[llvm-bugs] [Bug 50577] New: Partial ordering of function templates accepting template template parameters sensitive to existence of parameter list in requires expression

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50577

            Bug ID: 50577
           Summary: Partial ordering of function templates accepting
                    template template parameters sensitive to existence of
                    parameter list in requires expression
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: davidfromonline 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

The following translation unit is accepted by clang unless you define
AMBIGUOUS:

```
template<typename>
struct s {
};

#ifdef AMBIGUOUS

template<typename T>
concept true1 = requires(T) {
        requires true;
};

#else

template<typename T>
concept true1 = requires {
        requires true;
};

#endif

template<typename T>
concept true2 = requires {
        requires true;
};

template<template<typename...> typename Template, typename T> requires
true1<Template<int>> and true2<T>
void f(T) {
}
template<template<typename, auto...> typename Template, typename T> requires
true1<Template<int>>
void f(T) {
}


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

If AMBIGUOUS is defined, it is rejected with the error message

```
source>:35:2: error: call to 'f' is ambiguous
        f<s>(0);
        ^~~~
<source>:27:6: note: candidate function [with Template = s, T = int]
void f(T) {
     ^
<source>:30:6: note: candidate function [with Template = s, T = int]
void f(T) {
     ^
1 error generated.
Compiler returned: 1
```


See it live: https://godbolt.org/z/6ecMWxcq6

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