Return a value from xoptional_assembly_storage::data() (#2955)

# Checklist

- [x] The title and commit message(s) are descriptive.
- [x] Small commits made to fix your PR have been squashed to avoid
history pollution.
- [x] Tests have been added for new features or bug fixes.
- [x] API of new functions and classes are documented (no public API
changes).
# Description

Both overloads of `xoptional_assembly_storage::data()` contained
`pointer(front());` without `return`. C++ parses that statement as the
declaration of a local function named `front`, so the functions fall off
the end without returning a value:

```cpp
auto stor = xt::optional_assembly_storage(v, f);
stor.data()->value();  // undefined behavior, segfaults with GCC 15 at -O2
```

Calling both overloads in the same translation unit does not compile
either, because the two local declarations of `front` have different
return types. GCC reports the statement with `-Wparentheses` and
`-Wshadow`.

`data()` now returns the closure pointer to the front element.
`xoptional_assembly_storage.data` checks the mutable and const
overloads; it does not compile on master and passes with the fix.

Author: gouarin

include/xtensor/optional/xoptional_assembly_storage.hpp

@@ -320,13 +320,13 @@ namespace xt
     template <class VE, class FE>
     inline auto xoptional_assembly_storage<VE, FE>::data() noexcept -> pointer
     {
-        pointer(front());
+        return pointer(front());
     }
 
     template <class VE, class FE>
     inline auto xoptional_assembly_storage<VE, FE>::data() const noexcept -> const_pointer
     {
-        const_pointer(front());
+        return const_pointer(front());
     }
 
     template <class VE, class FE>

test/test_xoptional_assembly_storage.cpp

@@ -84,6 +84,24 @@ namespace xt
         ASSERT_EQ(stor[3], stor.back());
     }
 
+    TEST(xoptional_assembly_storage, data)
+    {
+        storage_type v = {56, 2, 3, 5};
+        flag_storage_type f = {false, true, true, true};
+        auto stor = optional_assembly_storage(v, f);
+
+        auto ptr = stor.data();
+        ASSERT_EQ(*ptr, stor.front());
+        ptr->value() = 12;
+        ptr->has_value() = true;
+        ASSERT_EQ(stor[0], 12);
+
+        const auto& cstor = stor;
+        auto cptr = cstor.data();
+        ASSERT_EQ(*cptr, cstor.front());
+        ASSERT_EQ(cptr->value(), 12);
+    }
+
     TEST(xoptional_assembly_storage, iterator)
     {
         storage_type v = {56, 2, 3, 5};