Make maybe_t conditionally copyable

This allows it to be used with both e.g. unique_ptr and std::vector.

src/fish_tests.cpp

@@ -5015,6 +5015,22 @@ void test_maybe() {
     std::string res = acquire_test.acquire();
     do_test(!acquire_test);
     do_test(res == "def");
+
+    // maybe_t<T> should be copyable iff T is copyable.
+    using copyable = std::shared_ptr<int>;
+    using noncopyable = std::unique_ptr<int>;
+    do_test(std::is_copy_assignable<maybe_t<copyable>>::value == true);
+    do_test(std::is_copy_constructible<maybe_t<copyable>>::value == true);
+    do_test(std::is_copy_assignable<maybe_t<noncopyable>>::value == false);
+    do_test(std::is_copy_constructible<maybe_t<noncopyable>>::value == false);
+
+    maybe_t<std::string> c1{"abc"};
+    maybe_t<std::string> c2 = c1;
+    do_test(c1.value() == "abc");
+    do_test(c2.value() == "abc");
+    c2 = c1;
+    do_test(c1.value() == "abc");
+    do_test(c2.value() == "abc");
 }
 
 void test_layout_cache() {

src/maybe.h

@@ -2,6 +2,116 @@
 #define FISH_MAYBE_H
 
 #include <cassert>
+#include <type_traits>
+
+namespace maybe_detail {
+// Template magic to make maybe_t<T> copyable iff T is copyable.
+// maybe_impl_t is the "too aggressive" implementation: it is always copyable.
+template <typename T>
+struct maybe_impl_t {
+    alignas(T) char storage[sizeof(T)];
+    bool filled = false;
+
+    T &value() {
+        assert(filled && "maybe_t does not have a value");
+        return *reinterpret_cast<T *>(storage);
+    }
+
+    const T &value() const {
+        assert(filled && "maybe_t does not have a value");
+        return *reinterpret_cast<const T *>(storage);
+    }
+
+    void reset() {
+        if (this->filled) {
+            value().~T();
+            this->filled = false;
+        }
+    }
+
+    T acquire() {
+        assert(filled && "maybe_t does not have a value");
+        T res = std::move(value());
+        reset();
+        return res;
+    }
+
+    maybe_impl_t() = default;
+
+    // Move construction/assignment from a T.
+    explicit maybe_impl_t(T &&v) : filled(true) { new (storage) T(std::forward<T>(v)); }
+    maybe_impl_t &operator=(T &&v) {
+        if (filled) {
+            value() = std::move(v);
+        } else {
+            new (storage) T(std::move(v));
+            filled = true;
+        }
+        return *this;
+    }
+
+    // Copy construction/assignment from a T.
+    explicit maybe_impl_t(const T &v) : filled(true) { new (storage) T(v); }
+    maybe_impl_t &operator=(const T &v) {
+        if (filled) {
+            value() = v;
+        } else {
+            new (storage) T(v);
+            filled = true;
+        }
+        return *this;
+    }
+
+    // Move construction/assignment from a maybe_impl.
+    maybe_impl_t(maybe_impl_t &&v) : filled(v.filled) {
+        if (filled) {
+            new (storage) T(std::move(v.value()));
+        }
+    }
+    maybe_impl_t &operator=(maybe_impl_t &&v) {
+        if (!v.filled) {
+            reset();
+        } else {
+            *this = std::move(v.value());
+        }
+        return *this;
+    }
+
+    // Copy construction/assignment from a maybe_impl.
+    maybe_impl_t(const maybe_impl_t &v) : filled(v.filled) {
+        if (v.filled) {
+            new (storage) T(v.value());
+        }
+    }
+    maybe_impl_t &operator=(const maybe_impl_t &v) {
+        if (&v == this) return *this;
+        if (!v.filled) {
+            reset();
+        } else {
+            *this = v.value();
+        }
+        return *this;
+    }
+
+    ~maybe_impl_t() { reset(); }
+};
+
+struct copyable_t {};
+struct noncopyable_t {
+    noncopyable_t() = default;
+    noncopyable_t(noncopyable_t &&) = default;
+    noncopyable_t &operator=(noncopyable_t &&) = default;
+    noncopyable_t(const noncopyable_t &) = delete;
+    noncopyable_t &operator=(const noncopyable_t &) = delete;
+};
+
+// conditionally_copyable_t is copyable iff T is copyable.
+// This enables conditionally copyable wrapper types by inheriting from it.
+template <typename T>
+using conditionally_copyable_t = typename std::conditional<std::is_copy_constructible<T>::value,
+                                                           copyable_t, noncopyable_t>::type;
+
+};  // namespace maybe_detail
 
