[ClangFormat] Future-proof Standard option, allow floating or pinning to arbitrary lang version
Summary:
The historical context:
- clang-format was written when C++11 was current,
and the main language-version concern was >> vs > > template-closers.
An option was added to allow selection of the 03/11 behavior, or auto-detection.
- there was no option to choose simply "latest standard" so anyone who didn't
ever want 03 behavior or auto-detection specified Cpp11.
- In r185149 this option started to affect lexer mode.
- no options were added to cover c++14, as parsing/formatting
didn't change that much. The usage of Cpp11 to mean "latest" became
codified e.g. in r206263
- c++17 added some new constructs. These were mostly backwards-compatible and so
not used in old programs, so having no way to turn them off was OK.
- c++20 added some new constructs and keywords (e.g. co_*) that changed the
meaning of existing programs, and people started to complain that
the c++20 parsing couldn't be turned off.
New plan:
- Default ('Auto') behavior remains unchanged: parse as latest, format
template-closers based on input.
- Add new 'Latest' option that more clearly expresses the intent "use
modern features" that many projects have chosen for their .clang-format files.
- Allow pinning to *any* language version, using the same name as clang -std:
c++03, c++11, c++14 etc. These set precise lexer options, and any
clang-format code depending on these can use a >= check.
- For backwards compatibility, `Cpp11` is an alias for `Latest`, not `c++11`.
This matches the historical documented semantics of this option.
This spelling (and `Cpp03`) are deprecated.
Reviewers: klimek, modocache
Subscribers: cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D67541
llvm-svn: 373439