Add -std=c2x, -std=gnu2x, -Wc11-c2x-compat, C2X _Static_assert support.

Now new features are starting to be added to a C2X draft (in the C2x
branch of the C standard git repository, no public WG14 document yet),
it's time to add -std=c2x and associated options to GCC for use in
enabling C2X features.

This patch adds the expected set of options: -std=c2x, -std=gnu2x,
-Wc11-c2x-compat.  A first C2X feature is added (the only one so far
in the repository that's obviously relevant to GCC): support (as in
C++) for the string constant to be omitted in _Static_assert.  This
feature is duly also supported as an extension in earlier standard
modes (diagnosed with -pedantic, unless -Wno-c11-c2x-compat is given,
or with -Wc11-c2x-compat even in C2X mode).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/
	* doc/cpp.texi (__STDC_VERSION__): Document C2X handling.
	* doc/invoke.texi (-std=c2x, -std=gnu2x): Document new options.
	* doc/standards.texi (C Language): Document C2X.
	* dwarf2out.c (highest_c_language), config/rl78/rl78.c
	(rl78_option_override): Handle "GNU C2X" language name.

gcc/c/
	* c-errors.c (pedwarn_c11): New function.
	* c-parser.c (disable_extension_diagnostics): Save
	warn_c11_c2x_compat and set it to 0.
	(restore_extension_diagnostics): Restore warn_c11_c2x_compat.
	(c_parser_static_assert_declaration_no_semi): Handle
	_Static_assert without string constant.
	* c-tree.h (pedwarn_c11): New prototype.

gcc/c-family/
	* c-common.c (flag_isoc2x): New variable.
	* c-common.h (clk_c): Update comment to reference C2X.
	(flag_isoc99, flag_isoc11): Update comments to reference future
	standard versions in general.
	(flag_isoc2x): Declare.
	* c-opts.c (set_std_c2x): New function.
	(c_common_handle_option): Handle -std=c2x and -std=gnu2x.
	(set_std_c89, set_std_c99, set_std_c11, set_std_c17): Set
	flag_isoc2x to 0.
	* c.opt (Wc11-c2x-compat, std=c2x, std=gnu2x): New options.

gcc/testsuite/
	* gcc.dg/c11-static-assert-7.c, gcc.dg/c11-static-assert-8.c,
	gcc.dg/c11-static-assert-9.c, gcc.dg/c2x-static-assert-1.c,
	gcc.dg/c2x-static-assert-2.c, gcc.dg/c99-static-assert-2.c,
	gcc.dg/gnu2x-static-assert-1.c: New tests.
	* gcc.dg/missing-symbol-3.c: Update expected fix-it text.

libcpp/
	* include/cpplib.h (enum c_lang): Add CLK_GNUC2X and CLK_STDC2X.
	* init.c (lang_defaults): Add GNUC2X and STDC2X entries.
	(cpp_init_builtins): Define __STDC_VERSION__ to 202000L for C2X.

From-SVN: r265251

3 files changed, 17 insertions, 1 deletions

diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index 5345574..cb5d5bd 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -1887,7 +1887,8 @@ the 1999 revision of the C standard; the value @code{201112L}
 signifies the 2011 revision of the C standard; the value
 @code{201710L} signifies the 2017 revision of the C standard (which is
 otherwise identical to the 2011 version apart from correction of
-defects).
+defects).  An unspecified value larger than @code{201710L} is used for
+the experimental @option{-std=c2x} and @option{-std=gnu2x} modes.
 
 This macro is not defined if the @option{-traditional-cpp} option is
 used, nor when compiling C++ or Objective-C@.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 7b08076..bf8bcfb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -1933,6 +1933,10 @@ same as C11 except for corrections of defects (all of which are also
 applied with @option{-std=c11}) and a new value of
 @code{__STDC_VERSION__}, and so is supported to the same extent as C11.
 
+@item c2x
+The next version of the ISO C standard, still under development.  The
+support for this version is experimental and incomplete.
+
 @item gnu90
 @itemx gnu89
 GNU dialect of ISO C90 (including some C99 features).
@@ -1950,6 +1954,11 @@ The name @samp{gnu1x} is deprecated.
 @itemx gnu18
 GNU dialect of ISO C17.  This is the default for C code.
 
+@item gnu2x
+The next version of the ISO C standard, still under development, plus
+GNU extensions.  The support for this version is experimental and
+incomplete.
+
 @item c++98
 @itemx c++03
 The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
diff --git a/gcc/doc/standards.texi b/gcc/doc/standards.texi
index bd9ff6b..26350cc 100644
--- a/gcc/doc/standards.texi
+++ b/gcc/doc/standards.texi
@@ -38,6 +38,8 @@ with some exceptions, and possibly with some extensions.
 @cindex C1X
 @cindex ISO C17
 @cindex C17
+@cindex ISO C2X
+@cindex C2X
 @cindex Technical Corrigenda
 @cindex TC1
 @cindex Technical Corrigendum 1
@@ -109,6 +111,10 @@ known as @dfn{C17} and is supported with @option{-std=c17} or
 @option{-std=c11}, and the only difference between the options is the
 value of @code{__STDC_VERSION__}.
 
+A further version of the C standard, known as @dfn{C2X}, is under
+development; experimental and incomplete support for this is enabled
+with @option{-std=c2x}.
+
 By default, GCC provides some extensions to the C language that, on
 rare occasions conflict with the C standard.  @xref{C
 Extensions,,Extensions to the C Language Family}.