Using Julia master (newer than 301db97), the following does not infer the correct type (see also the discussion in #34048):

using Test, LinearAlgebra
# @inferred mapreduce(norm, +, [rand(1)]);
@inferred mapreduce(norm, +, [rand(1)]; init = 0.);

Note that the third line works if you comment out the second line (start from a fresh Julia session), which makes testing this issue difficult.