ak.concatenate (mergemany) should preserve regular-type.

src/awkward/_v2/contents/indexedarray.py

@@ -563,9 +563,10 @@ def mergemany(self, others):
         nextindex = ak._v2.index.Index64.empty(total_length, self._nplike)
         parameters = self._parameters
 
+        parameters = self._parameters
         for array in head:
             parameters = ak._v2._util.merge_parameters(
-                self._parameters, array._parameters, True
+                parameters, array._parameters, True
             )
 
             if isinstance(

src/awkward/_v2/contents/indexedoptionarray.py

@@ -667,9 +667,10 @@ def mergemany(self, others):
         nextindex = ak._v2.index.Index64.empty(total_length, self._nplike)
         parameters = self._parameters
 
+        parameters = self._parameters
         for array in head:
             parameters = ak._v2._util.merge_parameters(
-                self._parameters, array._parameters, True
+                parameters, array._parameters, True
             )
 
             if isinstance(

src/awkward/_v2/contents/listarray.py

@@ -948,9 +948,10 @@ def mergemany(self, others):
 
         contents = []
 
+        parameters = self._parameters
         for array in head:
             parameters = ak._v2._util.merge_parameters(
-                self._parameters, array._parameters, True
+                parameters, array._parameters, True
             )
 
             if isinstance(

src/awkward/_v2/contents/numpyarray.py

@@ -482,9 +482,10 @@ def mergemany(self, others):
 
         contiguous_arrays = []
 
+        parameters = self._parameters
         for array in head:
             parameters = ak._v2._util.merge_parameters(
-                self._parameters, array._parameters, True
+                parameters, array._parameters, True
             )
             if isinstance(array, ak._v2.contents.emptyarray.EmptyArray):
                 pass

src/awkward/_v2/contents/recordarray.py

@@ -570,9 +570,10 @@ def mergemany(self, others):
             for_each_field.append([field])
 
         if self.is_tuple:
+            parameters = self._parameters
             for array in headless:
                 parameters = ak._v2._util.merge_parameters(
-                    self._parameters, array._parameters, True
+                    parameters, array._parameters, True
                 )
 
                 if isinstance(array, ak._v2.contents.recordarray.RecordArray):
@@ -607,7 +608,12 @@ def mergemany(self, others):
             these_fields = self._fields.copy()
             these_fields.sort()
 
+            parameters = self._parameters
             for array in headless:
+                parameters = ak._v2._util.merge_parameters(
+                    parameters, array._parameters, True
+                )
+
                 if isinstance(array, ak._v2.contents.recordarray.RecordArray):
                     if not array.is_tuple:
                         those_fields = array._fields.copy()

src/awkward/_v2/contents/regulararray.py

@@ -682,7 +682,31 @@ def mergeable(self, other, mergebool):
     def mergemany(self, others):
         if len(others) == 0:
             return self
-        return self.toListOffsetArray64(True).mergemany(others)
+
+        if any(x.is_OptionType for x in others):
+            return ak._v2.contents.UnmaskedArray(self).mergemany(others)
+
+        elif all(x.is_RegularType and x.size == self.size for x in others):
+            parameters = self._parameters
+            tail_contents = []
+            zeros_length = 0
+            for x in others:
+                parameters = ak._v2._util.merge_parameters(
+                    parameters, x._parameters, True
+                )
+                tail_contents.append(x._content[: x._length * x._size])
+                zeros_length += x._length
+
+            return RegularArray(
+                self._content.mergemany(tail_contents),
+                self._size,
+                zeros_length,
+                None,
+                parameters,
+            )
+
+        else:
+            return self.toListOffsetArray64(True).mergemany(others)
 
     def fill_none(self, value):
         return RegularArray(

src/awkward/_v2/contents/unionarray.py

@@ -883,9 +883,10 @@ def mergemany(self, others):
         length_so_far = 0
         parameters = self._parameters
 
