Fix the noexcept specification of xt::apply

include/xtensor/utils/xexception.hpp

@@ -340,6 +340,17 @@ namespace xt
 #define XTENSOR_ASSERT(expr)
 #endif
 
+    namespace detail
+    {
+        // True when a failing XTENSOR_ASSERT throws. A function containing an
+        // XTENSOR_ASSERT can only be noexcept when this is false.
+#if defined(XTENSOR_ENABLE_ASSERT) && !defined(XTENSOR_DISABLE_EXCEPTIONS)
+        inline constexpr bool assert_can_throw = true;
+#else
+        inline constexpr bool assert_can_throw = false;
+#endif
+    }
+
 #ifdef XTENSOR_ENABLE_CHECK_DIMENSION
 #define XTENSOR_CHECK_DIMENSION(S, ARGS) XTENSOR_TRY(check_dimension(S, ARGS))
 #else

include/xtensor/utils/xutils.hpp

@@ -28,6 +28,7 @@
 #include <xtl/xtype_traits.hpp>
 
 #include "../core/xtensor_config.hpp"
+#include "xexception.hpp"
 
 namespace xt
 {
@@ -47,8 +48,22 @@ namespace xt
     template <std::size_t I, class... Args>
     constexpr decltype(auto) argument(Args&&... args) noexcept;
 
+    namespace detail
+    {
+        // Whether calling F on each element type S and converting the result to R cannot throw.
+        template <class R, class F, class... S>
+        inline constexpr bool
+            is_nothrow_apply_call_v = (noexcept(static_cast<R>(std::declval<F&>()(std::declval<const S&>()))) && ...);
+
+        // apply also checks the index and falls back to R{}.
+        template <class R, class F, class... S>
+        inline constexpr bool is_nothrow_apply_v = is_nothrow_apply_call_v<R, F, S...>
+                                                   && std::is_nothrow_default_constructible<R>::value
+                                                   && !assert_can_throw;
+    }
+
     template <class R, class F, class... S>
-    R apply(std::size_t index, F&& func, const std::tuple<S...>& s) noexcept(noexcept(func(std::get<0>(s))));
+    R apply(std::size_t index, F&& func, const std::tuple<S...>& s) noexcept(detail::is_nothrow_apply_v<R, F, S...>);
 
     template <class T, class S>
     void nested_copy(T&& iter, const S& s);
@@ -252,7 +267,7 @@ namespace xt
 
     template <class R, class F, class... S>
     inline R
-    apply(std::size_t index, F&& func, const std::tuple<S...>& s) noexcept(noexcept(func(std::get<0>(s))))
+    apply(std::size_t index, F&& func, const std::tuple<S...>& s) noexcept(detail::is_nothrow_apply_v<R, F, S...>)
     {
         XTENSOR_ASSERT(sizeof...(S) > index);
         return std::apply(

test/test_xutils.cpp

@@ -9,6 +9,8 @@
 
 #include <complex>
 #include <initializer_list>
+#include <stdexcept>
+#include <string>
 #include <tuple>
 #include <type_traits>
 
@@ -113,9 +115,44 @@ namespace xt
             return i;
         };
         auto t = std::make_tuple(1, 2, 3);
-        static_assert(noexcept(apply<int>(1, func_ne, t)));
+        // The bound check on the index throws when assertions are enabled.
+        static_assert(noexcept(apply<int>(1, func_ne, t)) == !detail::assert_can_throw);
     }
 
+    struct throw_on_string
+    {
+        int operator()(int i) const noexcept
+        {
+            return i;
+        }
+
+        int operator()(const std::string&) const
+        {
+            throw std::runtime_error("throw_on_string");
+        }
+    };
+
+    TEST(utils, apply_propagates_exception)
+    {
+        // Only the call on the first element is noexcept: apply must not be.
+        auto t = std::make_tuple(1, std::string("a"));
+        static_assert(!noexcept(apply<int>(1, throw_on_string{}, t)));
+        EXPECT_EQ(apply<int>(0, throw_on_string{}, t), 1);
+        XT_EXPECT_THROW(apply<int>(1, throw_on_string{}, t), std::runtime_error);
+    }
+
+#ifdef XTENSOR_ENABLE_ASSERT
+    TEST(utils, apply_out_of_range)
+    {
+        auto func_ne = [](int i) noexcept
+        {
+            return i;
+        };
+        auto t = std::make_tuple(1, 2, 3);
+        XT_EXPECT_THROW(apply<int>(3, func_ne, t), std::runtime_error);
+    }
+#endif
+
     TEST(utils, conditional_cast)
     {
         EXPECT_TRUE((std::is_same<decltype(conditional_cast<false, double>(1)), int>::value));