Make any_container implicitly constructible from arithmetic values

This further reduces the constructors required in buffer_info/numpy by
removing the need for the constructors that take a single size_t and
just forward it on via an initializer_list to the container-accepting
constructor.

Unfortunately, in `array` one of the constructors runs into an ambiguity
problem with the deprecated `array(handle, bool)` constructor (because
both the bool constructor and the any_container constructor involve an
implicit conversion, so neither has precedence), so a forwarding
constructor is kept there (until the deprecated constructor is
eventually removed).

include/pybind11/buffer_info.h

@@ -36,7 +36,7 @@ struct buffer_info {
     }
 
     buffer_info(void *ptr, size_t itemsize, const std::string &format, size_t size)
-    : buffer_info(ptr, itemsize, format, 1, { size }, { itemsize }) { }
+    : buffer_info(ptr, itemsize, format, 1, size, itemsize) { }
 
     explicit buffer_info(Py_buffer *view, bool ownview_in = true)
     : buffer_info(view->buf, (size_t) view->itemsize, view->format, (size_t) view->ndim,

include/pybind11/common.h

@@ -660,7 +660,8 @@ static constexpr auto const_ = std::true_type{};
 NAMESPACE_BEGIN(detail)
 
 // Adaptor for converting arbitrary container arguments into a vector; implicitly convertible from
-// any standard container (or C-style array) supporting std::begin/std::end.
+// any standard container (or C-style array) supporting std::begin/std::end, any singleton
+// arithmetic type (if T is arithmetic), or explicitly constructible from an iterator pair.
 template <typename T>
 class any_container {
     std::vector<T> v;
@@ -680,6 +681,11 @@ public:
     template <typename TIn, typename = enable_if_t<std::is_convertible<TIn, T>::value>>
     any_container(const std::initializer_list<TIn> &c) : any_container(c.begin(), c.end()) { }
 
+    // Implicit conversion constructor from any arithmetic type (only participates if T is also
+    // arithmetic).
+    template <typename TIn, typename = enable_if_t<std::is_arithmetic<T>::value && std::is_arithmetic<TIn>::value>>
+    any_container(TIn singleton) : v(1, static_cast<T>(singleton)) { }
+
     // Avoid copying if given an rvalue vector of the correct type.
     any_container(std::vector<T> &&v) : v(std::move(v)) { }
 

include/pybind11/numpy.h

@@ -463,7 +463,7 @@ public:
           const void *ptr = nullptr, handle base = handle()) {
 
         if (strides->empty())
-            strides = default_strides(*shape, dt.itemsize());
+            *strides = default_strides(*shape, dt.itemsize());
 
         auto ndim = shape->size();
         if (ndim != strides->size())
@@ -499,9 +499,12 @@ public:
     array(const pybind11::dtype &dt, ShapeContainer shape, const void *ptr = nullptr, handle base = handle())
         : array(dt, std::move(shape), {}, ptr, base) { }
 
-    array(const pybind11::dtype &dt, size_t count, const void *ptr = nullptr,
+    // This constructor is only needed to avoid ambiguity with the deprecated (handle, bool)
-          handle base = handle())
+    // constructor that comes from PYBIND11_OBJECT_CVT; once that is gone, the above constructor can
-        : array(dt, ShapeContainer{{ count }}, ptr, base) { }
+    // handle it (because ShapeContainer is implicitly constructible from arithmetic types)
+    template <typename T, typename = detail::enable_if_t<std::is_arithmetic<T>::value && !std::is_same<bool, T>::value>>
+    array(const pybind11::dtype &dt, T count)
+        : array(dt, count, nullptr) { }
 
     template <typename T>
     array(ShapeContainer shape, StridesContainer strides, const T *ptr, handle base = handle())
@@ -511,10 +514,6 @@ public:
     array(ShapeContainer shape, const T *ptr, handle base = handle())
         : array(std::move(shape), {}, ptr, base) { }
 
-    template <typename T>
-    array(size_t count, const T *ptr, handle base = handle())
-        : array({{ count }}, ptr, base) { }
-
     explicit array(const buffer_info &info)
     : array(pybind11::dtype(info), info.shape, info.strides, info.ptr) { }
 
@@ -739,9 +738,6 @@ public:
     explicit array_t(ShapeContainer shape, const T *ptr = nullptr, handle base = handle())
         : array(std::move(shape), ptr, base) { }
 
-    explicit array_t(size_t count, const T *ptr = nullptr, handle base = handle())
-        : array(count, ptr, base) { }
-
     constexpr size_t itemsize() const {
         return sizeof(T);
     }