[clang][Interp] Emit dummy pointers for unknown static locals

We used to emit dummy pointers for unknown declarations in certain
cases in C, but this is also necessary in C++.

I'm limiting this to static local variables for now.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -3239,9 +3239,14 @@ bool ByteCodeExprGen<Emitter>::VisitDeclRefExpr(const DeclRefExpr *E) {
     return this->emitGetPtrThisField(Offset, E);
   }
 
-  // Lazily visit global declarations we haven't seen yet.
-  // This happens in C.
-  if (!Ctx.getLangOpts().CPlusPlus) {
+  // Try to lazily visit (or emit dummy pointers for) declarations
+  // we haven't seen yet.
+  if (Ctx.getLangOpts().CPlusPlus) {
+    if (const auto *VD = dyn_cast<VarDecl>(D); VD && VD->isStaticLocal()) {
+      if (std::optional<unsigned> I = P.getOrCreateDummy(D))
+        return this->emitGetPtrGlobal(*I, E);
+    }
+  } else {
     if (const auto *VD = dyn_cast<VarDecl>(D);
         VD && VD->getAnyInitializer() && VD->getType().isConstQualified()) {
       if (!this->visitVarDecl(VD))

clang/test/AST/Interp/functions.cpp

@@ -523,3 +523,23 @@ namespace Move {
   constexpr int A = std::move(5);
   static_assert(A == 5, "");
 }
+
+namespace StaticLocals {
+  void test() {
+    static int j; // both-note {{declared here}}
+    static_assert(&j != nullptr, ""); // both-warning {{always true}}
+
+    static_assert(j == 0, ""); // both-error {{not an integral constant expression}} \
+                               // both-note {{read of non-const variable 'j'}}
+
+    static int k = 0; // both-note {{declared here}}
+    static_assert(k == 0, ""); // both-error {{not an integral constant expression}} \
+                               // both-note {{read of non-const variable 'k'}}
+
+    static const int l = 12;
+    static_assert(l == 12, "");
+
+    static const int m; // both-error {{default initialization}}
+    static_assert(m == 0, "");
+  }
+}