[clang][Interp] Diagnose comparisons with weak pointers

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -801,6 +801,18 @@ inline bool CmpHelperEQ<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
     return true;
   }
 
+  for (const auto &P : {LHS, RHS}) {
+    if (P.isZero())
+      continue;
+    if (const ValueDecl *VD = P.getDeclDesc()->asValueDecl();
+        VD && VD->isWeak()) {
+      const SourceInfo &Loc = S.Current->getSource(OpPC);
+      S.FFDiag(Loc, diag::note_constexpr_pointer_weak_comparison)
+          << P.toDiagnosticString(S.getCtx());
+      return false;
+    }
+  }
+
   if (!Pointer::hasSameBase(LHS, RHS)) {
     S.Stk.push<BoolT>(BoolT::from(Fn(ComparisonCategoryResult::Unordered)));
     return true;

clang/test/AST/Interp/weak.cpp

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++20 -fexperimental-new-constant-interpreter -verify=expected,both %s
+// RUN: %clang_cc1 -std=c++20 -verify=ref,both %s
+
+
+
+
+/// FIXME: The new interpreter also emits the "address of weak declaration" note in the pointer-to-bool case.
+
+[[gnu::weak]] extern int a;
+int ha[(bool)&a]; // both-warning {{variable length arrays in C++ are a Clang extension}} \
+                  // expected-note {{comparison against address of weak declaration}} \
+                  // both-error {{variable length array declaration not allowed at file scope}}
+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}}