Tested x86_64-pc-linux-gnu.  Any objections?

-- 8< --

Several PRs complain about -Wswitch warning about a case for a bitwise
combination of enumerators.  Clang has an attribute flag_enum to prevent
this; let's adopt that approach as well.

This also recognizes the attribute as [[clang::flag_enum]], introducing
handling of the clang attribute namespace.

        PR c++/46457

gcc/c-family/ChangeLog:

        * c-attribs.cc (handle_flag_enum_attribute): New.
        (c_common_gnu_attributes): Add it.
        (c_common_clang_attributes, c_common_clang_attribute_table): New.
        * c-common.h: Declare c_common_clang_attribute_table.
        * c-warn.cc (c_do_switch_warnings): Handle flag_enum.

gcc/c/ChangeLog:

        * c-objc-common.h (c_objc_attribute_table): Add
        c_common_clang_attribute_table.

gcc/cp/ChangeLog:

        * cp-objcp-common.h (cp_objcp_attribute_table): Add
        c_common_clang_attribute_table.

gcc/testsuite/ChangeLog:

        * c-c++-common/attr-flag-enum-1.c: New test.

gcc/ChangeLog:

        * doc/extend.texi: Document flag_enum attribute.
        * doc/invoke.texi: Mention flag_enum in -Wswitch.

libstdc++-v3/ChangeLog:

        * include/bits/regex_constants.h: Use flag_enum.
---
 gcc/doc/extend.texi                           |  7 ++++
 gcc/doc/invoke.texi                           | 11 +++---
 gcc/c-family/c-common.h                       |  1 +
 gcc/c/c-objc-common.h                         |  1 +
 gcc/cp/cp-objcp-common.h                      |  1 +
 libstdc++-v3/include/bits/regex_constants.h   |  2 +-
 gcc/c-family/c-attribs.cc                     | 33 +++++++++++++++++
 gcc/c-family/c-warn.cc                        |  4 ++
 gcc/testsuite/c-c++-common/attr-flag-enum-1.c | 37 +++++++++++++++++++
 9 files changed, 91 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/attr-flag-enum-1.c

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 5845bcedf6e..5b9d8c51059 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -9187,6 +9187,13 @@ initialization will result in future breakage.
 GCC emits warnings based on this attribute by default; use
 @option{-Wno-designated-init} to suppress them.
 
+@cindex @code{flag_enum} type attribute
+@item flag_enum
+This attribute may be applied to an enumerated type to indicate that
+its enumerators are used in bitwise operations, so e.g. @option{-Wswitch}
+should not warn about a @code{case} that corresponds to a bitwise
+combination of enumerators.
+
 @cindex @code{hardbool} type attribute
 @item hardbool
 @itemx hardbool (@var{false_value})
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 43afb0984e5..7c6175efbc0 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -7672,9 +7672,9 @@ unless C++14 mode (or newer) is active.
 Warn whenever a @code{switch} statement has an index of enumerated type
 and lacks a @code{case} for one or more of the named codes of that
 enumeration.  (The presence of a @code{default} label prevents this
-warning.)  @code{case} labels outside the enumeration range also
-provoke warnings when this option is used (even if there is a
-@code{default} label).
+warning.)  @code{case} labels that do not correspond to enumerators also
+provoke warnings when this option is used, unless the enumeration is marked
+with the @code{flag_enum} attribute.
 This warning is enabled by @option{-Wall}.
 
