clang: Switch C compilations to C17 by default.

[nickdesaulniers]

Summary

Matches GCC 8.1 (2018).

Updates documentation+release notes as well.

See also https://reviews.llvm.org/rL220244.

[nickdesaulniers]

My hope is that we land this with time to soak for the clang-11 release.

[aaron.ballman]

LGTM -- C17 is C11 with bugfixes, so this seems like a sensible change, especially for compatibility with GCC.

[rsmith]

Looks good. Please consider updating the Clang 10 release notes to indicate that we intend to change the default in Clang 11.

[nickdesaulniers]

In D75383#1899211, @rsmith wrote:

Looks good. Please consider updating the Clang 10 release notes to indicate that we intend to change the default in Clang 11.

https://reviews.llvm.org/D75469

clang/docs/ReleaseNotes.rst

 - ...
 
 Windows Support
 ---------------
 
 C Language Changes in Clang
 ---------------------------
 
-- ...
-
-C11 Feature Support
-^^^^^^^^^^^^^^^^^^^
+- The default C language standard used when `-std=` is not specified has been
+  upgraded from gnu11 to gnu17.
 
-...
+- ...
 
 C++ Language Changes in Clang
 -----------------------------
 
 - Clang now implements a restriction on giving non-C-compatible anonymous
   structs a typedef name for linkage purposes, as described in C++ committee
   paper `P1766R1 <http://wg21.link/p1766r1>`. This paper was adopted by the
   C++ committee as a Defect Report resolution, so it is applied retroactively

clang/docs/UsersManual.rst

 .. _basicusage:
 
 Basic Usage
 -----------
 
 Intro to how to use a C compiler for newbies.
 
 compile + link compile then link debug info enabling optimizations
-picking a language to use, defaults to C11 by default. Autosenses based
+picking a language to use, defaults to C17 by default. Autosenses based
 on extension. using a makefile
 
 Command Line Options
 ====================
 
 This section is generally an index into other sections. It does not go
 into depth on the ones that are covered by other sections. However, the
 first part introduces the language selection and other high level

 Extensions supported by clang
 -----------------------------
 
 See :doc:`LanguageExtensions`.
 
 Differences between various standard modes
 ------------------------------------------
 
-clang supports the -std option, which changes what language mode clang
-uses. The supported modes for C are c89, gnu89, c99, gnu99, c11, gnu11,
-c17, gnu17, and various aliases for those modes. If no -std option is
-specified, clang defaults to gnu11 mode. Many C99 and C11 features are
+clang supports the -std option, which changes what language mode clang uses.
+The supported modes for C are c89, gnu89, c99, gnu99, c11, gnu11, c17, gnu17,
+c2x, gnu2x, and various aliases for those modes. If no -std option is
+specified, clang defaults to gnu17 mode. Many C99 and C11 features are
 supported in earlier modes as a conforming extension, with a warning. Use
 ``-pedantic-errors`` to request an error if a feature from a later standard
 revision is used in an earlier mode.
 
 Differences between all ``c*`` and ``gnu*`` modes:
 
 -  ``c*`` modes define "``__STRICT_ANSI__``".
 -  Target-specific defines not prefixed by underscores, like "linux",

clang/lib/Frontend/CompilerInvocation.cpp

if (LangStd == LangStandard::lang_unspecified) {
     case Language::C:
 #if defined(CLANG_DEFAULT_STD_C)
       LangStd = CLANG_DEFAULT_STD_C;
 #else
       // The PS4 uses C99 as the default C standard.
       if (T.isPS4())
         LangStd = LangStandard::lang_gnu99;
       else
-        LangStd = LangStandard::lang_gnu11;
+        LangStd = LangStandard::lang_gnu17;
 #endif
       break;
     case Language::ObjC:
 #if defined(CLANG_DEFAULT_STD_C)
       LangStd = CLANG_DEFAULT_STD_C;
 #else
       LangStd = LangStandard::lang_gnu11;
 #endif

