[clang][bytecode] Diagnose loads from weak variables (#109256)

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -560,13 +560,25 @@ bool CheckGlobalInitialized(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   return false;
 }
 
+static bool CheckWeak(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!Ptr.isWeak())
+    return true;
+
+  const auto *VD = Ptr.getDeclDesc()->asVarDecl();
+  assert(VD);
+  S.FFDiag(S.Current->getLocation(OpPC), diag::note_constexpr_var_init_weak)
+      << VD;
+  S.Note(VD->getLocation(), diag::note_declared_at);
+
+  return false;
+}
+
 bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
                AccessKinds AK) {
   if (!CheckLive(S, OpPC, Ptr, AK))
     return false;
   if (!CheckConstant(S, OpPC, Ptr))
     return false;
-
   if (!CheckDummy(S, OpPC, Ptr, AK))
     return false;
   if (!CheckExtern(S, OpPC, Ptr))
@@ -579,6 +591,8 @@ bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
     return false;
   if (!CheckTemporary(S, OpPC, Ptr, AK))
     return false;
+  if (!CheckWeak(S, OpPC, Ptr))
+    return false;
   if (!CheckMutable(S, OpPC, Ptr))
     return false;
   if (!CheckVolatile(S, OpPC, Ptr, AK))

clang/test/AST/ByteCode/weak.cpp

@@ -7,3 +7,11 @@ int ha[(bool)&a]; // both-warning {{variable length arrays in C++ are a Clang ex
 int ha2[&a == nullptr]; // both-warning {{variable length arrays in C++ are a Clang extension}} \
                         // both-note {{comparison against address of weak declaration '&a' can only be performed at runtime}} \
                         // both-error {{variable length array declaration not allowed at file scope}}
+
+extern const int W1 __attribute__((weak)) = 10; // both-note {{declared here}}
+static_assert(W1 == 10, ""); // both-error {{static assertion expression is not an integral constant expression}} \
+                             // both-note {{initializer of weak variable 'W1' is not considered constant because it may be different at runtime}}
+
+extern const int W2 __attribute__((weak)); // both-note {{declared here}}
+static_assert(W2 == 10, ""); // both-error {{static assertion expression is not an integral constant expression}} \
+                             // both-note {{initializer of 'W2' is unknown}}

clang/test/SemaCXX/weak-init.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 %s -verify -fsyntax-only
+// RUN: %clang_cc1 %s -verify -fsyntax-only -fexperimental-new-constant-interpreter
 
 extern const int W1 __attribute__((weak)) = 10; // expected-note {{declared here}}