[fan] Normalize trigger args to const remove_cvref_t<T> &

esphome/components/fan/__init__.py

@@ -358,26 +358,29 @@ async def fan_turn_on_to_code(config, action_id, template_arg, args):
         (CONF_DIRECTION, "set_direction", FanDirection),
     )
 
+    # Normalize trigger args to `const std::remove_cvref_t<T> &` so the
+    # apply lambda and any inner field lambdas (generated below via
+    # `process_lambda`) share one parameter spelling that's well-formed for
+    # any T (value, ref, or const-ref). Matches TurnOnAction::ApplyFn.
+    normalized_args = [
+        (cg.RawExpression(f"const std::remove_cvref_t<{cg.safe_exp(t)}> &"), n)
+        for t, n in args
+    ]
+
     fwd_args = ", ".join(name for _, name in args)
     body_lines: list[str] = []
     for conf_key, setter, type_ in FIELDS:
         if (value := config.get(conf_key)) is None:
             continue
         if isinstance(value, Lambda):
-            inner = await cg.process_lambda(value, args, return_type=type_)
+            inner = await cg.process_lambda(value, normalized_args, return_type=type_)
             body_lines.append(f"call.{setter}(({inner})({fwd_args}));")
         else:
             body_lines.append(f"call.{setter}({cg.safe_exp(value)});")
 
-    # Match TurnOnAction::ApplyFn signature: `const std::remove_reference_t<T> &`
-    # for each trigger arg so non-reference Ts stay no-copy (`const T &`) and
-    # reference Ts collapse correctly without producing `const T & &`.
     apply_args = [
         (FanCall.operator("ref"), "call"),
-        *(
-            (cg.RawExpression(f"const std::remove_reference_t<{cg.safe_exp(t)}> &"), n)
-            for t, n in args
-        ),
+        *normalized_args,
     ]
     apply_lambda = LambdaExpression(
         ["\n".join(body_lines)],

esphome/components/fan/automation.h

@@ -13,13 +13,15 @@ namespace fan {
 // Trigger args are forwarded to the apply function so user lambdas
 // (e.g. `speed: !lambda "return x;"`) keep working.
 //
-// Trigger args are forwarded as `const std::remove_reference_t<Ts> &...`
-// (instead of `const Ts &...`) so codegen can emit the same form in the
-// apply lambda's parameter list without producing `const T & &` for
-// triggers whose Ts already carries a reference (e.g. `std::string &`).
+// Trigger args are normalized to `const std::remove_cvref_t<Ts> &...` so
+// the codegen can emit a matching parameter list for both the apply lambda
+// and any inner field lambdas without producing invalid C++ source text
+// (e.g. `const T & &` if Ts already carries a reference, or `const const
+// T &` if Ts already carries a const). This keeps trigger args no-copy
+// regardless of whether the trigger supplies `T`, `T &`, or `const T &`.
 template<typename... Ts> class TurnOnAction : public Action<Ts...> {
  public:
-  using ApplyFn = void (*)(FanCall &, const std::remove_reference_t<Ts> &...);
+  using ApplyFn = void (*)(FanCall &, const std::remove_cvref_t<Ts> &...);
   TurnOnAction(Fan *state, ApplyFn apply) : state_(state), apply_(apply) {}
 
   void play(const Ts &...x) override {