rowsupport in checkzerobands for two matrices (#318)

* rowsupport in checkzerobands for two matrices

* skip zerobands check for higher dest bandwidth

Author: jishnub

src/generic/broadcast.jl

@@ -77,7 +77,7 @@ function checkzerobands(dest, f, A::AbstractMatrix)
     d_l, d_u = bandwidths(dest)
     l, u = bandwidths(A)
 
-    if (l,u) ≠ (d_l,d_u)
+    if !(d_l >= l && d_u >= u)
         for j = rowsupport(A)
             for k = max(1,j-u) : min(j-d_u-1,m)
                 iszero(f(A[k,j])) || throw(BandError(dest,j-k))
@@ -679,7 +679,9 @@ function checkzerobands(dest, f, (A,B)::Tuple{AbstractMatrix,AbstractMatrix})
     B_l, B_u = bandwidths(B)
     l, u = max(A_l,B_l), max(A_u,B_u)
 
-    for j = 1:n
+    rspA = rowsupport(A)
+    rspB = rowsupport(B)
+    for j = min(minimum(rspA), minimum(rspB)):max(maximum(rspA), maximum(rspB))
         for k = max(1,j-u) : min(j-d_u-1,m)
             iszero(f(A[k,j], B[k,j])) || throw(BandError(dest,b))
         end