[clang] [clang][bytecode] Limit "... variable whose value is not known" diags (PR #175287)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/175287

To variables with reference types.

>From 5a29e993713bb41f054f14a7dc645db006e62399 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Sat, 10 Jan 2026 07:13:40 +0100
Subject: [PATCH] [clang][bytecode] Limit "... variable whose value is not
 known" diags

To variables with reference types.
---
 clang/lib/AST/ByteCode/Interp.cpp     | 4 ++--
 clang/test/SemaTemplate/deduction.cpp | 5 ++---
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 0a083a4990856..7e901aa65af1c 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -137,7 +137,7 @@ static void diagnoseNonConstVariable(InterpState &S, CodePtr OpPC,
 static bool diagnoseUnknownDecl(InterpState &S, CodePtr OpPC,
                                 const ValueDecl *D) {
   // This function tries pretty hard to produce a good diagnostic. Just skip
-  // tha if nobody will see it anyway.
+  // that if nobody will see it anyway.
   if (!S.diagnosing())
     return false;
 
@@ -151,7 +151,7 @@ static bool diagnoseUnknownDecl(InterpState &S, CodePtr OpPC,
     }
 
     const SourceInfo &Loc = S.Current->getSource(OpPC);
-    if (S.getLangOpts().CPlusPlus23) {
+    if (S.getLangOpts().CPlusPlus23 && D->getType()->isReferenceType()) {
       S.FFDiag(Loc, diag::note_constexpr_access_unknown_variable, 1)
           << AK_Read << D;
       S.Note(D->getLocation(), diag::note_declared_at) << D->getSourceRange();
diff --git a/clang/test/SemaTemplate/deduction.cpp b/clang/test/SemaTemplate/deduction.cpp
index a209615c36479..7be3f732508d7 100644
--- a/clang/test/SemaTemplate/deduction.cpp
+++ b/clang/test/SemaTemplate/deduction.cpp
@@ -1,6 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++17
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++23
+// RUN: %clang_cc1 -fsyntax-only -verify %s %std_cxx11-
+// RUN: %clang_cc1 -fsyntax-only -verify %s %std_cxx11- -fexperimental-new-constant-interpreter
 
 // Template argument deduction with template template parameters.
 template<typename T, template<T> class A>