Promote a plain tensor to a trivial operator

Project.toml

@@ -1,6 +1,6 @@
 name = "ITensorBase"
 uuid = "4795dd04-0d67-49bb-8f44-b89c448a1dc7"
-version = "0.13.10"
+version = "0.13.11"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
 
 [workspace]

src/namedtensoroperator.jl

@@ -390,6 +390,20 @@ function operator(a::AbstractNamedTensor, output, input)
     return NamedTensorOperator(a, name.(output), name.(input))
 end
 
+# A plain tensor is a trivial (empty-pairing) operator, so an operator is the promotion of a
+# non-operator tensor. `convert` wraps a plain tensor as a trivial operator and `promote_rule`
+# makes the operator the common type, so `promote(state, operator)` returns two operators and a
+# mixed collection such as `[state, operator]` builds a homogeneous `Vector{<:NamedTensorOperator}`
+# instead of falling back to the abstract `typejoin`.
+function Base.convert(::Type{O}, a::NamedTensor) where {O <: NamedTensorOperator}
+    return operator(a, (), ())::O
+end
+function Base.promote_rule(
+        ::Type{<:NamedTensor{D}}, ::Type{O}
+    ) where {D, O <: NamedTensorOperator{D}}
+    return O
+end
+
 # Operator-preserving contraction. Contracting two named arrays sums over their
 # shared names, so the result keeps each operand's surviving output/input
 # structure. A non-operator tensor contributes no pairs (all its names are

test/test_operator.jl

@@ -473,3 +473,28 @@ end
     @test Pmat * Pmat' ≈ A
     @test Pmat * unnamed(state(Pinv)) ≈ I(n)
 end
+
+@testset "operator/state promotion" begin
+    t = NamedTensor(randn(2, 2), ("i", "j"))
+    o = operator(randn(2, 2), ("i",), ("j",))
+    O = typeof(o)
+
+    # A plain tensor promotes to the operator type (either argument order); the operator is
+    # the common type, so `promote` returns two operators.
+    @test promote_type(typeof(t), O) == O
+    @test promote_type(O, typeof(t)) == O
+    @test typeof.(promote(t, o)) == (O, O)
+
+    # `convert` wraps a plain tensor as a trivial empty-pairing operator, losslessly, and is a
+    # no-op on an operator.
+    to = convert(O, t)
+    @test to isa NamedTensorOperator
+    @test isempty(outputnames(to)) && isempty(inputnames(to))
+    @test state(to) === t
+    @test convert(O, o) === o
+
+    # A heterogeneous collection therefore homogenizes to the operator type.
+    v = [t, o]
+    @test eltype(v) == O
+    @test all(x -> x isa NamedTensorOperator, v)
+end