[llvm-bugs] [Bug 32165] New: [feature request] requires clause in class templates (Concepts TS)

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https://bugs.llvm.org/show_bug.cgi?id=32165

            Bug ID: 32165
           Summary: [feature request] requires clause in class templates
                    (Concepts TS)
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Formatter
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalobg88 at gmail.com
                CC: djasper at google.com, klimek at google.com,
                    llvm-bugs at lists.llvm.org

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.

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