 // A none_t is a helper type used to implicitly initialize maybe_t.
 // Example usage:
@@ -15,16 +125,15 @@ inline constexpr none_t none() { return none_t::none; }
 // Support for a maybe, also known as Optional.
 // This is a value-type class that stores a value of T in aligned storage.
 template <typename T>
-class maybe_t {
-    alignas(T) char storage[sizeof(T)];
-    bool filled = false;
+class maybe_t : private maybe_detail::conditionally_copyable_t<T> {
+    maybe_detail::maybe_impl_t<T> impl_;
 
    public:
     // return whether the receiver contains a value.
-    bool has_value() const { return filled; }
+    bool has_value() const { return impl_.filled; }
 
     // bool conversion indicates whether the receiver contains a value.
-    explicit operator bool() const { return filled; }
+    explicit operator bool() const { return impl_.filled; }
 
     // The default constructor constructs a maybe with no value.
     maybe_t() = default;
@@ -33,79 +142,46 @@ class maybe_t {
     /* implicit */ maybe_t(none_t n) { (void)n; }
 
     // Construct a maybe_t from a value T.
-    /* implicit */ maybe_t(T &&v) : filled(true) { new (storage) T(std::forward<T>(v)); }
+    /* implicit */ maybe_t(T &&v) : impl_(std::move(v)) {}
 
     // Construct a maybe_t from a value T.
-    /* implicit */ maybe_t(const T &v) : filled(true) { new (storage) T(v); }
+    /* implicit */ maybe_t(const T &v) : impl_(v) {}
 
     // Copy constructor.
-    maybe_t(const maybe_t &v) : filled(v.filled) {
-        if (v.filled) {
-            new (storage) T(v.value());
-        }
-    }
+    maybe_t(const maybe_t &v) = default;
 
     // Move constructor.
-    /* implicit */ maybe_t(maybe_t &&v) : filled(v.filled) {
-        if (v.filled) {
-            new (storage) T(std::move(v.value()));
-        }
-    }
+    /* implicit */ maybe_t(maybe_t &&v) = default;
 
     // Construct a value in-place.
     template <class... Args>
     void emplace(Args &&... args) {
         reset();
-        filled = true;
-        new (storage) T(std::forward<Args>(args)...);
+        impl_.filled = true;
+        new (impl_.storage) T(std::forward<Args>(args)...);
     }
 
     // Access the value.
-    T &value() {
-        assert(filled && "maybe_t does not have a value");
-        return *reinterpret_cast<T *>(storage);
-    }
+    T &value() { return impl_.value(); }
 
-    const T &value() const {
-        assert(filled && "maybe_t does not have a value");
-        return *reinterpret_cast<const T *>(storage);
-    }
+    const T &value() const { return impl_.value(); }
 
     // Transfer the value to the caller.
-    T acquire() {
-        assert(filled && "maybe_t does not have a value");
-        T res = std::move(value());
-        reset();
-        return res;
-    }
+    T acquire() { return impl_.acquire(); }
 
     // Clear the value.
-    void reset() {
-        if (filled) {
-            value().~T();
-            filled = false;
-        }
-    }
+    void reset() { impl_.reset(); }
 
     // Assign a new value.
     maybe_t &operator=(T &&v) {
-        if (filled) {
-            value() = std::move(v);
-        } else {
-            new (storage) T(std::move(v));
-            filled = true;
-        }
+        impl_ = std::move(v);
         return *this;
     }
 
-    maybe_t &operator=(maybe_t &&v) {
-        if (!v) {
-            reset();
-        } else {
-            *this = std::move(*v);
-        }
-        return *this;
-    }
+    // Note that defaulting these allows these to be conditionally deleted via
+    // conditionally_copyable_t().
+    maybe_t &operator=(const maybe_t &v) = default;
+    maybe_t &operator=(maybe_t &&v) = default;
 
     // Dereference support.
     const T *operator->() const { return &value(); }
@@ -127,8 +203,6 @@ class maybe_t {
     }
 
     bool operator!=(const maybe_t &rhs) const { return !(*this == rhs); }
-
-    ~maybe_t() { reset(); }
 };
 
 #endif