clang/test/Preprocessor/init-aarch64.c

 // AARCH64-NEXT: #define __SIZE_FMTx__ "lx"
 // AARCH64-NEXT: #define __SIZE_MAX__ 18446744073709551615UL
 // AARCH64-NEXT: #define __SIZE_TYPE__ long unsigned int
 // AARCH64-NEXT: #define __SIZE_WIDTH__ 64
 // AARCH64_CXX: #define __STDCPP_DEFAULT_NEW_ALIGNMENT__ 16UL
 // AARCH64-NEXT: #define __STDC_HOSTED__ 1
 // AARCH64-NEXT: #define __STDC_UTF_16__ 1
 // AARCH64-NEXT: #define __STDC_UTF_32__ 1
-// AARCH64_C: #define __STDC_VERSION__ 201112L
+// AARCH64_C: #define __STDC_VERSION__ 201710L
 // AARCH64-NEXT: #define __STDC__ 1
 // AARCH64-NEXT: #define __UINT16_C_SUFFIX__ 
 // AARCH64-NEXT: #define __UINT16_FMTX__ "hX"
 // AARCH64-NEXT: #define __UINT16_FMTo__ "ho"
 // AARCH64-NEXT: #define __UINT16_FMTu__ "hu"
 // AARCH64-NEXT: #define __UINT16_FMTx__ "hx"
 // AARCH64-NEXT: #define __UINT16_MAX__ 65535
 // AARCH64-NEXT: #define __UINT16_TYPE__ unsigned short

 // AARCH64-MSVC: #define __SIZEOF_WCHAR_T__ 2
 // AARCH64-MSVC: #define __SIZEOF_WINT_T__ 2
 // AARCH64-MSVC: #define __SIZE_MAX__ 18446744073709551615ULL
 // AARCH64-MSVC: #define __SIZE_TYPE__ long long unsigned int
 // AARCH64-MSVC: #define __SIZE_WIDTH__ 64
 // AARCH64-MSVC: #define __STDC_HOSTED__ 0
 // AARCH64-MSVC: #define __STDC_UTF_16__ 1
 // AARCH64-MSVC: #define __STDC_UTF_32__ 1
-// AARCH64-MSVC: #define __STDC_VERSION__ 201112L
+// AARCH64-MSVC: #define __STDC_VERSION__ 201710L
 // AARCH64-MSVC: #define __STDC__ 1
 // AARCH64-MSVC: #define __UINT16_C_SUFFIX__
 // AARCH64-MSVC: #define __UINT16_MAX__ 65535
 // AARCH64-MSVC: #define __UINT16_TYPE__ unsigned short
 // AARCH64-MSVC: #define __UINT32_C_SUFFIX__ U
 // AARCH64-MSVC: #define __UINT32_MAX__ 4294967295U
 // AARCH64-MSVC: #define __UINT32_TYPE__ unsigned int
 // AARCH64-MSVC: #define __UINT64_C_SUFFIX__ ULL

clang/test/Preprocessor/init.c

 // COMMON:#define __clang_version__ {{.*}}
 // COMMON:#define __llvm__ 1
 //
 // RUN: %clang_cc1 -E -dM -triple=x86_64-pc-win32 < /dev/null | FileCheck -match-full-lines -check-prefix C-DEFAULT %s
 // RUN: %clang_cc1 -E -dM -triple=x86_64-pc-linux-gnu < /dev/null | FileCheck -match-full-lines -check-prefix C-DEFAULT %s
 // RUN: %clang_cc1 -E -dM -triple=x86_64-apple-darwin < /dev/null | FileCheck -match-full-lines -check-prefix C-DEFAULT %s
 // RUN: %clang_cc1 -E -dM -triple=armv7a-apple-darwin < /dev/null | FileCheck -match-full-lines -check-prefix C-DEFAULT %s
 //
-// C-DEFAULT:#define __STDC_VERSION__ 201112L
+// C-DEFAULT:#define __STDC_VERSION__ 201710L
 //
 // RUN: %clang_cc1 -ffreestanding -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix FREESTANDING %s
 // FREESTANDING:#define __STDC_HOSTED__ 0
 //
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=gnu++20 -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX2A %s
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=gnu++2a -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX2A %s
 //
 // GXX2A:#define __GNUG__ 4

