[clang] [clang][bytecode] Check diagnosing() before calling diagnoseUnknownDecl() (PR #224906)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/224906

We can save some small preparations this way, e.g. calling `getRootVarDecl()`.

>From f0f2a53b052e60fb77d7a134070ac8cb4263c84e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Sun, 20 Sep 2026 12:14:04 +0200
Subject: [PATCH] checkdummy

---
 clang/lib/AST/ByteCode/Interp.cpp | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 49ebcc789b7dd..7afed0e05c886 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -129,9 +129,8 @@ static void diagnoseNonConstVariable(InterpState &S, CodePtr OpPC,
 static bool diagnoseUnknownDecl(InterpState &S, CodePtr OpPC,
                                 const ValueDecl *D, AccessKinds AK = AK_Read) {
   // This function tries pretty hard to produce a good diagnostic. Just skip
-  // that if nobody will see it anyway.
-  if (!S.diagnosing())
-    return false;
+  // that if nobody will see it anyway. This should be handled in the caller.
+  assert(S.diagnosing());
 
   if (isa<ParmVarDecl>(D)) {
     if (D->getType()->isReferenceType()) {
@@ -1346,6 +1345,9 @@ bool CheckDeleteSource(InterpState &S, CodePtr OpPC, const Expr *Source,
 /// We aleady know the given DeclRefExpr is invalid for some reason,
 /// now figure out why and print appropriate diagnostics.
 bool CheckDeclRef(InterpState &S, CodePtr OpPC, const DeclRefExpr *DR) {
+  if (!S.diagnosing())
+    return false;
+
   const ValueDecl *D = DR->getDecl();
   return diagnoseUnknownDecl(S, OpPC, D);
 }
@@ -1370,12 +1372,15 @@ bool CheckDummy(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
   if (!Ptr.isDummy())
     return true;
 
-  const VarDecl *D = Ptr.getRootVarDecl();
-  if (!D)
+  if (!S.diagnosing())
     return false;
 
-  if (AK == AK_Read || AK == AK_Increment || AK == AK_Decrement)
+  if (AK == AK_Read || AK == AK_Increment || AK == AK_Decrement) {
+    const VarDecl *D = Ptr.getRootVarDecl();
+    if (!D)
+      return false;
     return diagnoseUnknownDecl(S, OpPC, D, AK);
+  }
 
   if (AK == AK_Destroy || S.getLangOpts().CPlusPlus14)
     S.FFDiag(S.Current->getSource(OpPC), diag::note_constexpr_modify_global);