Support defining custom hashes for generated bitflags

Duplicates improvement made to Base's Enums:
JuliaLang/julia#49777 and JuliaLang/julia#49964

src/BitFlags.jl

@@ -27,6 +27,9 @@ Base.:&(x::T, y::T) where {T<:BitFlag} = T(Integer(x) & Integer(y))
 
 Base.broadcastable(x::BitFlag) = Ref(x)
 
+_bitflag_hash(x::BitFlag, h::UInt) = invoke(hash, Tuple{Any, UInt}, x, h)
+Base.hash(x::BitFlag, h::UInt) = _bitflag_hash(x, h)
+
 function Base.print(io::IO, x::T) where T<:BitFlag
     compact = get(io, :compact, false)::Bool
     xi = Integer(x)
@@ -267,7 +270,7 @@ function _bitflag_impl(__module__::Module, typename::Symbol, basetype::Type{<:Un
         Base.typemin(x::Type{$etypename}) = $etypename($lo)
         Base.typemax(x::Type{$etypename}) = $etypename($hi)
         let flag_hash = hash($etypename)
-            Base.hash(x::$etypename, h::UInt) = hash(flag_hash, hash(Integer(x), h))
+            BitFlags._bitflag_hash(x::$etypename, h::UInt) = hash(flag_hash, hash(Integer(x), h))
         end
         Base.instances(::Type{$etypename}) = ($(instances...),)
         $(flagconsts...)

test/runtests.jl

@@ -66,6 +66,11 @@ end
     # Hashing
     @test Int(flag2a) == Int(flag3a)    # same numerical value, but
     @test hash(flag2a) != hash(flag3a)  # unique hashes as BitFlag
+    @test which(hash, (Flag1, UInt)).sig != Tuple{typeof(hash), Flag1, UInt}
+    struct NonstandardBitFlag <: BitFlags.BitFlag{UInt8} end
+    let x = NonstandardBitFlag(), h = zero(UInt)
+        @test hash(x, h) == invoke(hash, Tuple{Any, UInt}, x, h)
+    end
 
     # Broadcasting
     @test [flag1a, flag1b] .| flag1c == [flag1a | flag1c, flag1b | flag1c]