1 files changed, 56 insertions, 1 deletions

diff --git a/numpy/core/_internal.py b/numpy/core/_internal.py
index 8942955f6..9a1787dde 100644
--- a/numpy/core/_internal.py
+++ b/numpy/core/_internal.py
@@ -10,7 +10,7 @@ import sys
 import platform
 import warnings
 
-from .multiarray import dtype, array, ndarray
+from .multiarray import dtype, array, ndarray, promote_types
 try:
     import ctypes
 except ImportError:
@@ -433,6 +433,61 @@ def _copy_fields(ary):
                   'formats': [dt.fields[name][0] for name in dt.names]}
     return array(ary, dtype=copy_dtype, copy=True)
 
+def _promote_fields(dt1, dt2):
+    """ Perform type promotion for two structured dtypes.
+
+    Parameters
+    ----------
+    dt1 : structured dtype
+        First dtype.
+    dt2 : structured dtype
+        Second dtype.
+
+    Returns
+    -------
+    out : dtype
+        The promoted dtype
+
+    Notes
+    -----
+    If one of the inputs is aligned, the result will be.  The titles of
+    both descriptors must match (point to the same field).
+    """
+    # Both must be structured and have the same names in the same order
+    if (dt1.names is None or dt2.names is None) or dt1.names != dt2.names:
+        raise TypeError("invalid type promotion")
+
+    # if both are identical, we can (maybe!) just return the same dtype.
+    identical = dt1 is dt2
+    new_fields = []
+    for name in dt1.names:
+        field1 = dt1.fields[name]
+        field2 = dt2.fields[name]
+        new_descr = promote_types(field1[0], field2[0])
+        identical = identical and new_descr is field1[0]
+
+        # Check that the titles match (if given):
+        if field1[2:] != field2[2:]:
+            raise TypeError("invalid type promotion")
+        if len(field1) == 2:
+            new_fields.append((name, new_descr))
+        else:
+            new_fields.append(((field1[2], name), new_descr))
+
+    res = dtype(new_fields, align=dt1.isalignedstruct or dt2.isalignedstruct)
+
+    # Might as well preserve identity (and metadata) if the dtype is identical
+    # and the itemsize, offsets are also unmodified.  This could probably be
+    # sped up, but also probably just be removed entirely.
+    if identical and res.itemsize == dt1.itemsize:
+        for name in dt1.names:
+            if dt1.fields[name][1] != res.fields[name][1]:
+                return res  # the dtype changed.
+        return dt1
+
+    return res
+
+
 def _getfield_is_safe(oldtype, newtype, offset):
     """ Checks safety of getfield for object arrays.