Restore old ITensor() behaviour

Author: kshyatt

src/itensor.jl

@@ -235,7 +235,7 @@ ITensor(is::Indices) = ITensor(Float64, is)
 ITensor(inds::Index...) = ITensor(Float64, IndexSet(inds...))
 
 # To fix ambiguity with QN Index version
-ITensor() = emptyITensor()
+ITensor() = ITensor(Float64, IndexSet())
 
 """
     ITensor([::Type{ElT} = Float64, ]::UndefInitializer, inds)
@@ -1230,25 +1230,13 @@ end
 -(A::ITensor) = itensor(-tensor(A))
 
 function (A::ITensor + B::ITensor)
-  if ndims(A) == 0 && ndims(B) > 0 && store(A) isa NDTensors.Empty
-    return copy(B)
-  end
-  if ndims(B) == 0 && ndims(A) > 0 && store(B) isa NDTensors.Empty
-    return copy(A)
-  end
   ndims(A) != ndims(B) && throw(DimensionMismatch("cannot add ITensors with different numbers of indices"))
   C = copy(A)
   C .+= B
   return C
 end
 
 function (A::ITensor - B::ITensor)
-  if ndims(A) == 0 && ndims(B) > 0 && store(A) isa NDTensors.Empty
-    return -copy(B)
-  end
-  if ndims(B) == 0 && ndims(A) > 0 && store(B) isa NDTensors.Empty
-    return copy(A)
-  end
   ndims(A) != ndims(B) && throw(DimensionMismatch("cannot subtract ITensors with different numbers of indices"))
   C = copy(A)
   C .-= B

test/itensor.jl

@@ -20,7 +20,7 @@ digits(::Type{T},x...) where {T} = T(sum([x[length(x)-k+1]*10^(k-1) for k=1:leng
 
   @testset "Default" begin
     A = ITensor()
-    @test store(A) isa NDTensors.Empty{Float64}
+    @test store(A) isa NDTensors.Dense{Float64}
   end
 
   @testset "Undef with index" begin
@@ -501,23 +501,24 @@ end
   a = [1.0; 2.0]
   A = itensor(a,i)
   B = ITensor()
-  @test A + B == A
+  @test_throws DimensionMismatch A + B
+  a = [1.0; 2.0]
   a = [1.0; 2.0]
   A = itensor(a,i)
   B = ITensor(2.0)
   @test_throws DimensionMismatch A - B
   a = [1.0; 2.0]
   A = itensor(a,i)
   B = ITensor()
-  @test A - B == A
+  @test_throws DimensionMismatch A - B
   a = [1.0; 2.0]
   A = itensor(a,i)
   B = ITensor(2.0)
   @test_throws DimensionMismatch B - A
   a = [1.0; 2.0]
   A = itensor(a,i)
   B = ITensor()
-  @test B - A == -A
+  @test_throws DimensionMismatch B - A
   a = [1.0; 2.0]
   b = [3.0; 4.0]
   A = itensor(a,i)