fix: check if for iterator does not return values

Fixes #736.

spec/statement/forin_spec.lua

@@ -188,6 +188,16 @@ describe("forin", function()
    }))
 
    describe("regression tests", function()
+      it("catches if iterator function does not return values (#736)", util.check_type_error([[
+         local function f()
+         end
+
+         for k, v in f() do
+         end
+      ]], {
+         { y = 4, msg = "expression in 'for' statement does not return any values" },
+      }))
+
       it("does not accept annotations (#701)", util.check_syntax_error([[
          for k <const>, v <const> in pairs(table as {string:any}) do
              k = "hello"

tl.lua

@@ -10795,8 +10795,14 @@ self:expand_type(node, values, elements) })
             assert(exptuple.typename == "tuple")
             local exptypes = exptuple.tuple
 
-            self:widen_all_unions(node)
             local exp1 = node.exps[1]
+            if #exptypes < 1 then
+               self.errs:invalid_at(exp1, "expression in 'for' statement does not return any values")
+               return
+            end
+
+            self:widen_all_unions(node)
+
             local args = a_type(node.exps, "tuple", { tuple = {
                node.exps[2] and exptypes[2],
                node.exps[3] and exptypes[3],

tl.tl

@@ -10795,8 +10795,14 @@ do
             assert(exptuple is TupleType)
             local exptypes = exptuple.tuple
 
-            self:widen_all_unions(node)
             local exp1 = node.exps[1]
+            if #exptypes < 1 then
+               self.errs:invalid_at(exp1, "expression in 'for' statement does not return any values")
+               return
+            end
+
+            self:widen_all_unions(node)
+
             local args = a_tuple(node.exps, {
                node.exps[2] and exptypes[2],
                node.exps[3] and exptypes[3]