[clang] [Clang] [NFC] Introduce helpers for defining compatibilty warnings (PR #132129)

[Aaron Ballman via cfe-commits]

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@@ -155,6 +155,38 @@ class DefaultWarnNoWerror {
 }
 class DefaultRemark { Severity DefaultSeverity = SEV_Remark; }
 
+// C++ compatibility warnings.
+multiclass CXXCompat<
+    string message,
+    int std_ver,
+    bit ext_warn = true,
+    string std_ver_override = ""#std_ver> {
+    // 'X is a C++YZ extension'.
+    def compat_pre_cxx#std_ver#_#NAME :
+        Diagnostic<!strconcat(message, " a C++", std_ver_override,  " extension"),
+                   CLASS_EXTENSION,
+                   !if(ext_warn, SEV_Warning, SEV_Ignored)>,
+        InGroup<!cast<DiagGroup>("CXX"#std_ver)>;
+
+    // 'X is incompatible with C++98' (if std_ver == 11).
+    // 'X is incompatible with C++ standards before C++YZ' (otherwise).
+    def compat_cxx#std_ver#_#NAME :
+        Warning<!if(!eq(std_ver, 11),
+                    !strconcat(message, " incompatible with C++98"),
+                    !strconcat(message, " incompatible with C++ standards before C++", std_ver_override))>,
+        InGroup<!cast<DiagGroup>(!if(!eq(std_ver, 11),
+                                     "CXX98Compat",
+                                     "CXXPre"#std_ver#"Compat"))>,
+        DefaultIgnore;
+}
+
+multiclass CXX11Compat<string message, bit ext_warn = true> : CXXCompat<message, 11, ext_warn>;
----------------
AaronBallman wrote:

The reason it's beneficial is because it helps the reader understand "this code is an extension" vs "this code is standard functionality". We could also accomplish that with the helper function call rather than the diagnostic name itself, though.

https://github.com/llvm/llvm-project/pull/132129