C++: add -std={c,gnu}++{current,future}

[Nathan Sidwell <nathan@acm.org>]

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Inspired by a user question.  Jason, thoughts?

Since C++ is such a moving target, Microsoft have /std:c++latest
(AFAICT clang does not), to select the currently implemented version
of the working paper.  But the use of 'std:latest' is somewhat
ambiguous -- the current std is C++20 -- that's the latest std, the
next std will more than likely but not necessarily be C++23.  So this
adds:

   -std=c++current -- the current std (c++20)
   -std=c++future -- the working paper (c++2b)

also adds gnu++current and gnu++future to select the gnu-extended
variants.

nathan

-- 
Nathan Sidwell

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From 9671f4d5e7efa130280b6d50fb4e9e8492d5b587 Mon Sep 17 00:00:00 2001
From: Nathan Sidwell <nathan@acm.org>
Date: Wed, 13 Jul 2022 12:11:40 -0700
Subject: [PATCH] C++: add -std={c,gnu}++{current,future}

Since C++ is such a moving target, Microsoft have /std:c++latest
(AFAICT clang does not), to select the currently implemented version
of the working paper.  But the use of 'std:latest' is somewhat
ambiguous -- the current std is C++20 -- that's the latest std, the
next std will more than likely but not necessarily be C++23.  So this
adds:

  -std=c++current -- the current std (c++20)
  -std=c++future -- the working paper (c++2b)

also adds gnu++current and gnu++future to select the gnu-extended
variants.

	gcc/
	* doc/invoke.texi (-std=): Document new c++ current & future
	options.
	gcc/c-family/
	* c.opt (-std={c,gnu}++{current,future}: New alias options.
	gcc/testsuite/
	* g++.dg/gnu-current.C: New.
	* g++.dg/gnu-future.C: New.
	* g++.dg/std-current.C: New.
	* g++.dg/std-future.C: New.
---
 gcc/c-family/c.opt                 | 16 ++++++++++++++++
 gcc/doc/invoke.texi                | 23 +++++++++++++++++++++++
 gcc/testsuite/g++.dg/gnu-current.C |  7 +++++++
 gcc/testsuite/g++.dg/gnu-future.C  |  7 +++++++
 gcc/testsuite/g++.dg/std-current.C | 11 +++++++++++
 gcc/testsuite/g++.dg/std-future.C  |  8 ++++++++
 6 files changed, 72 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/gnu-current.C
 create mode 100644 gcc/testsuite/g++.dg/gnu-future.C
 create mode 100644 gcc/testsuite/g++.dg/std-current.C
 create mode 100644 gcc/testsuite/g++.dg/std-future.C

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 44e1a60ce24..9292029a967 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -2321,6 +2321,14 @@ std=c++23
 C++ ObjC++ Undocumented
 Conform to the ISO 2023 C++ draft standard (experimental and incomplete support).
 
+std=c++current
+C++ ObjC++ Alias(std=c++20) Undocumented
+Conform to the current ISO C++ standard (C++20).
+
+std=c++future
+C++ ObjC++ Alias(std=c++23) Undocumented
+Conform to a future ISO C++ standard (C++2b, experimentatl and incomplete support).
+
 std=c11
 C ObjC
 Conform to the ISO 2011 C standard.
@@ -2407,6 +2415,14 @@ std=gnu++23
 C++ ObjC++ Undocumented
 Conform to the ISO 2023 C++ draft standard with GNU extensions (experimental and incomplete support).
 
+std=gnu++current
+C++ ObjC++ Alias(std=gnu++20) Undocumented
+Conform to the current ISO C++ standard with GNU extensions (C++20).
+
+std=gnu++future
+C++ ObjC++ Alias(std=gnu++23) Undocumented
+Conform to a future ISO C++ standard with GNU extensions (C++2b, experimentatl and incomplete support).
+
 std=gnu11
 C ObjC
 Conform to the ISO 2011 C standard with GNU extensions.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index d5ff1018372..1c0edb9df68 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -2462,6 +2462,17 @@ GNU dialect of @option{-std=c++17}.
 This is the default for C++ code.
 The name @samp{gnu++1z} is deprecated.
 
+@item gnu++current
+@itemx gnu++current
+GNU dialect of the current C++ standard, currently @option{-std=gnu++20}.
+The C++ version selected by this option is a moving target.
+
+@item gnu++future
+@itemx gnu++future
+GNU dialect of the next C++ standard, currently @option{-std=gnu++2b}.
+The C++ version selected by this option is a moving target (as are the
+semantics of that proposed version).
+
 @item c++20
 @itemx c++2a
 The 2020 ISO C++ standard plus amendments.
@@ -2487,6 +2498,18 @@ change in incompatible ways in future releases.
 GNU dialect of @option{-std=c++2b}.  Support is highly experimental,
 and will almost certainly change in incompatible ways in future
 releases.
+
+@item c++current
+@itemx c++current
+The current C++ standard, currently @option{-std=gnu++20}.
+The C++ version selected by this option is a moving target.
+
+@item c++future
+@itemx c++future
+The next C++ standard, currently @option{-std=gnu++2b}.
+The C++ version selected by this option is a moving target (as are the
+semantics of that proposed version).
+
 @end table
 
 @item -aux-info @var{filename}
diff --git a/gcc/testsuite/g++.dg/gnu-current.C b/gcc/testsuite/g++.dg/gnu-current.C
new file mode 100644
index 00000000000..c95c56d3ad8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/gnu-current.C
@@ -0,0 +1,7 @@
+// { dg-do compile }
+// { dg-options -std=gnu++current }
+
+static_assert (__cplusplus == 202002L, "time has moved on");
+#if __STRICT_ANSI__
+#error "__STRICT_ANSI__ surprisingly non-zero"
+#endif
diff --git a/gcc/testsuite/g++.dg/gnu-future.C b/gcc/testsuite/g++.dg/gnu-future.C
new file mode 100644
index 00000000000..366dcdeebc2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/gnu-future.C
@@ -0,0 +1,7 @@
+// { dg-do compile }
+// { dg-options -std=gnu++future }
+
+static_assert (__cplusplus > 202002L, "time has moved on");
+#if __STRICT_ANSI__
+#error "__STRICT_ANSI__ surprisingly non-zero"
+#endif
diff --git a/gcc/testsuite/g++.dg/std-current.C b/gcc/testsuite/g++.dg/std-current.C
new file mode 100644
index 00000000000..334ffd1d2c4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/std-current.C
@@ -0,0 +1,11 @@
+// { dg-do compile }
+// { dg-options -std=c++current }
+
+// Adust as tomorrow and tomorrow and tomorrow
+// Creeps in this petty pace from day to day
+// To the last syllable of recorded time.
+
+static_assert (__cplusplus == 202002L, "time has moved on");
+#if !__STRICT_ANSI__
+#error "__STRICT_ANSI__ surprisingly zero"
+#endif
diff --git a/gcc/testsuite/g++.dg/std-future.C b/gcc/testsuite/g++.dg/std-future.C
new file mode 100644
index 00000000000..dade97e6a71
--- /dev/null
+++ b/gcc/testsuite/g++.dg/std-future.C
@@ -0,0 +1,8 @@
+// { dg-do compile }
+// { dg-options -std=c++future }
+
+static_assert (__cplusplus > 202002L, "time has moved on");
+#if !__STRICT_ANSI__
+#error "__STRICT_ANSI__ surprisingly zero"
+#endif
+
-- 
2.30.2