Assert that exit never returns (JuliaLang#51857)

Closes JuliaLang#51856

base/initdefs.jl

@@ -25,7 +25,7 @@ Stop the program with an exit code. The default exit code is zero, indicating th
 program completed successfully. In an interactive session, `exit()` can be called with
 the keyboard shortcut `^D`.
 """
-exit(n) = ccall(:jl_exit, Cvoid, (Int32,), n)
+exit(n) = ccall(:jl_exit, Union{}, (Int32,), n)
 exit() = exit(0)
 
 const roottask = current_task()

test/compiler/inference.jl

@@ -5363,3 +5363,11 @@ function phic_type3()
 end
 @test Base.return_types(phic_type3) |> only === Union{Int, Float64}
 @test phic_type3() === 2
+
+# Test that `exit` returns `Union{}` (issue #51856)
+function test_exit_bottom(s)
+ n = tryparse(Int, s)
+ isnothing(n) && exit()
+ n
+end
+@test only(Base.return_types(test_exit_bottom, Tuple{String})) == Int