Add guarded calls to crl::on_main[_sync]().

Some guard types are included:
- std::shared_ptr, std::weak_ptr
- QPointer, QSharedPointer, QWeakPointer
- pointer to derived from QObject (using QPointer)
- gsl::not_null<pointer to derived from QObject>

Guard types can be added by library users through crl::guard_traits.
For example Telegram Desktop adds:
- base::weak_ptr
- pointer to derived from base::has_weak_ptr (using base::weak_ptr)
- gsl::not_null<pointer to derived from base::has_weak_ptr>

src/crl/common/crl_common_guards.h

@@ -0,0 +1,61 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop version of Telegram messaging app, see https://telegram.org
+
+Telegram Desktop is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+It is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+In addition, as a special exception, the copyright holders give permission
+to link the code of portions of this program with the OpenSSL library.
+
+Full license: https://github.com/telegramdesktop/tdesktop/blob/master/LICENSE
+Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
+*/
+#pragma once
+
+#include <crl/common/crl_common_config.h>
+
+#include <memory>
+
+namespace crl {
+
+template <typename T, typename Enable>
+struct guard_traits;
+
+
+template <typename T>
+struct guard_traits<std::weak_ptr<T>, void> {
+	static std::weak_ptr<T> create(const std::weak_ptr<T> &value) {
+		return value;
+	}
+	static std::weak_ptr<T> create(std::weak_ptr<T> &&value) {
+		return std::move(value);
+	}
+	static bool check(const std::weak_ptr<T> &guard) {
+		return guard.lock() != nullptr;
+	}
+
+};
+
+template <typename T>
+struct guard_traits<std::shared_ptr<T>, void> {
+	static std::weak_ptr<T> create(const std::shared_ptr<T> &value) {
+		return value;
+	}
+	static std::weak_ptr<T> create(std::shared_ptr<T> &&value) {
+		return value;
+	}
+	static bool check(const std::weak_ptr<T> &guard) {
+		return guard.lock() != nullptr;
+	}
+
+};
+
+} // namespace crl

src/crl/common/crl_common_on_main_guarded.h

@@ -0,0 +1,74 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop version of Telegram messaging app, see https://telegram.org
+
+Telegram Desktop is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+It is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+In addition, as a special exception, the copyright holders give permission
+to link the code of portions of this program with the OpenSSL library.
+
+Full license: https://github.com/telegramdesktop/tdesktop/blob/master/LICENSE
+Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
+*/
+#pragma once
+
+#include <crl/common/crl_common_config.h>
+
+#include <memory>
+#include <type_traits>
+
+namespace crl::details {
+
+template <typename T>
+constexpr std::size_t dependent_zero = 0;
+
+} // namespace crl::details
+
+namespace crl {
+
+template <typename T, typename Enable = void>
+struct guard_traits;
+
+template <
+	typename Guard,
+	typename Callable,
+	typename GuardTraits = guard_traits<std::decay_t<Guard>>,
+	typename = std::enable_if_t<
+		sizeof(GuardTraits) != details::dependent_zero<GuardTraits>>>
+inline void on_main(Guard &&guard, Callable &&callable) {
+	on_main([
+		passed_guard = GuardTraits::create(std::forward<Guard>(guard)),
+		passed_callable = std::forward<Callable>(callable)
+	]() mutable {
+		if (GuardTraits::check(passed_guard)) {
+			std::forward<Callable>(passed_callable)();
+		}
+	});
+}
+
+template <
+	typename Guard,
+	typename Callable,
+	typename GuardTraits = guard_traits<std::decay_t<Guard>>,
+	typename = std::enable_if_t<
+		sizeof(GuardTraits) != details::dependent_zero<GuardTraits>>>
+inline void on_main_sync(Guard &&guard, Callable &&callable) {
+	on_main_sync([
+		passed_guard = GuardTraits::create(std::forward<Guard>(guard)),
+		passed_callable = std::forward<Callable>(callable)
+	]() mutable {
+		if (GuardTraits::check(passed_guard)) {
+			std::forward<Callable>(passed_callable)();
+		}
+	});
+}
+
+} // namespace crl

src/crl/crl_on_main.h

@@ -31,3 +31,7 @@ Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
 #else // CRL_USE_QT
 #error "Configuration is not supported."
 #endif // !CRL_USE_WINAPI && !CRL_USE_DISPATCH && !CRL_USE_QT
+
+#include <crl/common/crl_common_on_main_guarded.h>
+#include <crl/common/crl_common_guards.h>
+#include <crl/qt/crl_qt_guards.h>

src/crl/qt/crl_qt_guards.h

@@ -0,0 +1,130 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop version of Telegram messaging app, see https://telegram.org
+
+Telegram Desktop is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+It is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+In addition, as a special exception, the copyright holders give permission
+to link the code of portions of this program with the OpenSSL library.
+
+Full license: https://github.com/telegramdesktop/tdesktop/blob/master/LICENSE
+Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
+*/
+#pragma once
+
+#include <crl/common/crl_common_config.h>
+
+#if __has_include(<QtCore/QPointer>)
+
+class QObject;
+
+template <typename T>
+class QPointer;
+
+template <typename T>
+class QWeakPointer;
+
+template <typename T>
+class QSharedPointer;
+
+#if __has_include(<gsl/gsl>)
+
+namespace gsl {
+
+template <typename T>
+class not_null;
+
+} // namespace gsl
+
+#endif // gsl
+
+namespace crl {
+
+template <typename T, typename Enable>
+struct guard_traits;
+
+template <typename T>
+struct guard_traits<QPointer<T>, void> {
+	static QPointer<T> create(const QPointer<T> &value) {
+		return value;
+	}
+	static QPointer<T> create(QPointer<T> &&value) {
+		return std::move(value);
+	}
+	static bool check(const QPointer<T> &guard) {
+		return guard.data() != nullptr;
+	}
+
+};
+
+template <typename T>
+struct guard_traits<
+	T*,
+	std::enable_if_t<
+		std::is_base_of_v<QObject, std::remove_cv_t<T>>>> {
+	static QPointer<T> create(T *value) {
+		return value;
+	}
+	static bool check(const QPointer<T> &guard) {
+		return guard.data() != nullptr;
+	}
+
+};
+
+#if __has_include(<gsl/gsl>)
+
+template <typename T>
+struct guard_traits<
+	gsl::not_null<T*>,
+	std::enable_if_t<
+		std::is_base_of_v<QObject, std::remove_cv_t<T>>>> {
+	static QPointer<T> create(gsl::not_null<T*> value) {
+		return value.get();
+	}
+	static bool check(const QPointer<T> &guard) {
+		return guard.data() != nullptr;
+	}
+
+};
+
+#endif // gsl
+
+template <typename T>
+struct guard_traits<QWeakPointer<T>, void> {
+	static QWeakPointer<T> create(const QWeakPointer<T> &value) {
+		return value;
+	}
+	static QWeakPointer<T> create(QWeakPointer<T> &&value) {
+		return std::move(value);
+	}
+	static bool check(const QWeakPointer<T> &guard) {
+		return guard.toStrongRef() != nullptr;
+	}
+
+};
+
+template <typename T>
+struct guard_traits<QSharedPointer<T>, void> {
+	static QWeakPointer<T> create(const QSharedPointer<T> &value) {
+		return value;
+	}
+	static QWeakPointer<T> create(QSharedPointer<T> &&value) {
+		return value;
+	}
+	static bool check(const QWeakPointer<T> &guard) {
+		return guard.toStrongRef() != nullptr;
+	}
+
+};
+
+} // namespace crl
+
+#endif // Qt