Clang 6.0.0 (In-Progress) Release Notes

Written by the LLVM Team


These are in-progress notes for the upcoming Clang 6 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 6.0.0. 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 Subversion 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 6.0.0?

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

  • ...

Improvements to Clang’s diagnostics

  • -Wpragma-pack is a new warning that warns in the following cases:
    • When a translation unit is missing terminating #pragma pack (pop) directives.
    • When leaving an included file that changes the current alignment value, i.e. when the alignment before #include is different to the alignment after #include.
    • -Wpragma-pack-suspicious-include (disabled by default) warns on an #include when the included file contains structures or unions affected by a non-default alignment that has been specified using a #pragma pack directive prior to the #include.
  • -Wobjc-messaging-id is a new, non-default warning that warns about message sends to unqualified id in Objective-C. This warning is useful for projects that would like to avoid any potential future compiler errors/warnings, as the system frameworks might add a method with the same selector which could make the message send to id ambiguous.
  • -Wtautological-compare now warns when comparing an unsigned integer and 0 regardless of whether the constant is signed or unsigned.”
  • -Wtautological-compare now warns about comparing a signed integer and 0 when the signed integer is coerced to an unsigned type for the comparison. -Wsign-compare was adjusted not to warn in this case.
  • -Wtautological-constant-compare is a new warning that warns on tautological comparisons between integer variable of the type T and the largest/smallest possible integer constant of that same type.
  • For C code, -Wsign-compare, -Wsign-conversion, -Wtautological-constant-compare and -Wtautological-constant-out-of-range-compare were adjusted to use the underlying datatype of enum.
  • -Wnull-pointer-arithmetic now warns about performing pointer arithmetic on a null pointer. Such pointer arithmetic has an undefined behavior if the offset is nonzero. It also now warns about arithmetic on a null pointer treated as a cast from integer to pointer (GNU extension).
  • -Wzero-as-null-pointer-constant was adjusted not to warn on null pointer constants that originate from system macros, except NULL macro.

Non-comprehensive list of changes in this release

  • Bitrig OS was merged back into OpenBSD, so Bitrig support has been removed from Clang/LLVM.
  • The default value of _MSC_VER was raised from 1800 to 1911, making it compatible with the Visual Studio 2015 and 2017 C++ standard library headers. Users should generally expect this to be regularly raised to match the most recently released version of the Visual C++ compiler.
  • clang now defaults to .init_array if no gcc installation can be found. If a gcc installation is found, it still prefers .ctors if the found gcc is older than 4.7.0.

New Compiler Flags

  • –autocomplete was implemented to obtain a list of flags and its arguments. This is used for shell autocompletion.
  • The -fdouble-square-bracket-attributes and corresponding -fno-double-square-bracket-attributes flags were added to enable or disable [[]] attributes in any language mode. Currently, only a limited number of attributes are supported outside of C++ mode. See the Clang attribute documentation for more information about which attributes are supported for each syntax.

Deprecated Compiler Flags

The following options are deprecated and ignored. They will be removed in future versions of Clang.

  • ...

New Pragmas in Clang

Clang now supports the ...

Attribute Changes in Clang

  • The presence of __attribute__((availability(...))) on a declaration no longer implies default visibility for that declaration on macOS.
  • ...

Windows Support

Clang’s support for building native Windows programs ...

C Language Changes in Clang

  • ...


C11 Feature Support


C++ Language Changes in Clang


C++1z Feature Support


Internal API Changes

These are major API changes that have happened since the 4.0.0 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.

  • ...

AST Matchers

The hasDeclaration matcher now works the same for Type and QualType and only ever looks through one level of sugaring in a limited number of cases.

There are two main patterns affected by this:

  • qualType(hasDeclaration(recordDecl(...))): previously, we would look through sugar like TypedefType to get at the underlying recordDecl; now, we need to explicitly remove the sugaring: qualType(hasUnqualifiedDesugaredType(hasDeclaration(recordDecl(...))))
  • hasType(recordDecl(...)): hasType internally uses hasDeclaration; previously, this matcher used to match for example TypedefTypes of the RecordType, but after the change they don’t; to fix, use:
  • templateSpecializationType(hasDeclaration(classTemplateDecl(...))): previously, we would directly match the underlying ClassTemplateDecl; now, we can explicitly match the ClassTemplateSpecializationDecl, but that requires to explicitly get the ClassTemplateDecl:


  • Option IndentPPDirectives added to indent preprocessor directives on conditionals.

    Before After
    #if FOO
    #if BAR
    #include <foo>
    #if FOO
    #  if BAR
    #    include <foo>
    #  endif
  • Option -verbose added to the command line. Shows the list of processed files.

  • Option IncludeBlocks added to merge and regroup multiple #include blocks during sorting.

    Before (Preserve) Merge Regroup
    #include "b.h"
    #include "a.b"
    #include <lib/main.h>
    #include "a.h"
    #include "b.h"
    #include <lib/main.h>
    #include "a.h"
    #include "b.h"
    #include <lib/main.h>

Static Analyzer

  • Static Analyzer can now properly detect and diagnose unary pre-/post- increment/decrement on an uninitialized value.


Undefined Behavior Sanitizer (UBSan)

  • A minimal runtime is now available. It is suitable for use in production environments, and has a small attack surface. It only provides very basic issue logging and deduplication, and does not support -fsanitize=vptr checking.

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 Subversion 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.