 @opindex Wswitch-default
@@ -7688,8 +7688,9 @@ case.
 @item -Wswitch-enum
 Warn whenever a @code{switch} statement has an index of enumerated type
 and lacks a @code{case} for one or more of the named codes of that
-enumeration.  @code{case} labels outside the enumeration range also
-provoke warnings when this option is used.  The only difference
+enumeration.  @code{case} labels that do not correspond to enumerators also
+provoke warnings when this option is used, unless the enumeration is marked
+with the @code{flag_enum} attribute.  The only difference
 between @option{-Wswitch} and this option is that this option gives a
 warning about an omitted enumeration code even if there is a
 @code{default} label.
diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index d3827573a36..027f077d51b 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -821,6 +821,7 @@ extern struct visibility_flags visibility_options;
 
 /* Attribute table common to the C front ends.  */
 extern const struct scoped_attribute_specs c_common_gnu_attribute_table;
+extern const struct scoped_attribute_specs c_common_clang_attribute_table;
 extern const struct scoped_attribute_specs c_common_format_attribute_table;
 
 /* Pointer to function to lazily generate the VAR_DECL for __FUNCTION__ etc.
diff --git a/gcc/c/c-objc-common.h b/gcc/c/c-objc-common.h
index 20af5a5bb94..365b5938803 100644
--- a/gcc/c/c-objc-common.h
+++ b/gcc/c/c-objc-common.h
@@ -79,6 +79,7 @@ static const scoped_attribute_specs *const 
c_objc_attribute_table[] =
 {
   &std_attribute_table,
   &c_common_gnu_attribute_table,
+  &c_common_clang_attribute_table,
   &c_common_format_attribute_table
 };
 
diff --git a/gcc/cp/cp-objcp-common.h b/gcc/cp/cp-objcp-common.h
index 0e6664cf9c3..e9c5ac40020 100644
--- a/gcc/cp/cp-objcp-common.h
+++ b/gcc/cp/cp-objcp-common.h
@@ -128,6 +128,7 @@ static const scoped_attribute_specs *const 
cp_objcp_attribute_table[] =
   &std_attribute_table,
   &cxx_gnu_attribute_table,
   &c_common_gnu_attribute_table,
+  &c_common_clang_attribute_table,
   &c_common_format_attribute_table
 };
 
diff --git a/libstdc++-v3/include/bits/regex_constants.h 
b/libstdc++-v3/include/bits/regex_constants.h
index 437895f1dc3..4148093bc4e 100644
--- a/libstdc++-v3/include/bits/regex_constants.h
+++ b/libstdc++-v3/include/bits/regex_constants.h
@@ -66,7 +66,7 @@ namespace regex_constants
    * elements @c ECMAScript, @c basic, @c extended, @c awk, @c grep, @c egrep
    * %set.
    */
-  enum syntax_option_type : unsigned int
+  enum [[gnu::flag_enum]] syntax_option_type : unsigned int
   {
     _S_icase           = 1 << 0,
     _S_nosubs          = 1 << 1,
diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
index cf27cd6d521..f3cf66ffb85 100644
--- a/gcc/c-family/c-attribs.cc
+++ b/gcc/c-family/c-attribs.cc
@@ -184,6 +184,7 @@ static tree handle_signed_bool_precision_attribute (tree *, 
tree, tree, int,
 static tree handle_hardbool_attribute (tree *, tree, tree, int, bool *);
 static tree handle_retain_attribute (tree *, tree, tree, int, bool *);
 static tree handle_fd_arg_attribute (tree *, tree, tree, int, bool *);
+static tree handle_flag_enum_attribute (tree *, tree, tree, int, bool *);
 static tree handle_null_terminated_string_arg_attribute (tree *, tree, tree, 
int, bool *);
 
 /* Helper to define attribute exclusions.  */
@@ -631,6 +632,8 @@ const struct attribute_spec c_common_gnu_attributes[] =
             handle_fd_arg_attribute, NULL},
   { "fd_arg_write",       1, 1, false, true, true, false,
             handle_fd_arg_attribute, NULL},         
+  { "flag_enum",             0, 0, false, true, false, false,
+                             handle_flag_enum_attribute, NULL },
   { "null_terminated_string_arg", 1, 1, false, true, true, false,
                              handle_null_terminated_string_arg_attribute, NULL}
 };
@@ -640,6 +643,17 @@ const struct scoped_attribute_specs 
c_common_gnu_attribute_table =
   "gnu", { c_common_gnu_attributes }
 };
 
+/* Attributes also recognized in the clang:: namespace.  */
+const struct attribute_spec c_common_clang_attributes[] = {
+  { "flag_enum",             0, 0, false, true, false, false,
+                             handle_flag_enum_attribute, NULL }
+};
+
+const struct scoped_attribute_specs c_common_clang_attribute_table =
+{
+  "clang", { c_common_clang_attributes }
+};
+
 /* Give the specifications for the format attributes, used by C and all
    descendants.
 
