From 0326b1e93a264fa1190047fba902ba285a032ff8 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Tue, 7 Apr 2026 14:40:32 -1000
Subject: [PATCH] [core] Restore = delete for stateful lambdas in TemplatableFn

---
 esphome/core/automation.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/esphome/core/automation.h b/esphome/core/automation.h
index 355968b284..7a054f9899 100644
--- a/esphome/core/automation.h
+++ b/esphome/core/automation.h
@@ -54,6 +54,9 @@ template<typename T, typename... X> class TemplatableFn {
       std::invocable<F, X...> &&std::convertible_to<std::invoke_result_t<F, X...>, T> &&std::is_empty_v<F>
           &&std::default_initializable<F> : f_([](X... x) -> T { return static_cast<T>(F{}(x...)); }) {}
 
+  // Reject stateful lambdas (non-empty, i.e. capturing) with a clear error
+  template<typename F> TemplatableFn(F) requires std::invocable<F, X...> &&(!std::is_empty_v<F>) = delete;
+
   bool has_value() const { return this->f_ != nullptr; }
 
   T value(X... x) const { return this->f_ ? this->f_(x...) : T{}; }