[clang][Interp] Fix dummy DeclRefExprs for function pointers

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -3722,8 +3722,16 @@ bool ByteCodeExprGen<Emitter>::VisitDeclRefExpr(const DeclRefExpr *E) {
     }
   }
 
-  if (std::optional<unsigned> I = P.getOrCreateDummy(D))
-    return this->emitGetPtrGlobal(*I, E);
+  if (std::optional<unsigned> I = P.getOrCreateDummy(D)) {
+    if (!this->emitGetPtrGlobal(*I, E))
+      return false;
+    // Convert the dummy pointer to another pointer type if we have to.
+    if (PrimType PT = classifyPrim(E); PT != PT_Ptr) {
+      if (!this->emitDecayPtr(PT_Ptr, PT, E))
+        return false;
+    }
+    return true;
+  }
 
   return this->emitInvalidDeclRef(E, E);
 }

clang/test/AST/Interp/functions.cpp

@@ -617,3 +617,9 @@ namespace {
   void bir [[clang::annotate("B", {1, 2, 3, 4})]] (); // both-error {{'annotate' attribute requires parameter 1 to be a constant expression}} \
                                                       // both-note {{subexpression not valid in a constant expression}}
 }
+
+namespace FuncPtrParam {
+  void foo(int(&a)()) {
+    *a; // both-warning {{expression result unused}}
+  }
+}