libstdc++: Remove #warning from <ciso646> for C++17 [PR120187]

Although <ciso646> was removed from C++20, it was not formally
deprecated in C++17. In contrast, <ctgmath>, <cstdalign>, etc. were
formally deprecated in C++17 before being removed in C++20.

Due to the widespread convention of including <ciso646> to detect
implementation-specific macros (such as _GLIBCXX_RELEASE) it causes
quite a lot of noise to issue deprecation warnings in C++17 mode. The
recommendation to include <version> instead does work for recent
compilers, even in C++17 mode, but isn't portable to older compilers
that don't provide <version> yet (e.g. GCC 8).

There are also potential objections to including <version> pre-C++20
when it wasn't defined by the standard. I don't have much sympathy for
this position, because including <ciso646> for implementation-specific
macros wasn't part of the C++17 standard either. It's no more
non-standard to rely on <version> being present and defining those
macros than to rely on <ciso646> defining them, and __has_include can be
used to detect whether <version> is present. However, <ciso646> is being
used in the wild by popular libraries like Abseil and we can't change
versions of those that have already been released.

This removes the #warning in <ciso646> for C++17 mode, so that we only
emit diagnostics for C++20 and later. With this change, including
<ciso646> in C++20 or later gives an error if _GLIBCXX_USE_DEPRECATED is
defined to zero, otherwise a warning if -Wdeprecated is enabled,
otherwise no diagnostic is given.

This also adds "@since C++11 (removed in C++20)" to the Doxygen @file
comments in all the relevant headers.

The test for <ciso646> needs to be updated to no longer expect a warning
for c++17_only. A new test is added to ensure that we get a warning
instead of an error when -D_GLIBCXX_USE_DEPRECATED=0 is not used.

libstdc++-v3/ChangeLog:

	PR libstdc++/120187
	* include/c_global/ciso646: Only give deprecated warning for
	C++20 and later.
	* include/c_global/ccomplex: Add @since to Doxygen comment.
	* include/c_global/cstdalign: Likewise.
	* include/c_global/cstdbool: Likewise.
	* include/c_global/ctgmath: Likewise.
	* testsuite/18_support/headers/ciso646/macros.cc: Remove
	dg-warning for c++17_only effective target.
	* testsuite/18_support/headers/ciso646/macros-2.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

7 files changed, 26 insertions, 8 deletions

diff --git a/libstdc++-v3/include/c_global/ccomplex b/libstdc++-v3/include/c_global/ccomplex
index 7044cf7..a39273f 100644
--- a/libstdc++-v3/include/c_global/ccomplex
+++ b/libstdc++-v3/include/c_global/ccomplex
@@ -24,6 +24,8 @@
 
 /** @file include/ccomplex
  *  This is a Standard C++ Library header.
+ *
+ *  @since C++11 (removed in C++20)
  */
 
 #ifndef _GLIBCXX_CCOMPLEX
diff --git a/libstdc++-v3/include/c_global/ciso646 b/libstdc++-v3/include/c_global/ciso646
index a663e04..6dec7df 100644
--- a/libstdc++-v3/include/c_global/ciso646
+++ b/libstdc++-v3/include/c_global/ciso646
@@ -28,6 +28,8 @@
  *
  *  This is the C++ version of the Standard C Library header @c iso646.h,
  *  which is empty in C++.
+ *
+ *  @since C++11 (removed in C++20)
  */
 #ifndef _GLIBCXX_CISO646
 #define _GLIBCXX_CISO646
@@ -38,13 +40,15 @@
 
 #include <bits/c++config.h>
 
-#if __cplusplus >= 202002L && ! _GLIBCXX_USE_DEPRECATED
-#  error "<ciso646> is not a standard header in C++20, use <version> to detect implementation-specific macros"
-#elif __cplusplus >= 201703L && defined __DEPRECATED
-#  pragma GCC diagnostic push
-#  pragma GCC diagnostic ignored "-Wc++23-extensions"
-#  warning "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros"
-#  pragma GCC diagnostic pop
+#if __cplusplus >= 202002L
+#  if ! _GLIBCXX_USE_DEPRECATED
+#    error "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
+#  elif defined __DEPRECATED
+#    pragma GCC diagnostic push
+#    pragma GCC diagnostic ignored "-Wc++23-extensions"
+#    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
+#    pragma GCC diagnostic pop
+#  endif
 #endif
 
 #endif
diff --git a/libstdc++-v3/include/c_global/cstdalign b/libstdc++-v3/include/c_global/cstdalign
index 92e0ad6..41ce506 100644
--- a/libstdc++-v3/include/c_global/cstdalign
+++ b/libstdc++-v3/include/c_global/cstdalign
@@ -24,6 +24,8 @@
 
 /** @file include/cstdalign
  *  This is a Standard C++ Library header.
+ *
+ *  @since C++11 (removed in C++20)
  */
 
 #ifndef _GLIBCXX_CSTDALIGN
diff --git a/libstdc++-v3/include/c_global/cstdbool b/libstdc++-v3/include/c_global/cstdbool
index e75f56c..5933d7d 100644
--- a/libstdc++-v3/include/c_global/cstdbool
+++ b/libstdc++-v3/include/c_global/cstdbool
@@ -24,6 +24,8 @@
 
 /** @file include/cstdbool
  *  This is a Standard C++ Library header.
+ *
+ *  @since C++11 (removed in C++20)
  */
 
 #ifndef _GLIBCXX_CSTDBOOL
diff --git a/libstdc++-v3/include/c_global/ctgmath b/libstdc++-v3/include/c_global/ctgmath
index 0a5a0e7..b708878 100644
--- a/libstdc++-v3/include/c_global/ctgmath
+++ b/libstdc++-v3/include/c_global/ctgmath
@@ -24,6 +24,8 @@
 
 /** @file include/ctgmath
  *  This is a Standard C++ Library header.
+ *
+ *  @since C++11 (removed in C++20)
  */
 
 #ifndef _GLIBCXX_CTGMATH
diff --git a/libstdc++-v3/testsuite/18_support/headers/ciso646/macros-2.cc b/libstdc++-v3/testsuite/18_support/headers/ciso646/macros-2.cc
new file mode 100644
index 0000000..a492924
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/headers/ciso646/macros-2.cc
@@ -0,0 +1,7 @@
+// { dg-options " -Wdeprecated -fno-operator-names" }
+// { dg-do preprocess }
+
+// Should get a warning for C++20 and up without -D_GLIBCXX_USE_DEPRECATED=0
+// { dg-warning "not a standard header" "" { target c++20 } 0 }
+
+#include "macros.cc"
diff --git a/libstdc++-v3/testsuite/18_support/headers/ciso646/macros.cc b/libstdc++-v3/testsuite/18_support/headers/ciso646/macros.cc
index ab3a041..0cb5168 100644
--- a/libstdc++-v3/testsuite/18_support/headers/ciso646/macros.cc
+++ b/libstdc++-v3/testsuite/18_support/headers/ciso646/macros.cc
@@ -3,7 +3,6 @@
 
 #include <ciso646>
 
-// { dg-warning "deprecated" "" { target c++17_only } 0 }
 // { dg-error "not a standard header" "" { target c++20 } 0 }
 
 #ifdef and