Fix fallback sign() issues in Base implementation

src/FixedPointNumbers.jl

@@ -6,7 +6,7 @@ import Base: ==, <, <=, -, +, *, /, ~, isapprox,
              zero, oneunit, one, typemin, typemax, floatmin, floatmax, eps, reinterpret,
              big, rationalize, float, trunc, round, floor, ceil, bswap, clamp,
              div, fld, cld, rem, mod, mod1, fld1, min, max, minmax,
-             signed, unsigned, copysign, flipsign, signbit,
+             signed, unsigned, sign, copysign, flipsign, signbit,
              length, widen, widemul
 
 import Random: Random, AbstractRNG, SamplerType, rand!
@@ -370,6 +370,12 @@ clamp(x, ::Type{X}) where {X <: FixedPoint} = clamp(x, typemin(X), typemax(X)) %
 signed(x::X) where {X <: FixedPoint} = signed(X)(signed(x.i), 0)
 unsigned(x::X) where {X <: FixedPoint} = unsigned(X)(unsigned(x.i), 0)
 
+function sign(x::X) where {T, X <: FixedPoint{T}}
+    T <: Unsigned && return iszero(x) ? x : oneunit(X)
+    # `Q0f7`, etc., result in an error for `oneunit(X)`.
+    x <= zero(X) && return x === zero(X) ? x : X(-1)
+    return oneunit(X)
+end
 function copysign(x::X, y::Real) where {T, X <: FixedPoint{T}}
     T <: Signed ? X(copysign(x.i, y), 0) : throw_not_a_signed_number_error(x)
 end

test/common.jl

@@ -383,6 +383,12 @@ function test_sign_related(TX::Type)
         @test_throws Exception unsigned(X)
         @test_throws Exception unsigned(X(0.5))
 
+        @test sign(X(0.0)) === zero(X)
+        if typemax(X) >= 1.0
+            @test sign(X(0.5)) === oneunit(X)
+        else
+            @test_throws ArgumentError sign(X(0.5))
+        end
         @test signbit(X(0.5)) === signbit(X(0.0)) === false
     end
 end

test/fixed.jl

@@ -620,10 +620,7 @@ end
     @test copysign(0.5Q0f7, -1) === -0.5Q0f7
     @test flipsign(0.5Q0f7, 0x1) === 0.5Q0f7
     @test flipsign(0.5Q0f7, -1) === -0.5Q0f7
-    @test_broken sign(0Q0f7) === zero(Q0f7)
-    @test_throws ArgumentError sign(0.5Q0f7)
-    @test sign(0Q1f6) === 0Q1f6
-    @test sign(0.5Q1f6) === 1Q1f6
+    @test sign(-0.5Q0f7) === -1Q0f7
     @test sign(-0.5Q1f6) === -1Q1f6
     @test signbit(-0.5Q0f7) === true
 end

test/normed.jl

@@ -602,8 +602,6 @@ end
     @test_throws ArgumentError copysign(1N0f8, -1)
     @test_throws ArgumentError flipsign(1N0f8, 0x1)
     @test_throws ArgumentError flipsign(1N0f8, -1)
-    @test_broken sign(0N0f8) === zero(N0f8)
-    @test_broken sign(0.5N0f8) === oneunit(N0f8)
 end
 
 @testset "bitwise" begin