From 71bc0a54ce249e08edbd3738a144338b81be5ad0 Mon Sep 17 00:00:00 2001
From: Matthew Fishman <mfishman@flatironinstitute.org>
Date: Thu, 23 Jul 2026 20:59:52 -0400
Subject: [PATCH] Add `Matrix`/`Vector` conversion for named tensors

## Summary

Adds `Matrix(::AbstractNamedTensor)` and `Vector(::AbstractNamedTensor)` for densifying a named tensor whose rank is fixed by its leg count (e.g. the two-leg result of `matricize`), which reads more clearly at the call site than the generic `Array`. Both are handled by a single `Array{<:Any, N}` method that defaults the eltype and forwards to the existing `Array{T, N}` conversion, so a rank mismatch (a non-two-leg tensor into `Matrix`, a non-one-leg tensor into `Vector`) errors as expected.

Also removes the `AbstractArray{T}` / `AbstractArray{T, N}` constructors, and the `_checkaxs` / `copyto_axcheck!` helpers they used: `AbstractNamedTensor` is not an `AbstractArray`, so those constructors returned a named tensor from an `AbstractArray` constructor, and nothing called them.
---
 Project.toml                  |  2 +-
 src/abstractnamedtensor.jl    | 19 ++-----------------
 test/test_nameddims_basics.jl |  9 +++++++++
 3 files changed, 12 insertions(+), 18 deletions(-)

diff --git a/Project.toml b/Project.toml
index 30cd9eb..c1a0f85 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "ITensorBase"
 uuid = "4795dd04-0d67-49bb-8f44-b89c448a1dc7"
-version = "0.13.3"
+version = "0.13.4"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
 
 [workspace]
diff --git a/src/abstractnamedtensor.jl b/src/abstractnamedtensor.jl
index 8ed058b..99d2c6f 100644
--- a/src/abstractnamedtensor.jl
+++ b/src/abstractnamedtensor.jl
@@ -271,17 +271,6 @@ end
 
 # Conversion
 
-# Copied from `Base` (defined in abstractarray.jl).
-@noinline function _checkaxs(axd, axs)
-    axd == axs || throw(DimensionMismatch("axes must agree, got $axd and $axs"))
-    return nothing
-end
-function copyto_axcheck!(dest, src)
-    _checkaxs(axes(dest), axes(src))
-    copyto!(dest, src)
-    return dest
-end
-
 # These are defined since the Base versions assume the eltype and ndims are known
 # at compile time, which isn't true for ITensors.
 # Workaround: `Array(::TensorKit.AbstractTensorMap)` is not defined yet (it should be), so a
@@ -295,12 +284,8 @@ function Base.Array(a::AbstractNamedTensor)
 end
 Base.Array{T}(a::AbstractNamedTensor) where {T} = Array{T}(unnamed(a))
 Base.Array{T, N}(a::AbstractNamedTensor) where {T, N} = Array{T, N}(unnamed(a))
-Base.AbstractArray{T}(a::AbstractNamedTensor) where {T} = AbstractArray{T, ndims(a)}(a)
-function Base.AbstractArray{T, N}(a::AbstractNamedTensor) where {T, N}
-    dest = similar(a, T)
-    copyto_axcheck!(unnamed(dest), unnamed(a))
-    return dest
-end
+# Catches the bare `Matrix`/`Vector` (`Array{T, N} where T`, ndims fixed), defaulting the eltype.
+Base.Array{<:Any, N}(a::AbstractNamedTensor) where {N} = Array{eltype(a), N}(a)
 
 # Read the parent's axes through TensorAlgebra's interface (not `Base.axes`) so a non-array
 # backend like a TensorMap, whose axes are its native spaces, is supported.
diff --git a/test/test_nameddims_basics.jl b/test/test_nameddims_basics.jl
index 1b53bc3..d78a824 100644
--- a/test/test_nameddims_basics.jl
+++ b/test/test_nameddims_basics.jl
@@ -89,6 +89,15 @@ end
         @test eltype(a′) ≡ elt
         @test a′ isa Matrix{elt}
         @test a′ == a
+        # `Matrix`/`Vector` densify when the rank matches the leg count.
+        m = Matrix(na)
+        @test m isa Matrix{elt}
+        @test m == a
+        @test_throws Exception Vector(na)                 # two legs, not a vector
+        v = nameddims(randn(elt, 4), ("i",))
+        @test Vector(v) isa Vector{elt}
+        @test Vector(v) == unnamed(v)
+        @test_throws Exception Matrix(v)                  # one leg, not a matrix
 
         if elt <: Real
             a = randn(elt, 3, 4)