Extended Description

The following code:

template <class T, class U, class... Rest>
requires CommonReference<T, U>()
struct common_reference<T, U, Rest...>
: common_reference<common_reference_t<T, U>, Rest...> {};

is reformatted to

template <class T, class U, class... Rest>
requires CommonReference<T, U>() struct common_reference<T, U, Rest...>
: common_reference<common_reference_t<T, U>, Rest...> {};

because clang-format does not understand the requires keyword in class templates.

It should either be formatted to

// A: requires clause in its own line:
template <class T, class U, class... Rest>
requires CommonReference<T, U>()
struct common_reference<T, U, Rest...>
: common_reference<common_reference_t<T, U>, Rest...> {};

// B: small require clause in line with template arguments
template <class T, class U, class... Rest> requires CommonReference<T, U>()
struct common_reference<T, U, Rest...>
: common_reference<common_reference_t<T, U>, Rest...> {};

Also, long requires clauses are not handled properly

template <class T, class U, class... Rest>
requires CommonReference<T, U>() &&
CommonReference<T, U>() &&
CommonReference<T, U>()
struct common_reference<T, U, Rest...>
: common_reference<common_reference_t<T, U>, Rest...> {};

is reformatted to:

template <class T, class U, class... Rest>
requires CommonReference<T, U>() && CommonReference<T, U>() &&
CommonReference<T, U>() struct common_reference<T, U, Rest...>
: common_reference<common_reference_t<T, U>, Rest...> {};

Following customization parameters might make sense, but I haven't given this much thought:

• indentation of the require clause with respect to the template arguments/struct (e.g. 1 space, 4 spaces...). Maybe we could reuse ContinuationIndentWidth/AccessModifierOffset for this, but I'd rather have it as a standalone option.
• AllowShortRequireClausesOnASingleLine: whether short requires clauses should be wrapped into the same line as the template arguments

And well probably stuff like BreakBeforeBinaryOperators should work here properly.