Fix #36531 - Error in abstract_iteration (#36532)

The issue here is passing a `Vargarg` to `precise_container_type`, which
doesn't really make sense. Instead, we need to have the caller unwrap
the vararg, request the precise container type of the inner type and
then re-wrap the answer in a vararg.

(cherry picked from commit 63179af)

base/compiler/abstractinterpretation.jl

@@ -529,6 +529,7 @@ function abstract_iteration(@nospecialize(itft), @nospecialize(itertype), vtypes
     else
         return Any[Vararg{Any}]
     end
+    @assert !isvarargtype(itertype)
     stateordonet = abstract_call_known(iteratef, nothing, Any[itft, itertype], vtypes, sv)
     # Return Bottom if this is not an iterator.
     # WARNING: Changes to the iteration protocol must be reflected here,
@@ -591,7 +592,22 @@ function abstract_apply(@nospecialize(itft), @nospecialize(aft), aargtypes::Vect
     for i = 1:nargs
         ctypes´ = []
         for ti in (splitunions ? uniontypes(aargtypes[i]) : Any[aargtypes[i]])
-            cti = precise_container_type(itft, ti, vtypes, sv)
+            if !isvarargtype(ti)
+                cti = precise_container_type(itft, ti, vtypes, sv)
+            else
+                cti = precise_container_type(itft, unwrapva(ti), vtypes, sv)
+                # We can't represent a repeating sequence of the same types,
+                # so tmerge everything together to get one type that represents
+                # everything.
+                argt = cti[end]
+                if isvarargtype(argt)
+                    argt = unwrapva(argt)
+                end
+                for i in 1:(length(cti)-1)
+                    argt = tmerge(argt, cti[i])
+                end
+                cti = Any[Vararg{argt}]
+            end
             if _any(t -> t === Bottom, cti)
                 continue
             end

test/compiler/inference.jl

@@ -2602,3 +2602,8 @@ _use_unstable_kw_2() = _unstable_kw(x = 2, y = rand())
 end
 _construct_structwithsplatnew() = StructWithSplatNew(("",))
 @test Base.return_types(_construct_structwithsplatnew) == Any[StructWithSplatNew]
+
+# Issue #36531, double varargs in abstract_iteration
+f36531(args...) = tuple((args...)...)
+@test @inferred(f36531(1,2,3)) == (1,2,3)
+@test code_typed(f36531, Tuple{Vararg{Int}}) isa Vector