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

[Amr Hesham 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())
+    return;
+
   FPOptions FPO = Call->getFPFeaturesInEffect(getLangOpts());
-  bool HasIdentifier = FDecl->getIdentifier() != nullptr;
-  bool IsNaNOrIsUnordered =
-      IsStdFunction(FDecl, "isnan") || IsStdFunction(FDecl, "isunordered");
-  bool IsSpecialNaN =
-      HasIdentifier && IsInfOrNanFunction(FDecl->getName(), MathCheck::NaN);
-  if ((IsNaNOrIsUnordered || IsSpecialNaN) && FPO.getNoHonorNaNs()) {
+  if (FPO.getNoHonorNaNs() &&
+      (IsStdFunction(FDecl, "isnan") || IsStdFunction(FDecl, "isunordered") ||
+       IsInfOrNanFunction(FDecl->getName(), MathCheck::NaN))) {
----------------
AmrDeveloper wrote:

I don't mind but my point was that functions are readable for example, for the first boolean, we can easily know the meaning from the condition itself.

```cpp
bool IsInfOrIsFinite =
        IsStdFunction(FDecl, "isinf") || IsStdFunction(FDecl, "isfinite");
    bool IsInfinityOrIsSpecialInf =
        HasIdentifier && ((FDecl->getName() == "infinity") ||
                          IsInfOrNanFunction(FDecl->getName(), MathCheck::Inf));
```


Also I am thinking of performing the checking only if `FPO.getNoHonorInfs()` is true, so to do this with variables either I need to introduce another variable for each case to perform other conditions then if it true, or merge it with `IsInfOrIsFinite` so it will be like `IsNoHonorInfsAndIsInfOrIsFinite` and `IsNoHonorInfsAndIsInfinityOrIsSpecialInf` then in the if condition makes sure one of them is true and I think that's less readable that the current code.


What do you think? @zahiraam 

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