Fix #356.

Author: tkoolen

src/SMatrix.jl

@@ -54,7 +54,7 @@ end
 end
 
 @inline convert(::Type{SMatrix{S1,S2}}, a::StaticArray{<:Any, T}) where {S1,S2,T} = SMatrix{S1,S2,T}(Tuple(a))
-@inline SMatrix(a::StaticMatrix) = SMatrix{size(typeof(a),1),size(typeof(a),2)}(Tuple(a))
+@inline SMatrix(a::StaticMatrix{S1, S2}) where {S1, S2} = SMatrix{S1, S2}(Tuple(a))
 
 # Simplified show for the type
 show(io::IO, ::Type{SMatrix{N, M, T}}) where {N, M, T} = print(io, "SMatrix{$N,$M,$T}")

test/SMatrix.jl

@@ -67,6 +67,8 @@
         @test isa(@SMatrix(randexp(Float32, 2, 2)), SMatrix{2, 2, Float32})
 
         @test isa(SMatrix(@SMatrix zeros(4,4)), SMatrix{4, 4, Float64})
+
+        @inferred SMatrix(rand(SMatrix{3, 3})) # issue 356
     end
 
     @testset "Methods" begin