Dtype field ordering for NumPy 1.14 (#1837)

* Test dtype field order in numpy dtype tests

When running tests with NumPy 1.14 or later this test exposes the
"invalid buffer descriptor" error reported in #1274.

* Create dtype_ptr with ordered fields

include/pybind11/numpy.h

@@ -1066,8 +1066,14 @@ inline PYBIND11_NOINLINE void register_structured_dtype(
     if (numpy_internals.get_type_info(tinfo, false))
         pybind11_fail("NumPy: dtype is already registered");
 
+    // Use ordered fields because order matters as of NumPy 1.14:
+    // https://docs.scipy.org/doc/numpy/release.html#multiple-field-indexing-assignment-of-structured-arrays
+    std::vector<field_descriptor> ordered_fields(std::move(fields));
+    std::sort(ordered_fields.begin(), ordered_fields.end(),
+        [](const field_descriptor &a, const field_descriptor &b) { return a.offset < b.offset; });
+
     list names, formats, offsets;
-    for (auto field : *fields) {
+    for (auto& field : ordered_fields) {
         if (!field.descr)
             pybind11_fail(std::string("NumPy: unsupported field dtype: `") +
                             field.name + "` @ " + tinfo.name());
@@ -1084,9 +1090,6 @@ inline PYBIND11_NOINLINE void register_structured_dtype(
     // - https://github.com/numpy/numpy/pull/7798
     // Because of this, we won't use numpy's logic to generate buffer format
     // strings and will just do it ourselves.
-    std::vector<field_descriptor> ordered_fields(std::move(fields));
-    std::sort(ordered_fields.begin(), ordered_fields.end(),
-        [](const field_descriptor &a, const field_descriptor &b) { return a.offset < b.offset; });
     ssize_t offset = 0;
     std::ostringstream oss;
     // mark the structure as unaligned with '^', because numpy and C++ don't

tests/test_numpy_dtypes.cpp

@@ -29,6 +29,13 @@ std::ostream& operator<<(std::ostream& os, const SimpleStruct& v) {
     return os << "s:" << v.bool_ << "," << v.uint_ << "," << v.float_ << "," << v.ldbl_;
 }
 
+struct SimpleStructReordered {
+    bool bool_;
+    float float_;
+    uint32_t uint_;
+    long double ldbl_;
+};
+
 PYBIND11_PACKED(struct PackedStruct {
     bool bool_;
     uint32_t uint_;
@@ -255,6 +262,7 @@ TEST_SUBMODULE(numpy_dtypes, m) {
     py::class_<SimpleStruct>(m, "SimpleStruct");
 
     PYBIND11_NUMPY_DTYPE(SimpleStruct, bool_, uint_, float_, ldbl_);
+    PYBIND11_NUMPY_DTYPE(SimpleStructReordered, bool_, uint_, float_, ldbl_);
     PYBIND11_NUMPY_DTYPE(PackedStruct, bool_, uint_, float_, ldbl_);
     PYBIND11_NUMPY_DTYPE(NestedStruct, a, b);
     PYBIND11_NUMPY_DTYPE(PartialStruct, bool_, uint_, float_, ldbl_);