[llvm-bugs] [Bug 44670] New: Rejects valid code with requires clause referring to function parameter when called from variadic function

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44670

            Bug ID: 44670
           Summary: Rejects valid code with requires clause referring to
                    function parameter when called from variadic function
           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

```
struct s {};

constexpr bool f(s) {
        return true;
}

template<typename T>
bool a(T t) requires(f(t)) {
    return true;
}

template<typename... Args>
bool b(Args ... args) {
        (..., a(args));
        return true;
}

//bool x = a(s());
bool y = b(s());
```


when compiled against 8d6884a15e86419c51ba456f8b6f5223696800a1 with the command
`clang++ -std=c++2a -o /dev/null -c main.cpp` should compile with no errors.
Instead, it outputs


```
main.cpp:14:8: error: no matching function for call to 'a'
        (..., a(args));
              ^
main.cpp:19:10: note: in instantiation of function template specialization
'b<s>' requested here
bool y = b(s());
         ^
main.cpp:8:6: note: candidate function not viable: constraints not satisfied
bool a(T t) requires(f(t)) {
     ^
main.cpp:8:24: note: because substituted constraint expression is ill-formed:
reference to local variable 't' declared in enclosing function 'a<s>'
bool a(T t) requires(f(t)) {
                       ^
1 error generated.
```


Uncommenting the direct call to `a` (the variable `x`) causes clang to accept
the code.

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