Fix BandedMatrix pad for fewer rows/cols (#446)

Author: jishnub

src/LinearAlgebra/helper.jl

@@ -261,7 +261,7 @@ end
 
 function pad(A::BandedMatrix, n::Integer, m::Integer)
     B = BandedMatrix{eltype(A)}(undef, (n,m), bandwidths(A))
-    copyto!(B.data, A.data)
+    copyto!(B.data, firstindex(B.data), A.data, firstindex(A.data), min(length(A.data), length(B.data)))
     B.data[length(A.data)+1:end] .= 0
     return B
 end

test/runtests.jl

@@ -84,6 +84,25 @@ end
                 @test pad(a, 2) == @view(a[1:2])
             end
         end
+
+        @testset "BandedMatrix" begin
+            B = BandedMatrix(0=>[1:4;])
+            B11 = pad(B, 1, 1)
+            @test B11 isa BandedMatrix
+            @test B11 == BandedMatrix(0=>[1])
+            B16 = pad(B, 1, 6)
+            @test B16 isa BandedMatrix
+            @test B16 == BandedMatrix((0=>[1],), (1,6), (0,0))
+            B61 = pad(B, 6, 1)
+            @test B61 isa BandedMatrix
+            @test B61 == BandedMatrix((0=>[1],), (6,1), (0,0))
+            B66 = pad(B, 6, 6)
+            @test B66 isa BandedMatrix
+            @test B66 == BandedMatrix(0=>[1:4;0;0])
+            B23 = pad(B, 2, 3)
+            @test B23 isa BandedMatrix
+            @test B23 == BandedMatrix((0=>[1,2],), (2,3), (0,0))
+        end
     end
     @testset "nocat" begin
         D = Derivative()