Loosen linkdim(::AbstractITensorNetwork, ::AbstractEdge) signature

The previous signature
`linkdim(tn::AbstractITensorNetwork{V}, edge::AbstractEdge{V}) where V`
required the network's vertex-type parameter and the edge's vertex-type
parameter to match exactly. For graphs with abstract V (e.g. the
`NamedGraph{Tuple}` produced by `named_binary_tree`, where vertices are
tuples of varying arity), downstream operations can produce edges whose
V parameter is a strict subtype — for example, `subgraph(g, vs)` where
`vs` came from a Dict's keys and concretized to
`Tuple{Int64, Vararg{Int64}}`. In that situation dispatch failed with
`MethodError` even though `linkinds` itself accepts the edge fine.

Drop the `{V}` constraint so `linkdim` accepts any `AbstractEdge`
alongside any `AbstractITensorNetwork`. Behavior is unchanged for cases
that previously dispatched; previously-failing cases now go through.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mtfishman

src/abstractitensornetwork.jl

@@ -556,7 +556,7 @@ function linkdim(tn::AbstractITensorNetwork, edge::Pair)
     return linkdim(tn, edgetype(tn)(edge))
 end
 
-function linkdim(tn::AbstractITensorNetwork{V}, edge::AbstractEdge{V}) where {V}
+function linkdim(tn::AbstractITensorNetwork, edge::AbstractEdge)
     ls = linkinds(tn, edge)
     return prod([isnothing(l) ? 1 : dim(l) for l in ls])
 end

test/test_itensornetwork.jl

@@ -250,4 +250,16 @@ const elts = (Float32, Float64, Complex{Float32}, Complex{Float64})
         tn = random_tensornetwork(rng, is; link_space = 3)
         @test_broken swapprime(tn, 0, 2)
     end
+
+    # Regression test for https://github.com/ITensor/ITensorNetworks.jl/pull/373.
+    @testset "linkdim accepts an AbstractEdge whose vertex type is a subtype" begin
+        i = Index(2)
+        A = ITensor(i)
+        B = ITensor(i)
+        tn = ITensorNetwork{Any}(Dictionary([1, 2], [A, B]))
+        @test tn isa ITensorNetwork{Any}
+        e = NamedEdge(1 => 2)
+        @test e isa NamedEdge{Int}
+        @test ITensorNetworks.linkdim(tn, e) == 2
+    end
 end