Add diagnostic groups for attribute extensions

Some users have a need to control attribute extension diagnostics
independent of other extension diagnostics. Consider something like use
of [[nodiscard]] within C++11:
```
[[nodiscard]]
int f();
```
If compiled with -Wc++17-extensions enabled, this will produce warning:
use of the 'nodiscard' attribute is a C++17 extension. This diagnostic
is correct -- using [[nodiscard]] in C++11 mode is a C++17 extension.
And the behavior of __has_cpp_attribute(nodiscard) is also correct --
we support [[nodiscard]] in C++11 mode as a conforming extension. But
this makes use of -Werror or -pedantic-errors` builds more onerous.

This patch adds diagnostic groups for attribute extensions so that
users can selectively disable attribute extension diagnostics. I
believe this is preferable to requiring users to specify additional
flags because it means -Wc++17-extensions continues to be the way we
enable all C++17-related extension diagnostics. It would be quite easy
for someone to use that flag thinking they're protected from some
portability issues without realizing it skipped attribute extensions if
we went the other way.

This addresses PR33518.

4 files changed, 33 insertions, 6 deletions

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 5ca1e1e2a889..6b0ce5bfd595 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -53,6 +53,11 @@ Improvements to Clang's diagnostics
 
 - -Wbitwise-instead-of-logical (part of -Wbool-operation) warns about use of bitwise operators with boolean operands which have side effects.
 
+- Added diagnostic groups to control diagnostics for attribute extensions by
+  adding groups ``-Wc++N-attribute-extensions`` (where ``N`` is the standard
+  release being diagnosed against). These new groups are automatically implied
+  when passing ``-Wc++N-extensions``. Resolves PR33518.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 22cd9344d193..c0642efaee4e 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1046,6 +1046,13 @@ def : DiagGroup<"unused-local-typedefs", [UnusedLocalTypedef]>;
 def NonGCC : DiagGroup<"non-gcc",
     [SignCompare, Conversion, LiteralRange]>;
 
+def CXX14Attrs : DiagGroup<"c++14-attribute-extensions">;
+def CXX17Attrs : DiagGroup<"c++17-attribute-extensions">;
+def CXX20Attrs : DiagGroup<"c++20-attribute-extensions">;
+def FutureAttrs : DiagGroup<"future-attribute-extensions", [CXX14Attrs,
+                                                            CXX17Attrs,
+                                                            CXX20Attrs]>;
+
 // A warning group for warnings about using C++11 features as extensions in
 // earlier C++ versions.
 def CXX11 : DiagGroup<"c++11-extensions", [CXX11ExtraSemi, CXX11InlineNamespace,
@@ -1053,15 +1060,15 @@ def CXX11 : DiagGroup<"c++11-extensions", [CXX11ExtraSemi, CXX11InlineNamespace,
 
 // A warning group for warnings about using C++14 features as extensions in
 // earlier C++ versions.
-def CXX14 : DiagGroup<"c++14-extensions", [CXX14BinaryLiteral]>;
+def CXX14 : DiagGroup<"c++14-extensions", [CXX14BinaryLiteral, CXX14Attrs]>;
 
 // A warning group for warnings about using C++17 features as extensions in
 // earlier C++ versions.
-def CXX17 : DiagGroup<"c++17-extensions">;
+def CXX17 : DiagGroup<"c++17-extensions", [CXX17Attrs]>;
 
 // A warning group for warnings about using C++20 features as extensions in
 // earlier C++ versions.
-def CXX20 : DiagGroup<"c++20-extensions", [CXX20Designator]>;
+def CXX20 : DiagGroup<"c++20-extensions", [CXX20Designator, CXX20Attrs]>;
 
 // A warning group for warnings about using C++2b features as extensions in
 // earlier C++ versions.
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 640286d684e8..2ab99f5f4c9e 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8626,11 +8626,11 @@ def warn_unused_volatile : Warning<
   InGroup<DiagGroup<"unused-volatile-lvalue">>;
 
 def ext_cxx14_attr : Extension<
-  "use of the %0 attribute is a C++14 extension">, InGroup<CXX14>;
+  "use of the %0 attribute is a C++14 extension">, InGroup<CXX14Attrs>;
 def ext_cxx17_attr : Extension<
-  "use of the %0 attribute is a C++17 extension">, InGroup<CXX17>;
+  "use of the %0 attribute is a C++17 extension">, InGroup<CXX17Attrs>;
 def ext_cxx20_attr : Extension<
-  "use of the %0 attribute is a C++20 extension">, InGroup<CXX20>;
+  "use of the %0 attribute is a C++20 extension">, InGroup<CXX20Attrs>;
 
 def warn_unused_comparison : Warning<
   "%select{equality|inequality|relational|three-way}0 comparison result unused">,
diff --git a/clang/test/SemaCXX/attr-extension-diags.cpp b/clang/test/SemaCXX/attr-extension-diags.cpp
new file mode 100644
index 000000000000..9a5f6977396c
--- /dev/null
+++ b/clang/test/SemaCXX/attr-extension-diags.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++11 -verify=ext -fsyntax-only -Wfuture-attribute-extensions %s
+// RUN: %clang_cc1 -std=c++11 -verify -fsyntax-only -Wno-future-attribute-extensions %s
+// RUN: %clang_cc1 -std=c++11 -verify -fsyntax-only -Wno-c++14-attribute-extensions -Wno-c++17-attribute-extensions -Wno-c++20-attribute-extensions %s
+
+// expected-no-diagnostics
+
+[[deprecated]] int func1(); // ext-warning {{use of the 'deprecated' attribute is a C++14 extension}}
+[[deprecated("msg")]] int func2(); // ext-warning {{use of the 'deprecated' attribute is a C++14 extension}}
+
+[[nodiscard]] int func3(); // ext-warning {{use of the 'nodiscard' attribute is a C++17 extension}}
+[[nodiscard("msg")]] int func4(); // ext-warning {{use of the 'nodiscard' attribute is a C++20 extension}}
+
+void func5() {
+  if (true) [[likely]]; // ext-warning {{use of the 'likely' attribute is a C++20 extension}}
+}