 // MIPS32BE:#define __SIZEOF_SIZE_T__ 4
 // MIPS32BE:#define __SIZEOF_WCHAR_T__ 4
 // MIPS32BE:#define __SIZEOF_WINT_T__ 4
 // MIPS32BE:#define __SIZE_MAX__ 4294967295U
 // MIPS32BE:#define __SIZE_TYPE__ unsigned int
 // MIPS32BE:#define __SIZE_WIDTH__ 32
 // MIPS32BE-CXX:#define __STDCPP_DEFAULT_NEW_ALIGNMENT__ 8U
 // MIPS32BE:#define __STDC_HOSTED__ 0
-// MIPS32BE-C:#define __STDC_VERSION__ 201112L
+// MIPS32BE-C:#define __STDC_VERSION__ 201710L
 // MIPS32BE:#define __STDC__ 1
 // MIPS32BE:#define __UINT16_C_SUFFIX__
 // MIPS32BE:#define __UINT16_MAX__ 65535
 // MIPS32BE:#define __UINT16_TYPE__ unsigned short
 // MIPS32BE:#define __UINT32_C_SUFFIX__ U
 // MIPS32BE:#define __UINT32_MAX__ 4294967295U
 // MIPS32BE:#define __UINT32_TYPE__ unsigned int
 // MIPS32BE:#define __UINT64_C_SUFFIX__ ULL

 // MIPSN32BE: #define __SIZE_FMTx__ "x"
 // MIPSN32BE: #define __SIZE_MAX__ 4294967295U
 // MIPSN32BE: #define __SIZE_TYPE__ unsigned int
 // MIPSN32BE: #define __SIZE_WIDTH__ 32
 // MIPSN32BE-CXX: #define __STDCPP_DEFAULT_NEW_ALIGNMENT__ 16U
 // MIPSN32BE: #define __STDC_HOSTED__ 0
 // MIPSN32BE: #define __STDC_UTF_16__ 1
 // MIPSN32BE: #define __STDC_UTF_32__ 1
-// MIPSN32BE-C: #define __STDC_VERSION__ 201112L
+// MIPSN32BE-C: #define __STDC_VERSION__ 201710L
 // MIPSN32BE: #define __STDC__ 1
 // MIPSN32BE: #define __UINT16_C_SUFFIX__
 // MIPSN32BE: #define __UINT16_FMTX__ "hX"
 // MIPSN32BE: #define __UINT16_FMTo__ "ho"
 // MIPSN32BE: #define __UINT16_FMTu__ "hu"
 // MIPSN32BE: #define __UINT16_FMTx__ "hx"
 // MIPSN32BE: #define __UINT16_MAX__ 65535
 // MIPSN32BE: #define __UINT16_TYPE__ unsigned short

 // MIPSN32EL: #define __SIZE_FMTu__ "u"
 // MIPSN32EL: #define __SIZE_FMTx__ "x"
 // MIPSN32EL: #define __SIZE_MAX__ 4294967295U
 // MIPSN32EL: #define __SIZE_TYPE__ unsigned int
 // MIPSN32EL: #define __SIZE_WIDTH__ 32
 // MIPSN32EL: #define __STDC_HOSTED__ 0
 // MIPSN32EL: #define __STDC_UTF_16__ 1
 // MIPSN32EL: #define __STDC_UTF_32__ 1
-// MIPSN32EL: #define __STDC_VERSION__ 201112L
+// MIPSN32EL: #define __STDC_VERSION__ 201710L
 // MIPSN32EL: #define __STDC__ 1
 // MIPSN32EL: #define __UINT16_C_SUFFIX__
 // MIPSN32EL: #define __UINT16_FMTX__ "hX"
 // MIPSN32EL: #define __UINT16_FMTo__ "ho"
 // MIPSN32EL: #define __UINT16_FMTu__ "hu"
 // MIPSN32EL: #define __UINT16_FMTx__ "hx"
 // MIPSN32EL: #define __UINT16_MAX__ 65535
 // MIPSN32EL: #define __UINT16_TYPE__ unsigned short

