[llvm-bugs] [Bug 49964] New: [concepts] ambiguous overload accepted as valid

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49964

            Bug ID: 49964
           Summary: [concepts] ambiguous overload accepted as valid
           Product: clang
           Version: 11.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldalessandro at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Clang accepts the following concept code that gcc considers ambiguous (reduced
testcase due to Patrick Palka).

```
  template <class T> concept C1 = true;
  template <class T> concept C2 = C1<T> && true;

  template <C1 T, C1 U> int f(T, U);
  template <C1 T, C2 U> int f(U, T);

  int x = f(0, 0); // expect error: ambiguous call
```

Live: https://godbolt.org/z/6K93d1Ye4

I originally submitted this as a gcc bug
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963) but Patrick and Jason
Merrill determined that gcc's behavior is correct.

According to their reading of the standard (I don't have the background to make
this statement), this call to f should be ambiguous because the matching
between the template argument index and function argument index is different in
its two declarations.

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