Clang 13.0.0 (In-Progress) Release Notes

Written by the LLVM Team


These are in-progress notes for the upcoming Clang 13 release. Release notes for previous releases can be found on the Download Page.


This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 13.0.0. Here we describe the status of Clang in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see the LLVM documentation. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about Clang or LLVM, including information about the latest release, please see the Clang Web Site or the LLVM Web Site.

Note that if you are reading this file from a Git checkout or the main Clang web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

What’s New in Clang 13.0.0?

Some of the major new features and improvements to Clang are listed here. Generic improvements to Clang as a whole or to its underlying infrastructure are described first, followed by language-specific sections with improvements to Clang’s support for those languages.

Major New Features

  • Guaranteed tail calls are now supported with statement attributes [[clang::musttail]] in C++ and __attribute__((musttail)) in C. The attribute is applied to a return statement (not a function declaration), and an error is emitted if a tail call cannot be guaranteed, for example if the function signatures of caller and callee are not compatible. Guaranteed tail calls enable a class of algorithms that would otherwise use an arbitrary amount of stack space.

Improvements to Clang’s diagnostics

New Compiler Flags

  • -Wreserved-identifier emits warning when user code uses reserved identifiers.

Modified Compiler Flags

  • -Wshadow now also checks for shadowed structured bindings
  • -B <prefix> (when <prefix> is a directory) was overloaded to additionally detect GCC installations under <prefix> (lib{,32,64}/gcc{,-cross}/$triple). This behavior was incompatible with GCC, caused interop issues with --gcc-toolchain, and was thus dropped. Specify --gcc-toolchain=<dir> instead. -B’s other GCC-compatible semantics are preserved: $prefix/$triple-$file and $prefix$file are searched for executables, libraries, includes, and data files used by the compiler.

Removed Compiler Flags

  • The clang-cl /fallback flag, which made clang-cl invoke Microsoft Visual C++ on files it couldn’t compile itself, has been removed.
  • -Wreturn-std-move-in-c++11, which checked whether an entity is affected by CWG1579 to become implicitly movable, has been removed.

C++ Language Changes in Clang

  • The oldest supported GNU libstdc++ is now 4.8.3 (released 2014-05-22). Clang workarounds for bugs in earlier versions have been removed.

C++20 Feature Support

C++2b Feature Support

Internal API Changes

These are major API changes that have happened since the 12.0.0 release of Clang. If upgrading an external codebase that uses Clang as a library, this section should help get you past the largest hurdles of upgrading.

Build System Changes

These are major changes to the build system that have happened since the 12.0.0 release of Clang. Users of the build system should adjust accordingly.

  • The option LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA no longer exists. There were two releases with that flag forced off, and no uses were added that forced it on. The recommended replacement is clangd.


  • Option SpacesInLineCommentPrefix has been added to control the number of spaces in a line comments prefix.

  • Option SortIncludes has been updated from a bool to an enum with backwards compatibility. In addition to the previous true/false states (now CaseSensitive/Never), a third state has been added (CaseInsensitive) which causes an alphabetical sort with case used as a tie-breaker.

    // Never (previously false)
    #include "B/A.h"
    #include "A/B.h"
    #include "a/b.h"
    #include "A/b.h"
    #include "B/a.h"
    // CaseSensitive (previously true)
    #include "A/B.h"
    #include "A/b.h"
    #include "B/A.h"
    #include "B/a.h"
    #include "a/b.h"
    // CaseInsensitive
    #include "A/B.h"
    #include "A/b.h"
    #include "a/b.h"
    #include "B/A.h"
    #include "B/a.h"
  • BasedOnStyle: InheritParentConfig allows to use the .clang-format of the parent directories to overwrite only parts of it.

  • Option IndentAccessModifiers has been added to be able to give access modifiers their own indentation level inside records.

  • Option ShortNamespaceLines has been added to give better control over FixNamespaceComments when determining a namespace length.

  • Support for Whitesmiths has been improved, with fixes for namespace blocks and case blocks and labels.

  • Option EmptyLineAfterAccessModifier has been added to remove, force or keep new lines after access modifiers.

  • Checks for newlines in option EmptyLineBeforeAccessModifier are now based on the formatted new lines and not on the new lines in the file. (Fixes

  • Option SpacesInAngles has been improved, it now accepts Leave value that allows to keep spaces where they are already present.

  • Option AllowShortIfStatementsOnASingleLine has been improved, it now accepts AllIfsAndElse value that allows to put “else if” and “else” short statements on a single line. (Fixes

New Issues Found

Python Binding Changes

The following methods have been added:

Additional Information

A wide variety of additional information is available on the Clang web page. The web page contains versions of the API documentation which are up-to-date with the Git version of the source code. You can access versions of these documents specific to this release by going into the “clang/docs/” directory in the Clang tree.

If you have any questions or comments about Clang, please feel free to contact us via the mailing list.