@@ -5017,6 +5031,25 @@ handle_fd_arg_attribute (tree *node, tree name, tree 
args,
   return NULL_TREE;
 }
 
+/* Handle the "flag_enum" attribute.  */
+
+static tree
+handle_flag_enum_attribute (tree *node, tree ARG_UNUSED(name), tree args,
+                           int ARG_UNUSED (flags), bool *no_add_attrs)
+{
+  if (args)
+    warning (OPT_Wattributes, "%qE attribute arguments ignored", name);
+  else if (TREE_CODE (*node) != ENUMERAL_TYPE)
+    warning (OPT_Wattributes, "%qE attribute ignored on non-enum", name);
+  else
+    goto ok;
+
+  *no_add_attrs = true;
+
+ ok:
+  return NULL_TREE;
+}
+
 /* Handle the "null_terminated_string_arg" attribute.  */
 
 static tree
diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
index 5e4fb7f0f0a..47e0a6bfa07 100644
--- a/gcc/c-family/c-warn.cc
+++ b/gcc/c-family/c-warn.cc
@@ -1808,6 +1808,10 @@ c_do_switch_warnings (splay_tree cases, location_t 
switch_location,
                  TREE_PURPOSE (chain));
     }
 
+  /* Attribute flag_enum means bitwise combinations are OK.  */
+  if (lookup_attribute ("flag_enum", TYPE_ATTRIBUTES (type)))
+    return;
+
   /* Warn if there are case expressions that don't correspond to
      enumerators.  This can occur since C and C++ don't enforce
      type-checking of assignments to enumeration variables.
diff --git a/gcc/testsuite/c-c++-common/attr-flag-enum-1.c 
b/gcc/testsuite/c-c++-common/attr-flag-enum-1.c
new file mode 100644
index 00000000000..4eb78b1d8ee
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/attr-flag-enum-1.c
@@ -0,0 +1,37 @@
+/* { dg-additional-options -Wswitch } */
+
+enum E0 { a0 = 1, b0 = 2 };
+void f0 (enum E0 e) {
+  switch (e) {
+  case !(a0|b0):               /* { dg-warning "not in enumerated type" } */
+  case a0|b0:                  /* { dg-warning "not in enumerated type" } */
+  default:;
+  }
+}
+
+enum __attribute ((flag_enum)) E1 { a1 = 1, b1 = 2 };
+void f1 (enum E1 e) {
+  switch (e) {
+  case !(a1|b1):               /* { dg-bogus "not in enumerated type" } */
+  case a1|b1:                  /* { dg-bogus "not in enumerated type" } */
+  default:;
+  }
+}
+
+enum [[gnu::flag_enum]] E2 { a2 = 1, b2 = 2 };
+void f2 (enum E2 e) {
+  switch (e) {
+  case !(a2|b2):               /* { dg-bogus "not in enumerated type" } */
+  case a2|b2:                  /* { dg-bogus "not in enumerated type" } */
+  default:;
+  }
+}
+
+enum [[clang::flag_enum]] E3 { a3 = 1, b3 = 2 };
+void f3 (enum E3 e) {
+  switch (e) {
+  case !(a3|b3):               /* { dg-bogus "not in enumerated type" } */
+  case a3|b3:                  /* { dg-bogus "not in enumerated type" } */
+  default:;
+  }
+}

base-commit: 3775f71c8909b3531fe002138814fa2504ec2e8b
-- 
2.46.0

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