[clang] [Clang] Fix warning for non std functions with name `infinity` (PR #123417)

[Zahira Ammarguellat via cfe-commits]

================
@@ -8454,26 +8454,43 @@ static bool IsInfOrNanFunction(StringRef calleeName, MathCheck Check) {
   llvm_unreachable("unknown MathCheck");
 }
 
+static bool IsInfinityFunction(const FunctionDecl *FDecl) {
+  if (FDecl->getName() != "infinity")
+    return false;
+
+  if (const CXXMethodDecl *MDecl = dyn_cast<CXXMethodDecl>(FDecl)) {
+    const CXXRecordDecl *RDecl = MDecl->getParent();
+    if (RDecl->getName() != "numeric_limits")
+      return false;
+
+    if (const NamespaceDecl *NSDecl =
+            dyn_cast<NamespaceDecl>(RDecl->getDeclContext()))
+      return NSDecl->isStdNamespace();
+  }
+
+  return false;
+}
+
 void Sema::CheckInfNaNFunction(const CallExpr *Call,
                                const FunctionDecl *FDecl) {
+  if (!FDecl->getIdentifier())
----------------
zahiraam wrote:

Wouldn't something like this work?
```suggestion
  if (!FDecl->getIdentifier())
  return;
FPOptions FPO = Call->getFPFeaturesInEffect(getLangOpts());
bool IsNaNOrIsUnordered =
    IsStdFunction(FDecl, "isnan") || IsStdFunction(FDecl, "isunordered");
bool IsSpecialNaN =
    IsInfOrNanFunction(FDecl->getName(), MathCheck::NaN);
bool IsInfOrIsFinite =
    IsStdFunction(FDecl, "isinf") || IsStdFunction(FDecl, "isfinite");
bool IsInfinityOrIsSpecialInf =
    IsInfOrNanFunction(FDecl->getName(), MathCheck::Inf);

if ((IsNaNOrIsUnordered || IsSpecialNaN) && FPO.getNoHonorNaNs()) {
  Diag(Call->getBeginLoc(), diag::warn_fp_nan_inf_when_disabled)
      << 1 << 0 << Call->getSourceRange();
  return;
}
if ((IsInfOrIsFinite || IsInfinityFunction(FDecl) ||
     IsInfinityOrIsSpecialInf) &&
    FPO.getNoHonorInfs()) {
  Diag(Call->getBeginLoc(), diag::warn_fp_nan_inf_when_disabled)
      << 0 << 0 << Call->getSourceRange();
}
```

https://github.com/llvm/llvm-project/pull/123417