provide type-inferred length

resolves #138
Need to check correct general computation of length of Axis.

src/axis.jl

@@ -142,6 +142,7 @@ Base.merge(axs::Axis...) = Axis(merge(indexmap.(axs)...))
 
 Base.firstindex(ax::AbstractAxis) = first(viewindex(first(indexmap(ax))))
 Base.lastindex(ax::AbstractAxis) = last(viewindex(last(indexmap(ax))))
+Base.length(ax::AbstractAxis) = lastindex(ax) - firstindex(ax) + 1
 
 Base.keys(ax::AbstractAxis) = keys(indexmap(ax))
 

src/componentarray.jl

@@ -235,6 +235,10 @@ last_index(x) = last(x)
 last_index(x::ViewAxis) = last_index(viewindex(x))
 last_index(x::AbstractAxis) = last_index(last(indexmap(x)))
 
+# length information is in Axis, use it to make SVector creation type stable
+Base.length(ca::ComponentArray) = prod(length.(getaxes(ca)))
+Base.size(ca::ComponentArray) = map(length, getaxes(ca))
+
 # Reduce singleton dimensions
 remove_nulls() = ()
 remove_nulls(x1, args...) = (x1, remove_nulls(args...)...)

test/runtests.jl

@@ -598,6 +598,15 @@ end
     @test convert(Cholesky{Float32,Matrix{Float32}}, chol).factors isa Matrix{Float32}
 end
 
+@testset "length typestable" begin
+    # function boundary, so that cv is type-inferred
+    test_create_svector = (cv) -> SVector{length(cv)}(cv)
+    @inferred test_create_svector(ComponentVector(a=1:3));
+    @inferred test_create_svector(cmat);
+    test_create_smatrix = (cmat) -> SMatrix{size(cmat)...}(cmat)
+    @test (@inferred test_create_smatrix(cmat)) isa SMatrix
+end;
+
 @testset "Autodiff" begin
     include("autodiff_tests.jl")
 end