gcc, libcpp: Add warning switch for "#pragma once in main file" [PR89808]

This patch adds a warning switch for "#pragma once in main file".  The
warning option name is Wpragma-once-outside-header, which is the same
as Clang provides.

	PR preprocessor/89808

gcc/c-family/ChangeLog:

	* c.opt (Wpragma_once_outside_header): Define new option.
	* c.opt.urls: Regenerate.

gcc/ChangeLog:

	* doc/invoke.texi (Warning Options): Document
	-Wno-pragma-once-outside-header.

libcpp/ChangeLog:

	* include/cpplib.h (cpp_warning_reason): Define
	CPP_W_PRAGMA_ONCE_OUTSIDE_HEADER.
	* directives.cc (do_pragma_once): Use
	CPP_W_PRAGMA_ONCE_OUTSIDE_HEADER.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wno-pragma-once-outside-header.C: New test.
	* g++.dg/warn/Wpragma-once-outside-header.C: New test.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Marek Polacek <polacek@redhat.com>

gcc/c-family/c.opt

@@ -1214,6 +1214,10 @@ Wpragmas
 C ObjC C++ ObjC++ Var(warn_pragmas) Init(1) Warning
 Warn about misuses of pragmas.
 
+Wpragma-once-outside-header
+C ObjC C++ ObjC++ Var(warn_pragma_once_outside_header) CppReason(CPP_W_PRAGMA_ONCE_OUTSIDE_HEADER) Init(1) Warning
+Warn about #pragma once outside of a header.
+
 Wprio-ctor-dtor
 C ObjC C++ ObjC++ Var(warn_prio_ctor_dtor) Init(1) Warning
 Warn if constructor or destructors with priorities from 0 to 100 are used.

gcc/c-family/c.opt.urls

@@ -684,6 +684,9 @@ UrlSuffix(gcc/Warning-Options.html#index-Wno-pointer-to-int-cast)
 Wpragmas
 UrlSuffix(gcc/Warning-Options.html#index-Wno-pragmas)
 
+Wpragma-once-outside-header
+UrlSuffix(gcc/Warning-Options.html#index-Wno-pragma-once-outside-header)
+
 Wprio-ctor-dtor
 UrlSuffix(gcc/Warning-Options.html#index-Wno-prio-ctor-dtor)
 

gcc/doc/invoke.texi

@@ -395,8 +395,8 @@ Objective-C and Objective-C++ Dialects}.
 -Wpacked  -Wno-packed-bitfield-compat  -Wpacked-not-aligned  -Wpadded
 -Wparentheses  -Wno-pedantic-ms-format
 -Wpointer-arith  -Wno-pointer-compare  -Wno-pointer-to-int-cast
--Wno-pragmas  -Wno-prio-ctor-dtor  -Wredundant-decls
--Wrestrict  -Wno-return-local-addr  -Wreturn-type
+-Wno-pragmas  -Wno-pragma-once-outside-header  -Wno-prio-ctor-dtor
+-Wredundant-decls  -Wrestrict  -Wno-return-local-addr  -Wreturn-type
 -Wno-scalar-storage-order  -Wsequence-point
 -Wshadow  -Wshadow=global  -Wshadow=local  -Wshadow=compatible-local
 -Wno-shadow-ivar
@@ -8147,6 +8147,12 @@ Do not warn about misuses of pragmas, such as incorrect parameters,
 invalid syntax, or conflicts between pragmas.  See also
 @option{-Wunknown-pragmas}.
 
+@opindex Wno-pragma-once-outside-header
+@opindex Wpragma-once-outside-header
+@item -Wno-pragma-once-outside-header
+Do not warn when @code{#pragma once} is used in a file that is not a header
+file, such as a main file.
+
 @opindex Wno-prio-ctor-dtor
 @opindex Wprio-ctor-dtor
 @item -Wno-prio-ctor-dtor

gcc/testsuite/g++.dg/warn/Wno-pragma-once-outside-header.C

@@ -0,0 +1,5 @@
+// { dg-do assemble  }
+// { dg-options "-Wno-pragma-once-outside-header" }
+
+#pragma once
+int main() {}

gcc/testsuite/g++.dg/warn/Wpragma-once-outside-header.C

@@ -0,0 +1,6 @@
+// { dg-do assemble  }
+// { dg-options "-Werror=pragma-once-outside-header" }
+// { dg-message "some warnings being treated as errors" "" {target "*-*-*"} 0 }
+
+#pragma once  // { dg-error "'#pragma once' in main file" }
+int main() {}

libcpp/directives.cc

@@ -2074,7 +2074,8 @@ static void
 do_pragma_once (cpp_reader *pfile)
 {
   if (_cpp_in_main_source_file (pfile))
-    cpp_error (pfile, CPP_DL_WARNING, "#pragma once in main file");
+    cpp_warning (pfile, CPP_W_PRAGMA_ONCE_OUTSIDE_HEADER,
+		 "%<#pragma once%> in main file");
 
   check_eol (pfile, false);
   _cpp_mark_file_once_only (pfile, pfile->buffer->file);

libcpp/include/cpplib.h

@@ -714,7 +714,8 @@ enum cpp_warning_reason {
   CPP_W_BIDIRECTIONAL,
   CPP_W_INVALID_UTF8,
   CPP_W_UNICODE,
-  CPP_W_HEADER_GUARD
+  CPP_W_HEADER_GUARD,
+  CPP_W_PRAGMA_ONCE_OUTSIDE_HEADER
 };
 
 /* Callback for header lookup for HEADER, which is the name of a