Fix Vararg methods widening with unused type variable (#48953)

Avoid possible nested `Vararg`.

Fix #48950

Author: N5N3

src/gf.c

@@ -642,8 +642,7 @@ static jl_value_t *inst_varargp_in_env(jl_value_t *decl, jl_svec_t *sparams)
                 int T_has_tv = T && jl_has_typevar(T, v);
                 int N_has_tv = N && jl_has_typevar(N, v); // n.b. JL_VARARG_UNBOUND check means this should be false
                 assert(!N_has_tv || N == (jl_value_t*)v);
-                if (T_has_tv)
-                    vm = jl_type_unionall(v, T);
+                vm = T_has_tv ? jl_type_unionall(v, T) : T;
                 if (N_has_tv)
                     N = NULL;
                 vm = (jl_value_t*)jl_wrap_vararg(vm, N); // this cannot throw for these inputs

test/core.jl

@@ -7973,3 +7973,6 @@ let spec = only(methods(g47476)).specializations
     @test any(mi -> mi !== nothing && Base.isvatuple(mi.specTypes), spec)
     @test all(mi -> mi === nothing || !Base.has_free_typevars(mi.specTypes), spec)
 end
+
+f48950(::Union{Int,d}, ::Union{c,Nothing}...) where {c,d} = 1
+@test f48950(1, 1, 1) == 1