Activate helpful error message for undefined-Size conversions

Adding `S<:Tuple` ensured a method would be higher in the methodtable.

Author: timholy

src/traits.jl

@@ -68,7 +68,7 @@ end
 
 Base.show(io::IO, ::Size{S}) where {S} = print(io, "Size", S)
 
-#= There seems to be a subtyping/specialization bug...
+Size(a::T) where {T<:AbstractArray} = Size(T)
 function Size(::Type{SA}) where {SA <: StaticArray} # A nice, default error message for when S not defined
     error("""
         The size of type `$SA` is not known.
@@ -82,9 +82,8 @@ function Size(::Type{SA}) where {SA <: StaticArray} # A nice, default error mess
             SMatrix(m)      # this error
             SMatrix{3,3}(m) # correct - size is inferrable
         """)
-end =#
-Size(a::T) where {T<:AbstractArray} = Size(T)
-Size(::Type{<:StaticArray{S}}) where {S} = Size(S)
+end
+Size(::Type{SA}) where {SA <: StaticArray{S}} where {S<:Tuple} = Size(S)  # S defined as a Tuple
 @pure Size(::Type{<:AbstractArray{<:Any, N}}) where {N} = Size(ntuple(_ -> Dynamic(), N))
 
 struct Length{L}

test/core.jl

@@ -133,6 +133,13 @@
         @test @inferred(convert(Array, ma)) == a
         @test @inferred(convert(Array{Int}, ma)) == a
         @test @inferred(convert(Array{Int,2}, ma)) == a
+
+        try
+            convert(SVector, [1,2,3])
+        catch err
+            @test isa(err, ErrorException)
+            @test startswith(err.msg, "The size of type `StaticArrays.SArray{Tuple{S},T,1,S} where T where S` is not known.")
+        end
     end
     @test_throws Exception Length{2.5}()
     @test Length(2) == Length{2}()