some type safety for Fix

Disallows any type `Fix{x}` where `!(x isa Int) || (x < 0)`.

Follows up on PR JuliaLang#54653.

The goals are similar as in the closed PR JuliaLang#56518, but less ambitious.

I hope it's not too late in the release cycle to consider this change
as it seems not to be invasive or disruptive.

base/operators.jl

@@ -1170,17 +1170,15 @@ A type representing a partially-applied version of a function `f`, with the argu
     `Fix{1}(Fix{2}(f, 4), 4)` fixes the first and second arg, while `Fix{2}(Fix{1}(f, 4), 4)`
     fixes the first and third arg.
 """
-struct Fix{N,F,T} <: Function
+struct Fix{N,F,T,NTuple{N,Nothing}<:NTup<:NTuple{N,Nothing}} <: Function
     f::F
     x::T
 
     function Fix{N}(f::F, x) where {N,F}
-        if !(N isa Int)
-            throw(ArgumentError(LazyString("expected type parameter in `Fix` to be `Int`, but got `", N, "::", typeof(N), "`")))
-        elseif N < 1
-            throw(ArgumentError(LazyString("expected `N` in `Fix{N}` to be integer greater than 0, but got ", N)))
+        if N < 1
+            throw(ArgumentError("expected `N` in `Fix{N}` to be integer greater than 0, but got 0"))
         end
-        new{N,_stable_typeof(f),_stable_typeof(x)}(f, x)
+        new{N,_stable_typeof(f),_stable_typeof(x),NTuple{N,Nothing}}(f, x)
     end
 end
 
@@ -1196,12 +1194,12 @@ end
 """
 Alias for `Fix{1}`. See [`Fix`](@ref Base.Fix).
 """
-const Fix1{F,T} = Fix{1,F,T}
+const Fix1{F,T} = Fix{1,F,T,NTuple{1,Nothing}}
 
 """
 Alias for `Fix{2}`. See [`Fix`](@ref Base.Fix).
 """
-const Fix2{F,T} = Fix{2,F,T}
+const Fix2{F,T} = Fix{2,F,T,NTuple{2,Nothing}}
 
 
 """

test/functional.jl

@@ -348,9 +348,11 @@ end
             @test f() == 'a'
         end
         @testset "Dummy-proofing" begin
+            @test_throws TypeError Fix{1, <:Any, <:Any, Union{}}
+            @test_throws Exception Fix{-1}
             @test_throws ArgumentError("expected `N` in `Fix{N}` to be integer greater than 0, but got 0") Fix{0}(>, 1)
-            @test_throws ArgumentError("expected type parameter in `Fix` to be `Int`, but got `0.5::Float64`") Fix{0.5}(>, 1)
-            @test_throws ArgumentError("expected type parameter in `Fix` to be `Int`, but got `1::UInt64`") Fix{UInt64(1)}(>, 1)
+            @test_throws TypeError Fix{0.5}
+            @test_throws TypeError Fix{UInt64(1)}
         end
         @testset "Specialize to structs not in `Base`" begin
             struct MyStruct