Remove ambiguity from "not safe to write" compiler error message

src/libponyc/expr/operator.c

@@ -480,6 +480,18 @@ bool expr_assign(pass_opt_t* opt, ast_t* ast)
       }
     }
 
+    if(ast_child(left) != NULL)
+    {
+      ast_error(opt->check.errors, ast,
+        "not safe to write right side to left side");
+      ast_error_continue(opt->check.errors, r_type, "right side type: %s",
+        ast_print_type(r_type));
+      ast_error_continue(opt->check.errors, ast_child(left), "left side type: %s",
+        ast_print_type(ast_type(ast_child(left))));
+      ast_free_unattached(wl_type);
+      return false;
+    }
+
     ast_error(opt->check.errors, ast,
       "not safe to write right side to left side");
     ast_error_continue(opt->check.errors, wl_type, "right side type: %s",

test/libponyc/badpony.cc

@@ -1287,3 +1287,21 @@ TEST_F(BadPonyTest, CantAssignErrorExpression)
   TEST_ERRORS_1(src,
     "right side must be something that can be assigned")
 }
+
+TEST_F(BadPonyTest, NotSafeToWrite)
+{
+  // From issue #4290
+  const char* src =
+    "class Foo\n"
+    "class X\n"
+      "var foo: Foo ref = Foo\n"
+
+    "actor Main\n"
+      "new create(env: Env) =>\n"
+        "let x = X\n"
+        "let foo: Foo ref = Foo\n"
+        "x.foo = foo";
+
+  TEST_ERRORS_1(src,
+    "not safe to write right side to left side")
+}