Turn the copy of a read-only memory view always into a writable memory view.

Closes #2134.

Cython/Compiler/MemoryView.py

@@ -484,18 +484,23 @@ def copy_c_or_fortran_cname(memview):
     return "__pyx_memoryview_copy_slice_%s_%s" % (
             memview.specialization_suffix(), c_or_f)
 
+
 def get_copy_new_utility(pos, from_memview, to_memview):
-    if from_memview.dtype != to_memview.dtype:
-        return error(pos, "dtypes must be the same!")
+    if (from_memview.dtype != to_memview.dtype and
+            not (from_memview.dtype.is_const and from_memview.dtype.const_base_type == to_memview.dtype)):
+        error(pos, "dtypes must be the same!")
+        return
     if len(from_memview.axes) != len(to_memview.axes):
-        return error(pos, "number of dimensions must be same")
+        error(pos, "number of dimensions must be same")
+        return
     if not (to_memview.is_c_contig or to_memview.is_f_contig):
-        return error(pos, "to_memview must be c or f contiguous.")
+        error(pos, "to_memview must be c or f contiguous.")
+        return
 
     for (access, packing) in from_memview.axes:
         if access != 'direct':
-            return error(
-                    pos, "cannot handle 'full' or 'ptr' access at this time.")
+            error(pos, "cannot handle 'full' or 'ptr' access at this time.")
+            return
 
     if to_memview.is_c_contig:
         mode = 'c'
@@ -516,6 +521,7 @@ def get_copy_new_utility(pos, from_memview, to_memview):
             dtype_is_object=int(to_memview.dtype.is_pyobject)),
         requires=[copy_contents_new_utility])
 
+
 def get_axes_specs(env, axes):
     '''
     get_axes_specs(env, axes) -> list of (access, packing) specs for each axis.

Cython/Compiler/PyrexTypes.py

@@ -711,8 +711,12 @@ class MemoryViewSliceType(PyrexType):
             to_axes_c = follow_dim * (ndim - 1) + contig_dim
             to_axes_f = contig_dim + follow_dim * (ndim -1)
 
-            to_memview_c = MemoryViewSliceType(self.dtype, to_axes_c)
-            to_memview_f = MemoryViewSliceType(self.dtype, to_axes_f)
+            dtype = self.dtype
+            if dtype.is_const:
+                dtype = dtype.const_base_type
+
+            to_memview_c = MemoryViewSliceType(dtype, to_axes_c)
+            to_memview_f = MemoryViewSliceType(dtype, to_axes_f)
 
             for to_memview, cython_name in [(to_memview_c, "copy"),
                                             (to_memview_f, "copy_fortran")]:

tests/memoryview/numpy_memoryview_readonly.pyx

@@ -110,3 +110,17 @@ def test_assign_ro_to_rw(x):
     cdef double[:] rw = np.empty_like(ro)
     rw[:] = ro
     return rw[2]
+
+
+def test_copy():
+    """
+    >>> test_copy()
+    (1.0, 2.0, 1.0, 1.0, 1.0, 2.0)
+    """
+    cdef const double[:] ro = np.ones(3)
+    cdef const double[:] ro2 = ro.copy()
+    cdef double[:] rw = ro.copy()
+    cdef double[:] rw2 = ro2.copy()
+    rw[1] = 2
+    rw2[2] = 2
+    return rw[0], rw[1], rw[2], rw2[0], rw2[1], rw2[2]