[clang] Better diagnostics when assertion fails in `consteval` (PR #130458)

[JJ Marr via cfe-commits]

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@@ -5975,9 +5975,22 @@ static bool CheckConstexprFunction(EvalInfo &Info, SourceLocation CallLoc,
                                         Definition->hasAttr<MSConstexprAttr>())))
     return true;
 
-  if (Info.getLangOpts().CPlusPlus11) {
-    const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
+  const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
+  // Special note for the assert() macro, as the normal error message falsely
+  // implies we cannot use an assertion during constant evaluation.
+  if (CallLoc.isMacroID() && DiagDecl->getIdentifier()) {
+    // FIXME: Instead of checking for an implementation-defined function,
+    // check and evaluate the assert() macro.
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jj-marr wrote:

I have no idea how to access the preprocessor from this method and I am too inexperienced to know where to start. 
If you can give me a hint on where to look I would appreciate that. Do I just do a big refactor and pass it in as a parameter to the function? Or is there some method on an object I already have? 

https://github.com/llvm/llvm-project/pull/130458