fix #27361, error adding method using e.g. Tuple{3} (#27404)

Author: JeffBezanson

src/subtype.c

@@ -2332,8 +2332,10 @@ static int sub_msp(jl_value_t *a, jl_value_t *b, jl_typeenv_t *env)
 {
     JL_GC_PUSH2(&a, &b);
     while (env != NULL) {
-        a = jl_type_unionall(env->var, a);
-        b = jl_type_unionall(env->var, b);
+        if (jl_is_type(a) || jl_is_typevar(a))
+            a = jl_type_unionall(env->var, a);
+        if (jl_is_type(b) || jl_is_typevar(b))
+            b = jl_type_unionall(env->var, b);
         env = env->prev;
     }
     int sub = jl_subtype(a, b);

test/specificity.jl

@@ -202,3 +202,8 @@ let A = Tuple{Vector, AbstractVector},
     @test args_morespecific(B, C)
     @test args_morespecific(A, C)
 end
+
+# issue #27361
+f27361(::M) where M <: Tuple{2} = nothing
+f27361(::M) where M <: Tuple{3} = nothing
+@test length(methods(f27361)) == 2