inference: eliminate more potential widenconst(::TypeofVararg) cases

Follows #42583.
The missing case within `const_prop_function_heuristic` was originally
reported from RationalAI's test suite. I added the other two cases by
some code review.

base/compiler/abstractinterpretation.jl

@@ -766,14 +766,14 @@ function const_prop_function_heuristic(
         # but highly worthwhile to inline promote of a constant
         length(argtypes) > 2 || return false
         t1 = widenconst(argtypes[2])
-        all_same = true
         for i in 3:length(argtypes)
-            if widenconst(argtypes[i]) !== t1
-                all_same = false
-                break
+            at = argtypes[i]
+            ty = isvarargtype(at) ? unwraptv(at) : widenconst(at)
+            if ty !== t1
+                return true
             end
         end
-        return !all_same
+        return false
     end
     return true
 end
@@ -1344,9 +1344,12 @@ function abstract_call_known(interp::AbstractInterpreter, @nospecialize(f),
         return CallMeta(Any, false)
     elseif f === Core.kwfunc
         if la == 2
-            ft = widenconst(argtypes[2])
-            if isa(ft, DataType) && isdefined(ft.name, :mt) && isdefined(ft.name.mt, :kwsorter)
-                return CallMeta(Const(ft.name.mt.kwsorter), MethodResultPure())
+            aty = argtypes[2]
+            if !isvarargtype(aty)
+                ft = widenconst(aty)
+                if isa(ft, DataType) && isdefined(ft.name, :mt) && isdefined(ft.name.mt, :kwsorter)
+                    return CallMeta(Const(ft.name.mt.kwsorter), MethodResultPure())
+                end
             end
         end
         return CallMeta(Any, false)
@@ -1366,8 +1369,12 @@ function abstract_call_known(interp::AbstractInterpreter, @nospecialize(f),
         return CallMeta(typevar_tfunc(n, lb_var, ub_var), false)
     elseif f === UnionAll
         return CallMeta(abstract_call_unionall(argtypes), false)
-    elseif f === Tuple && la == 2 && !isconcretetype(widenconst(argtypes[2]))
-        return CallMeta(Tuple, false)
+    elseif f === Tuple && la == 2
+        aty = argtypes[2]
+        ty = isvarargtype(aty) ? unwrapva(aty) : widenconst(aty)
+        if !isconcretetype(ty)
+            return CallMeta(Tuple, false)
+        end
     elseif is_return_type(f)
         return return_type_tfunc(interp, argtypes, sv)
     elseif la == 2 && istopfunction(f, :!)

test/compiler/inference.jl

@@ -3667,6 +3667,35 @@ let
     @test argtypes[11] == Tuple{Integer,Integer}
 end
 
+# make sure not to call `widenconst` on `TypeofVararg` objects
+@testset "unhandled Vararg" begin
+    struct UnhandledVarargCond
+        val::Bool
+    end
+    function Base.:+(a::UnhandledVarargCond, xs...)
+        if a.val
+            return nothing
+        else
+            s = 0
+            for x in xs
+                s += x
+            end
+            return s
+        end
+    end
+    @test Base.return_types((Vector{Int},)) do xs
+        +(UnhandledVarargCond(false), xs...)
+    end |> only === Int
+
+    @test (Base.return_types((Vector{Any},)) do xs
+        Core.kwfunc(xs...)
+    end; true)
+
+    @test Base.return_types((Vector{Vector{Int}},)) do xs
+        Tuple(xs...)
+    end |> only === Tuple{Vararg{Int}}
+end
+
 # issue #42646
 @test only(Base.return_types(getindex, (Array{undef}, Int))) >: Union{} # check that it does not throw