From 2c6ebe312b2374b7b070401fcceb0488ac390b59 Mon Sep 17 00:00:00 2001
From: Hisham Muhammad <hisham@gobolinux.org>
Date: Fri, 26 Jan 2024 17:50:18 -0300
Subject: [PATCH] fix: check if for iterator does not return values

Fixes #736.
---
 spec/statement/forin_spec.lua | 10 ++++++++++
 tl.lua                        |  8 +++++++-
 tl.tl                         |  8 +++++++-
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/spec/statement/forin_spec.lua b/spec/statement/forin_spec.lua
index 9f93f131b..c473b1c55 100644
--- a/spec/statement/forin_spec.lua
+++ b/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"
diff --git a/tl.lua b/tl.lua
index 053e6edde..5541e8ee2 100644
--- a/tl.lua
+++ b/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],
diff --git a/tl.tl b/tl.tl
index d6dd29776..a021db2f3 100644
--- a/tl.tl
+++ b/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]