 // PPC-DARWIN:#define __SIZEOF_SHORT__ 2
 // PPC-DARWIN:#define __SIZEOF_SIZE_T__ 4
 // PPC-DARWIN:#define __SIZEOF_WCHAR_T__ 4
 // PPC-DARWIN:#define __SIZEOF_WINT_T__ 4
 // PPC-DARWIN:#define __SIZE_MAX__ 4294967295UL
 // PPC-DARWIN:#define __SIZE_TYPE__ long unsigned int
 // PPC-DARWIN:#define __SIZE_WIDTH__ 32
 // PPC-DARWIN:#define __STDC_HOSTED__ 0
-// PPC-DARWIN:#define __STDC_VERSION__ 201112L
+// PPC-DARWIN:#define __STDC_VERSION__ 201710L
 // PPC-DARWIN:#define __STDC__ 1
 // PPC-DARWIN:#define __UINT16_C_SUFFIX__
 // PPC-DARWIN:#define __UINT16_MAX__ 65535
 // PPC-DARWIN:#define __UINT16_TYPE__ unsigned short
 // PPC-DARWIN:#define __UINT32_C_SUFFIX__ U
 // PPC-DARWIN:#define __UINT32_MAX__ 4294967295U
 // PPC-DARWIN:#define __UINT32_TYPE__ unsigned int
 // PPC-DARWIN:#define __UINT64_C_SUFFIX__ ULL

 // X86_64-CLOUDABI:#define __SSE2_MATH__ 1
 // X86_64-CLOUDABI:#define __SSE2__ 1
 // X86_64-CLOUDABI:#define __SSE_MATH__ 1
 // X86_64-CLOUDABI:#define __SSE__ 1
 // X86_64-CLOUDABI:#define __STDC_HOSTED__ 0
 // X86_64-CLOUDABI:#define __STDC_ISO_10646__ 201206L
 // X86_64-CLOUDABI:#define __STDC_UTF_16__ 1
 // X86_64-CLOUDABI:#define __STDC_UTF_32__ 1
-// X86_64-CLOUDABI:#define __STDC_VERSION__ 201112L
+// X86_64-CLOUDABI:#define __STDC_VERSION__ 201710L
 // X86_64-CLOUDABI:#define __STDC__ 1
 // X86_64-CLOUDABI:#define __UINT16_C_SUFFIX__
 // X86_64-CLOUDABI:#define __UINT16_FMTX__ "hX"
 // X86_64-CLOUDABI:#define __UINT16_FMTo__ "ho"
 // X86_64-CLOUDABI:#define __UINT16_FMTu__ "hu"
 // X86_64-CLOUDABI:#define __UINT16_FMTx__ "hx"
 // X86_64-CLOUDABI:#define __UINT16_MAX__ 65535
 // X86_64-CLOUDABI:#define __UINT16_TYPE__ unsigned short

 // WEBASSEMBLY-NEXT:#define __STDC_HOSTED__ 0
 // WEBASSEMBLY-NOT:#define __STDC_MB_MIGHT_NEQ_WC__
 // WEBASSEMBLY-NOT:#define __STDC_NO_ATOMICS__
 // WEBASSEMBLY-NOT:#define __STDC_NO_COMPLEX__
 // WEBASSEMBLY-NOT:#define __STDC_NO_VLA__
 // WEBASSEMBLY-NOT:#define __STDC_NO_THREADS__
 // WEBASSEMBLY-NEXT:#define __STDC_UTF_16__ 1
 // WEBASSEMBLY-NEXT:#define __STDC_UTF_32__ 1
-// WEBASSEMBLY-NEXT:#define __STDC_VERSION__ 201112L
+// WEBASSEMBLY-NEXT:#define __STDC_VERSION__ 201710L
 // WEBASSEMBLY-NEXT:#define __STDC__ 1
 // WEBASSEMBLY-NEXT:#define __UINT16_C_SUFFIX__
 // WEBASSEMBLY-NEXT:#define __UINT16_FMTX__ "hX"
 // WEBASSEMBLY-NEXT:#define __UINT16_FMTo__ "ho"
 // WEBASSEMBLY-NEXT:#define __UINT16_FMTu__ "hu"
 // WEBASSEMBLY-NEXT:#define __UINT16_FMTx__ "hx"
 // WEBASSEMBLY-NEXT:#define __UINT16_MAX__ 65535
 // WEBASSEMBLY-NEXT:#define __UINT16_TYPE__ unsigned short

