Fix fd(::IOStream) returning an integer

NEWS.md

@@ -99,6 +99,7 @@ Standard library changes
 ------------------------
 
 * `gcdx(0, 0)` now returns `(0, 0, 0)` instead of `(0, 1, 0)` ([#40989]).
+* `fd` returns a `RawFD` instead of an `Int` ([#55080]).
 
 #### StyledStrings
 

base/iostream.jl

@@ -47,17 +47,18 @@ macro _lock_ios(s, expr)
 end
 
 """
-    fd(stream)
+    fd(stream) -> RawFD
 
 Return the file descriptor backing the stream or file. Note that this function only applies
 to synchronous `File`'s and `IOStream`'s not to any of the asynchronous streams.
 
-File descriptors should typically be represented as [`RawFD`](@ref) objects, rather
-than as `Int`s, to ensure that they are properly interpreted by Julia functions.
+`RawFD` objects can be passed directly to other languages via the `ccall` interface.
 
-Note that `RawFD` objects can be passed directly to other languages via the `ccall` interface.
+!!! compat "Julia 1.12"
+    Prior to 1.12, this function returned an `Int` instead of a `RawFD`. You may use
+    `RawFD(fd(x))` to produce a `RawFD` in all Julia versions.
 """
-fd(s::IOStream) = Int(ccall(:jl_ios_fd, Clong, (Ptr{Cvoid},), s.ios))
+fd(s::IOStream) = RawFD(ccall(:jl_ios_fd, Clong, (Ptr{Cvoid},), s.ios))
 
 stat(s::IOStream) = stat(fd(s))
 

test/iostream.jl

@@ -190,3 +190,7 @@ end
     @test all(T -> T <: Union{UInt, Int}, Base.return_types(unsafe_write, (IO, Ptr{UInt8}, UInt)))
     @test all(T -> T === Bool, Base.return_types(eof, (IO,)))
 end
+
+@testset "fd" begin
+    @test open(fd, tempname(), "w") isa RawFD
+end