fix ismersene check (#164)

* fix ismersene check

* add test

Author: oscardssmith

src/Primes.jl

@@ -592,7 +592,7 @@ true
 function ismersenneprime(M::Integer; check::Bool = true)
     if check
         d = ndigits(M, base=2)
-        M >= 0 && isprime(d) && (M >> d == 0) ||
+        (M >= 0 && d == count_ones(M) && isprime(d)) ||
             throw(ArgumentError("The argument given is not a valid Mersenne Number (`M = 2^p - 1`)."))
     end
     M < 7 && return M == 3

test/runtests.jl

@@ -262,6 +262,7 @@ end
 @test ismersenneprime(8191)
 @test_throws ArgumentError ismersenneprime(9)
 @test_throws ArgumentError ismersenneprime(9, check=true)
+@test_throws ArgumentError ismersenneprime(17, check=true)
 # test the following does not throw
 ismersenneprime(9, check=false)