Make Extrema example compilable

src/core.jl

@@ -205,7 +205,7 @@ not defined for `R_{X}`, this happens automatically.
 complete(f, result) = f(result)
 complete(rf::Reduction, result) =
     # Not using dispatch to avoid ambiguity
-    if result isa PrivateState{typeof(rf)}
+    if ownsstate(rf, result)
         complete(rf.inner, unwrap(rf, result)[2])
     else
         complete(rf.inner, result)
@@ -214,10 +214,10 @@ complete(rf::Reduction, result) =
 combine(f, a, b) = f(a, b)
 combine(rf::Reduction, a, b) =
     # Not using dispatch to avoid ambiguity
-    if a isa PrivateState{typeof(rf)}
+    if ownsstate(rf, a)
         # TODO: make sure this branch is compiled out
         error("Stateful transducer ", rf.xform, " does not support `combine`")
-    elseif b isa PrivateState{typeof(rf)}
+    elseif ownsstate(rf, b)
         error("""
         Some thing went wrong in two ways:
         * `combine(rf, a, b)` is called but type of `a` and `b` are different.
@@ -236,6 +236,13 @@ end
 PrivateState(rf::Reduction, state, result) =
     PrivateState{typeof(rf), typeof(state), typeof(result)}(state, result)
 
+ownsstate(::Any, ::Any) = false
+ownsstate(::R, ::PrivateState{T}) where {R, T} = R === T
+# Using `result isa PrivateState{typeof(rf)}` makes it impossible to
+# compile Extrema examples in ../examples/tutorial_missings.jl (it
+# took more than 10 min).  See also:
+# https://github.com/JuliaLang/julia/issues/30125
+
 """
     unwrap(rf, result)