name lookup for non-type template parameter

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44615

            Bug ID: 44615
           Summary: -fconcepts-ts causes incorrect name lookup for
                    non-type template parameter
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: david at doublewise.net
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The following code

```
namespace s {
        using t = int;
};

template<s::t>
void f();
```

when compiled with

`clang++ -Xclang -fconcepts-ts -std=c++2a -c main.cpp`

at revision bb9b964


does not actually look up `t` in `s`. Instead, it looks it up in the global
scope, and thus it gives an error of

```
main.cpp:6:13: error: unknown type name 't'; did you mean 's::t'?
template<s::t>
            ^
            s::t
main.cpp:2:8: note: 's::t' declared here
        using t = int;
              ^
1 error generated.
```

If I define a `t` in the global scope as well, it picks up that name and
compiles, using that variable instead. This is demonstrated by the following
program:


```
#include <type_traits>

namespace s {
        using t = int;
};

using t = short;

template<s::t x>
void f() {
        static_assert(std::is_same_v<decltype(x), int>);
}

void g() {
        f<0>();
}
```


This behavior occurs only if `-Xclang -fconcepts-ts` is passed, but it seems
that this flag is necessary to actually get all of the concepts implementation.

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