Implement conversion between compatible GradVectors

src/grad_vector.jl

@@ -90,6 +90,19 @@ end
 
 _exp_prop_convert_state(::GradVector) = Vector{ComplexF64}
 
+
+function Base.convert(
+    ::Type{GradVector{num_controls,T2}},
+    Ψ̃::GradVector{num_controls,T1}
+) where {T1,T2,num_controls}
+    # Used, e.g., for Cheby initialization
+    return GradVector{num_controls,T2}(
+        convert(T2, Ψ̃.state),
+        T2[convert(T2, Ψ′) for Ψ′ in Ψ̃.grad_states]
+    )
+end
+
+
 supports_inplace(::Type{GradVector{N,T}}) where {N,T} = supports_inplace(T)
 
 supports_vector_interface(::Type{GradVector{N,T}}) where {N,T} =

test/test_interface.jl

@@ -6,7 +6,7 @@ using QuantumControl.Interfaces: check_generator
 using QuantumPropagators.Interfaces:
     check_state, check_operator, supports_matrix_interface, supports_vector_interface
 using QuantumGradientGenerators: GradGenerator, GradVector, GradgenOperator
-using StaticArrays: SVector, SMatrix
+using StaticArrays: SVector, SMatrix, MVector
 using LinearAlgebra: norm, dot, mul!, I
 
 
@@ -31,6 +31,18 @@ end
 
     @test norm(2.2 * Ψ̃ - Ψ̃ * 2.2) < 1e-14
 
+    Ψ̃2 = similar(Ψ̃)
+    @test Ψ̃2 isa GradVector{2,<:MVector}
+
+    # We've had propagators use code like
+    #
+    #   v0::ST = similar(Ψ::ST)
+    #
+    # which relies on being able to convert mutable types back to their
+    # immutable version
+    Ψ̃3 = convert(typeof(Ψ̃), Ψ̃2)
+    @test typeof(Ψ̃3) == typeof(Ψ̃)
+
 end