Add test for undefined behavior of unsafe_trunc

test/fixed.jl

@@ -282,6 +282,22 @@ end
 
     @test -1 % Q0f7 === Q0f7(-1)
     @test -2 % Q0f7 === Q0f7(0)
+
+    # issue #288
+    _fixed(::Val, x) = x % Q0f7
+    _fixed(::Val{16}, x) = x % Q0f15
+    buf = IOBuffer()
+    for v in (8, 16)
+        show(buf, _fixed(Val(v), 1.0))
+        print(buf, " ")
+    end
+    str = String(take!(buf))
+    if str == "-1.0Q0f7 -1.0Q0f15 " # just leave it in the report
+        @test str == "-1.0Q0f7 -1.0Q0f15 "
+    else
+        @test_broken str == "-1.0Q0f7 -1.0Q0f15 "
+        @warn "broken result: " * repr(str)
+    end
 end
 
 @testset "neg" begin

test/normed.jl

@@ -279,6 +279,22 @@ end
     # issue #211
     @test big"1.2" % N0f8 === 0.196N0f8
     @test reinterpret(BigFloat(0x0_01234567_89abcdef) % N63f1) === 0x01234567_89abcdef
+
+    # issue #288
+    _normed(::Val, x) = x % N0f8
+    _normed(::Val{16}, x) = x % N0f16
+    buf = IOBuffer()
+    for v in (8, 16)
+        show(buf, _normed(Val(v), 1.0))
+        print(buf, " ")
+    end
+    str = String(take!(buf))
+    if str == "1.0N0f8 1.0N0f16 " # just leave it in the report
+        @test str == "1.0N0f8 1.0N0f16 "
+    else
+        @test_broken str == "1.0N0f8 1.0N0f16 "
+        @warn "broken result: " * repr(str)
+    end
 end
 
 @testset "arithmetic" begin