[clang] [clang][bytecode] Don't diagnose const assignments... (PR #192593)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/192593

... when we're in CPCE mode.

>From 68781e1e325e847dce870dcf81cd4351e6e08444 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Fri, 17 Apr 2026 07:01:19 +0200
Subject: [PATCH] [clang][bytecode] Don't diagnose const assignments...

... when we're in CPCE mode.
---
 clang/lib/AST/ByteCode/Interp.cpp | 8 +++++---
 clang/test/AST/ByteCode/cxx20.cpp | 9 +++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 17d194355208d..f5fb9f6b7bbfa 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -579,9 +579,11 @@ bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   if (llvm::is_contained(S.InitializingBlocks, Ptr.block()))
     return true;
 
-  const QualType Ty = Ptr.getType();
-  const SourceInfo &Loc = S.Current->getSource(OpPC);
-  S.FFDiag(Loc, diag::note_constexpr_modify_const_type) << Ty;
+  if (!S.checkingPotentialConstantExpression()) {
+    const QualType Ty = Ptr.getType();
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.FFDiag(Loc, diag::note_constexpr_modify_const_type) << Ty;
+  }
   return false;
 }
 
diff --git a/clang/test/AST/ByteCode/cxx20.cpp b/clang/test/AST/ByteCode/cxx20.cpp
index 4c4624d7505e2..43115416451ec 100644
--- a/clang/test/AST/ByteCode/cxx20.cpp
+++ b/clang/test/AST/ByteCode/cxx20.cpp
@@ -1345,3 +1345,12 @@ namespace ExpandOnOPTEPointers {
   }
   static_assert(test());
 }
+
+namespace ConstIntPotentialConstantExpr {
+  /// NO error about a constexpr function that's never a constant expression.
+  constexpr int Const() {
+    const int a = 10; // both-note {{declared const here}}
+    a = 20; // both-error {{cannot assign to variable 'a' with const-qualified type 'const int'}}
+    return 1;
+  }
+}