 // RISCV32: #define __SIZEOF_WCHAR_T__ 4
 // RISCV32: #define __SIZEOF_WINT_T__ 4
 // RISCV32: #define __SIZE_MAX__ 4294967295U
 // RISCV32: #define __SIZE_TYPE__ unsigned int
 // RISCV32: #define __SIZE_WIDTH__ 32
 // RISCV32: #define __STDC_HOSTED__ 0
 // RISCV32: #define __STDC_UTF_16__ 1
 // RISCV32: #define __STDC_UTF_32__ 1
-// RISCV32: #define __STDC_VERSION__ 201112L
+// RISCV32: #define __STDC_VERSION__ 201710L
 // RISCV32: #define __STDC__ 1
 // RISCV32: #define __UINT16_C_SUFFIX__
 // RISCV32: #define __UINT16_MAX__ 65535
 // RISCV32: #define __UINT16_TYPE__ unsigned short
 // RISCV32: #define __UINT32_C_SUFFIX__ U
 // RISCV32: #define __UINT32_MAX__ 4294967295U
 // RISCV32: #define __UINT32_TYPE__ unsigned int
 // RISCV32: #define __UINT64_C_SUFFIX__ ULL

 // RISCV64: #define __SIZEOF_WCHAR_T__ 4
 // RISCV64: #define __SIZEOF_WINT_T__ 4
 // RISCV64: #define __SIZE_MAX__ 18446744073709551615UL
 // RISCV64: #define __SIZE_TYPE__ long unsigned int
 // RISCV64: #define __SIZE_WIDTH__ 64
 // RISCV64: #define __STDC_HOSTED__ 0
 // RISCV64: #define __STDC_UTF_16__ 1
 // RISCV64: #define __STDC_UTF_32__ 1
-// RISCV64: #define __STDC_VERSION__ 201112L
+// RISCV64: #define __STDC_VERSION__ 201710L
 // RISCV64: #define __STDC__ 1
 // RISCV64: #define __UINT16_C_SUFFIX__
 // RISCV64: #define __UINT16_MAX__ 65535
 // RISCV64: #define __UINT16_TYPE__ unsigned short
 // RISCV64: #define __UINT32_C_SUFFIX__ U
 // RISCV64: #define __UINT32_MAX__ 4294967295U
 // RISCV64: #define __UINT32_TYPE__ unsigned int
 // RISCV64: #define __UINT64_C_SUFFIX__ UL

clang/www/compatibility.html

 
 <!-- ======================================================================= -->
 <h2 id="c">C compatibility</h2>
 <!-- ======================================================================= -->
 
 <!-- ======================================================================= -->
 <h3 id="inline">C99 inline functions</h3>
 <!-- ======================================================================= -->
-<p>By default, Clang builds C code in GNU C11 mode, so it uses standard C99
+<p>By default, Clang builds C code in GNU C17 mode, so it uses standard C99
 semantics for the <code>inline</code> keyword. These semantics are different
 from those in GNU C89 mode, which is the default mode in versions of GCC
 prior to 5.0. For example, consider the following code:</p>
 <pre>
 inline int add(int i, int j) { return i + j; }
 
 int main() {
   int i = add(4, 5);