fix issue involving bandwidths larger than dimensions

src/generic/AbstractBandedMatrix.jl

@@ -414,21 +414,31 @@ function LinearAlgebra.vcat(x::AbstractBandedMatrix...)
 
     #instantiate the returned banded matrix with zeros and required bandwidths/dimensions
     m = size(x[1], 2)
-    l, u = bandwidths(x[1])
+    l,u = -m, typemin(Int64)
     n = 0
+    isempty = true
 
+    #Check for dimension error and calculate bandwidths
     for A in x
         if size(A, 2) != m
             sizes = Tuple(size(b, 2) for b in x)
             throw(DimensionMismatch("number of columns of each matrix must match (got $sizes)"))
         end
 
-        u = max(u, bandwidth(A, 2) - n)
-        l = max(l, n + bandwidth(A, 1))
+        l_A, u_A = bandwidths(A)
+        if l_A + u_A >= 0
+            isempty = false
+            u = max(u, min(m - 1, u_A) - n)
+            l = max(l, min(size(A, 1) - 1, l_A) + n)
+        end
+
         n += size(A, 1)
     end
 
     type = promote_type(eltype.(x)...)
+    if isempty
+        return BandedMatrix{type}(undef, (n, m), bandwidths(Zeros(1)))
+    end
     ret = BandedMatrix(Zeros{type}(n, m), (l, u))
 
     #Populate the banded matrix

test/test_cat.jl

@@ -1,6 +1,6 @@
 module TestCat
 
-using BandedMatrices, LinearAlgebra, Test, Random, FillArrays
+using BandedMatrices, LinearAlgebra, Test, Random, FillArrays, SparseArrays
 
 @testset "vcat" begin
     @testset "banded matrices" begin
@@ -14,13 +14,15 @@ using BandedMatrices, LinearAlgebra, Test, Random, FillArrays
         @test eltype(vcat(b, c)) == Float64
         @test vcat(b, c) == vcat(Matrix(b), Matrix(c))
 
-        for i = 1:3
-            a = brand(Float64, rand(1:10), 5, rand(1:10),rand(-4:4))
-            b = brand(Float64, rand(1:10), 5, rand(1:10),rand(-4:4))
-            c = brand(Float64, rand(1:10), 5, rand(1:10),rand(-4:4))
+        for i in ((1,2), (-3,4), (0,-1))
+            a = BandedMatrix(ones(Float64, rand(1:10), 5), i)
+            b = BandedMatrix(ones(Int64, rand(1:10), 5), i)
+            c = BandedMatrix(ones(Int32, rand(1:10), 5), i)
             d = vcat(a, b, c)
-            @test d == vcat(Matrix(a), Matrix(b), Matrix(c))
-            @test bandwidths(d) == (bandwidth(c, 1) + size(a, 1) + size(b, 1), bandwidth(a, 2))
+            sd = vcat(sparse(a), sparse(b), sparse(c))
+            @test eltype(d) == Float64
+            @test d == sd
+            @test bandwidths(d) == bandwidths(sd)
         end
     end