Fix mapreduce() in a corner case

mapreduce_seq_impl() does not exist in Base for a long time, but this went
unnoticed because since that code path is only used in very special conditions.
Another consequence of this bug is that the return type of mapreduce_pairwise_impl()
was inferred as Any. Add a test which would have triggered the failure.

Also rename a variable to fix a type instability due to using it for two
different things.

src/reduce.jl

@@ -22,8 +22,8 @@ function mapreduce_seq_impl_skipna(f, op, T, A::DataArray, ifirst::Int, ilast::I
 
     while i < ilast
         i += 1
-        @inbounds na = Base.unsafe_bitgetindex(chunks, i)
-        na && continue
+        @inbounds na_el = Base.unsafe_bitgetindex(chunks, i)
+        na_el && continue
         @inbounds d = data[i]
         v = op(v, f(d))
     end
@@ -36,7 +36,7 @@ function mapreduce_pairwise_impl_skipna{T}(f, op, A::DataArray{T}, bytefirst::In
         ifirst = 64*(bytefirst-1)+1
         ilast = min(64*bytelast, length(A))
         # Fall back to Base implementation if no NAs in block
-        return ilast - ifirst + 1 == n_notna ? Base.mapreduce_seq_impl(f, op, A.data, ifirst, ilast) :
+        return ilast - ifirst + 1 == n_notna ? Base.mapreduce_impl(f, op, A.data, ifirst, ilast) :
                                                mapreduce_seq_impl_skipna(f, op, T, A, ifirst, ilast)
     end
 

test/reduce.jl

@@ -76,6 +76,12 @@ end
         @test sum(da2; skipna=true) ≈ sum(dropna(da2))
     end
 
+    # Check that mapreduce with skipna=true works when a full block has no NA
+    da = DataArray(1:2049)
+    da[1] = NA
+    @test isna(sum(da))
+    @test sum(da, skipna=true) === 2100224
+
     ## other reductions
 
     _varuc(x; kw...) = var(x; corrected=false, kw...)