[clang] [clang][bytecode] Don't diagnose const extern reads in CPCE mode (PR #137285)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/137285

They might become constexpr later.

>From af0ed78bbab6b944d51b35ab2d2055c9f781662c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Fri, 25 Apr 2025 07:40:30 +0200
Subject: [PATCH] [clang][bytecode] Don't diagnose const extern reads in CPCE
 mode

They might become constexpr later.
---
 clang/lib/AST/ByteCode/Descriptor.cpp | 10 +++++-----
 clang/lib/AST/ByteCode/Descriptor.h   |  2 +-
 clang/lib/AST/ByteCode/Interp.cpp     | 14 ++++++++------
 clang/lib/AST/ByteCode/Program.cpp    |  8 ++++----
 4 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Descriptor.cpp b/clang/lib/AST/ByteCode/Descriptor.cpp
index 6bc51596f441f..fc389e1c18c66 100644
--- a/clang/lib/AST/ByteCode/Descriptor.cpp
+++ b/clang/lib/AST/ByteCode/Descriptor.cpp
@@ -358,13 +358,13 @@ Descriptor::Descriptor(const DeclTy &D, PrimType Type, MetadataSize MD,
 
 /// Primitive unknown-size arrays.
 Descriptor::Descriptor(const DeclTy &D, PrimType Type, MetadataSize MD,
-                       bool IsTemporary, UnknownSize)
+                       bool IsTemporary, bool IsConst, UnknownSize)
     : Source(D), ElemSize(primSize(Type)), Size(UnknownSizeMark),
       MDSize(MD.value_or(0)),
-      AllocSize(MDSize + sizeof(InitMapPtr) + alignof(void *)), IsConst(true),
-      IsMutable(false), IsTemporary(IsTemporary), IsArray(true),
-      CtorFn(getCtorArrayPrim(Type)), DtorFn(getDtorArrayPrim(Type)),
-      MoveFn(getMoveArrayPrim(Type)) {
+      AllocSize(MDSize + sizeof(InitMapPtr) + alignof(void *)),
+      IsConst(IsConst), IsMutable(false), IsTemporary(IsTemporary),
+      IsArray(true), CtorFn(getCtorArrayPrim(Type)),
+      DtorFn(getDtorArrayPrim(Type)), MoveFn(getMoveArrayPrim(Type)) {
   assert(Source && "Missing source");
 }
 
diff --git a/clang/lib/AST/ByteCode/Descriptor.h b/clang/lib/AST/ByteCode/Descriptor.h
index 532b407c2c788..251443475efc8 100644
--- a/clang/lib/AST/ByteCode/Descriptor.h
+++ b/clang/lib/AST/ByteCode/Descriptor.h
@@ -184,7 +184,7 @@ struct Descriptor final {
              bool IsConst, bool IsTemporary, bool IsMutable);
 
   /// Allocates a descriptor for an array of primitives of unknown size.
-  Descriptor(const DeclTy &D, PrimType Type, MetadataSize MDSize,
+  Descriptor(const DeclTy &D, PrimType Type, MetadataSize MDSize, bool IsConst,
              bool IsTemporary, UnknownSize);
 
   /// Allocates a descriptor for an array of composites.
diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 23fd894bd6a3e..9d7cea0de0182 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -396,10 +396,12 @@ bool CheckExtern(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
       (Ptr.getDeclDesc()->asVarDecl() == S.EvaluatingDecl))
     return true;
 
-  if (!S.checkingPotentialConstantExpression() && S.getLangOpts().CPlusPlus) {
-    const auto *VD = Ptr.getDeclDesc()->asValueDecl();
-    diagnoseNonConstVariable(S, OpPC, VD);
-  }
+  if (S.checkingPotentialConstantExpression() && S.getLangOpts().CPlusPlus &&
+      Ptr.isConst())
+    return false;
+
+  const auto *VD = Ptr.getDeclDesc()->asValueDecl();
+  diagnoseNonConstVariable(S, OpPC, VD);
   return false;
 }
 
@@ -740,12 +742,12 @@ bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
                AccessKinds AK) {
   if (!CheckLive(S, OpPC, Ptr, AK))
     return false;
+  if (!CheckExtern(S, OpPC, Ptr))
+    return false;
   if (!CheckConstant(S, OpPC, Ptr))
     return false;
   if (!CheckDummy(S, OpPC, Ptr, AK))
     return false;
-  if (!CheckExtern(S, OpPC, Ptr))
-    return false;
   if (!CheckRange(S, OpPC, Ptr, AK))
     return false;
   if (!CheckActive(S, OpPC, Ptr, AK))
diff --git a/clang/lib/AST/ByteCode/Program.cpp b/clang/lib/AST/ByteCode/Program.cpp
index fdb7b960ff06d..2d9ed58effe16 100644
--- a/clang/lib/AST/ByteCode/Program.cpp
+++ b/clang/lib/AST/ByteCode/Program.cpp
@@ -166,11 +166,11 @@ unsigned Program::getOrCreateDummy(const DeclTy &D) {
 
   Descriptor *Desc;
   if (std::optional<PrimType> T = Ctx.classify(QT))
-    Desc = createDescriptor(D, *T, nullptr, std::nullopt, /*IsTemporary=*/true,
-                            /*IsMutable=*/false);
+    Desc = createDescriptor(D, *T, /*SourceTy=*/nullptr, std::nullopt,
+                            /*IsConst=*/QT.isConstQualified());
   else
     Desc = createDescriptor(D, QT.getTypePtr(), std::nullopt,
-                            /*IsTemporary=*/true, /*IsMutable=*/false);
+                            /*IsConst=*/QT.isConstQualified());
   if (!Desc)
     Desc = allocateDescriptor(D);
 
@@ -431,7 +431,7 @@ Descriptor *Program::createDescriptor(const DeclTy &D, const Type *Ty,
     if (isa<IncompleteArrayType>(ArrayType) ||
         isa<VariableArrayType>(ArrayType)) {
       if (std::optional<PrimType> T = Ctx.classify(ElemTy)) {
-        return allocateDescriptor(D, *T, MDSize, IsTemporary,
+        return allocateDescriptor(D, *T, MDSize, IsConst, IsTemporary,
                                   Descriptor::UnknownSize{});
       } else {
         const Descriptor *Desc = createDescriptor(