+        parameters = self._parameters
         for array in head:
             parameters = ak._v2._util.merge_parameters(
-                self._parameters, array._parameters, True
+                parameters, array._parameters, True
             )
             if isinstance(array, ak._v2.contents.unionarray.UnionArray):
                 union_tags = ak._v2.index.Index(array.tags)

tests/v2/test_1586-concatenate-should-preserve-regulararray.py

@@ -0,0 +1,78 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+import pytest  # noqa: F401
+import numpy as np  # noqa: F401
+import awkward as ak  # noqa: F401
+
+from awkward._v2.types import ArrayType, RegularType, OptionType, NumpyType
+
+
+def test_simple():
+    a = ak._v2.from_numpy(np.array([[1, 2], [3, 4], [5, 6]]), regulararray=True)
+    b = ak._v2.from_numpy(np.array([[7, 8], [9, 10]]), regulararray=True)
+    c = a.layout.merge(b.layout)
+    assert isinstance(c, ak._v2.contents.RegularArray)
+    assert c.size == 2
+    assert ak._v2.operations.to_list(c) == [[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]
+
+
+def test_regular_regular():
+    a1 = ak._v2.from_json("[[0.0, 1.1], [2.2, 3.3]]")
+    a2 = ak._v2.from_json("[[4.4, 5.5], [6.6, 7.7], [8.8, 9.9]]")
+    a1 = ak._v2.to_regular(a1, axis=1)
+    a2 = ak._v2.to_regular(a2, axis=1)
+    c = ak._v2.concatenate([a1, a2])
+    assert c.tolist() == [[0.0, 1.1], [2.2, 3.3], [4.4, 5.5], [6.6, 7.7], [8.8, 9.9]]
+    assert c.type == ArrayType(RegularType(NumpyType("float64"), 2), 5)
+
+
+def test_regular_option():
+    a1 = ak._v2.from_json("[[0.0, 1.1], [2.2, 3.3]]")
+    a2 = ak._v2.from_json("[[4.4, 5.5], [6.6, 7.7], null, [8.8, 9.9]]")
+    a1 = ak._v2.to_regular(a1, axis=1)
+    a2 = ak._v2.to_regular(a2, axis=1)
+    c = ak._v2.concatenate([a1, a2])
+    assert c.tolist() == [
+        [0.0, 1.1],
+        [2.2, 3.3],
+        [4.4, 5.5],
+        [6.6, 7.7],
+        None,
+        [8.8, 9.9],
+    ]
+    assert c.type == ArrayType(OptionType(RegularType(NumpyType("float64"), 2)), 6)
+
+
+def test_option_regular():
+    a1 = ak._v2.from_json("[[0.0, 1.1], null, [2.2, 3.3]]")
+    a2 = ak._v2.from_json("[[4.4, 5.5], [6.6, 7.7], [8.8, 9.9]]")
+    a1 = ak._v2.to_regular(a1, axis=1)
+    a2 = ak._v2.to_regular(a2, axis=1)
+    c = ak._v2.concatenate([a1, a2])
+    assert c.tolist() == [
+        [0.0, 1.1],
+        None,
+        [2.2, 3.3],
+        [4.4, 5.5],
+        [6.6, 7.7],
+        [8.8, 9.9],
+    ]
+    assert c.type == ArrayType(OptionType(RegularType(NumpyType("float64"), 2)), 6)
+
+
+def test_option_option():
+    a1 = ak._v2.from_json("[[0.0, 1.1], null, [2.2, 3.3]]")
+    a2 = ak._v2.from_json("[[4.4, 5.5], [6.6, 7.7], null, [8.8, 9.9]]")
+    a1 = ak._v2.to_regular(a1, axis=1)
+    a2 = ak._v2.to_regular(a2, axis=1)
+    c = ak._v2.concatenate([a1, a2])
+    assert c.tolist() == [
+        [0.0, 1.1],
+        None,
+        [2.2, 3.3],
+        [4.4, 5.5],
+        [6.6, 7.7],
+        None,
+        [8.8, 9.9],
+    ]
+    assert c.type == ArrayType(OptionType(RegularType(NumpyType("float64"), 2)), 7)