Add missing unordered_map template defaults (GH-3686)

Cython/Includes/libcpp/unordered_map.pxd

 from .utility cimport pair
 
 cdef extern from "<unordered_map>" namespace "std" nogil:
-    cdef cppclass unordered_map[T, U]:
+    cdef cppclass unordered_map[T, U, HASH=*, PRED=*, ALLOCATOR=*]:
         ctypedef T key_type
         ctypedef U mapped_type
         ctypedef pair[const T, U] value_type

tests/run/cpp_stl_cpp11.pyx

@@ -140,6 +140,11 @@
     return "pass"
 
 
+cdef extern from "cpp_unordered_map_helper.h":
+    cdef cppclass IntVectorHash:
+        pass
+
+
 def test_unordered_map_functionality():
     """
     >>> test_unordered_map_functionality()
@@ -153,6 +158,8 @@
         unordered_map[int, int] int_map2
         unordered_map[int, int*] intptr_map
         const int* intptr
+        unordered_map[vector[int], int, IntVectorHash] int_vector_map
+        vector[int] intvec
     assert int_map[1] == 2
     assert int_map.size() == 1
     assert int_map.erase(1) == 1 # returns number of elements erased
@@ -183,4 +190,12 @@
 
     intptr_map[0] = NULL
     intptr = intptr_map.const_at(0)
+
+    intvec = [1, 2]
+    int_vector_map[intvec] = 3
+    intvec = [4, 5]
+    int_vector_map[intvec] = 6
+    assert int_vector_map[intvec] == 6
+    intvec = [1, 2]
+    assert int_vector_map[intvec] == 3
     return "pass"

tests/run/cpp_unordered_map_helper.h

+#include <functional>
+#include <vector>
+
+struct IntVectorHash {
+  size_t operator()(const std::vector<int>& v) const {
+    std::hash<int> hasher;
+    size_t seed = 0;
+    for (int i : v) {
+      seed ^= hasher(i) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+    }
+    return seed;
+  }
+};