fix: union specialization with interface subtyping produces a valid type

spec/lang/operator/is_spec.lua

@@ -777,4 +777,29 @@ end]]))
           print("Result: "..x)
       end
    ]]))
+
+   it("union specialization with interface subtyping produces a valid type (regression test)", util.check([[
+      local interface IA
+         where self.t == "a"
+
+         t: string
+      end
+
+      local interface IB
+         where self.t == "b"
+
+         t: string
+      end
+
+      local record R is IA
+         where self.t == "r"
+
+         rfield: string
+      end
+
+      local t: IA | IB
+
+      if t and t is R and t.rfield then
+      end
+   ]]))
 end)

tl.lua

@@ -9597,6 +9597,8 @@ a.types[i], b.types[i]), }
    end
 
    function TypeChecker:same_in_all_union_entries(u, check)
+      assert(#u.types > 0)
+
       local t1, f = check(u.types[1])
       if not t1 then
          return nil
@@ -10844,16 +10846,17 @@ a.types[i], b.types[i]), }
                   table.insert(out, t)
                end
             end
-            return unite(w, out)
-         else
-            if self:is_a(t1, t2) then
-               return t1
-            elseif self:is_a(t2, t1) then
-               return t2
-            else
-               return a_type(w, "nil", {})
+            if #out > 0 then
+               return unite(w, out)
             end
          end
+         if self:is_a(t1, t2) then
+            return t1
+         elseif self:is_a(t2, t1) then
+            return t2
+         else
+            return a_type(w, "nil", {})
+         end
       end
 
       function TypeChecker:resolve_if_union(t)

tl.tl

@@ -9597,6 +9597,8 @@ do
    end
 
    function TypeChecker:same_in_all_union_entries<T is Type, F is Type>(u: UnionType, check: function(Type): (T, F)): F
+      assert(#u.types > 0)
+
       local t1, f = check(u.types[1])
       if not t1 then
          return nil
@@ -10844,16 +10846,17 @@ do
                   table.insert(out, t)
                end
             end
-            return unite(w, out)
-         else
-            if self:is_a(t1, t2) then
-               return t1
-            elseif self:is_a(t2, t1) then
-               return t2
-            else
-               return a_type(w, "nil", {}) -- because of implicit nil in all unions
+            if #out > 0 then
+               return unite(w, out)
             end
          end
+         if self:is_a(t1, t2) then
+            return t1
+         elseif self:is_a(t2, t1) then
+            return t2
+         else
+            return a_type(w, "nil", {}) -- because of implicit nil in all unions
+         end
       end
 
       function TypeChecker:resolve_if_union(t: Type): Type