diff --git a/meson.build b/meson.build
index 6231175e6..e90bf213b 100644
--- a/meson.build
+++ b/meson.build
@@ -405,6 +405,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Wdeprecated-copy',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
   # at ~1% overhead in `nix search`.
   #
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index aa7c6926a..df4492993 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -185,6 +185,8 @@ public:
 
     HintFmt(const HintFmt & hf) : fmt(hf.fmt) {}
 
+    HintFmt & operator=(HintFmt const & rhs) = default;
+
     std::string str() const
     {
         return fmt.str();
diff --git a/src/libutil/ref.hh b/src/libutil/ref.hh
index 5d0c3696d..03aa64273 100644
--- a/src/libutil/ref.hh
+++ b/src/libutil/ref.hh
@@ -77,6 +77,8 @@ public:
         return ref<T2>((std::shared_ptr<T2>) p);
     }
 
+    ref<T> & operator=(ref<T> const & rhs) = default;
+
     bool operator == (const ref<T> & other) const
     {
         return p == other.p;