[ITensor] Make checkflux pass for dense ITensors (#1285)

src/qn/flux.jl

@@ -68,8 +68,12 @@ allfluxequal(T::Tensor) = allequal(flux(T, b) for b in nzblocks(T))
 
 Check that fluxes of all non-zero blocks of a blocked or symmetric Tensor
 are equal. Throws an error if one or more blocks have a different flux.
+If the tensor is dense (is not blocked) then `checkflux` returns `nothing`.
 """
-checkflux(T::Tensor) = allfluxequal(T) ? nothing : error("Fluxes not all equal")
+function checkflux(T::Tensor)
+  (!hasqns(T) || isempty(T)) && return nothing
+  return allfluxequal(T) ? nothing : error("Fluxes not all equal")
+end
 
 """
     checkflux(T::Tensor, flux)

test/base/test_itensor.jl

@@ -1838,6 +1838,11 @@ end
     n = uniqueindex(N, A)
     @test dim(n) == dim(j) - dim(i)
   end
+
+  @testset "checkflux test" begin
+    # Calling checkflux should not error (issue #1283)
+    @test ITensors.checkflux(randomITensor(Index(2))) == nothing
+  end
 end # End Dense ITensor basic functionality
 
 # Disable debug checking once tests are completed