From 810a95efe06c2b4ac1ac0b34e5026d9b7ce59017 Mon Sep 17 00:00:00 2001
From: Hisham Muhammad <hisham@gobolinux.org>
Date: Sat, 21 Oct 2023 16:00:38 -0300
Subject: [PATCH] fix: avoid assertion error if typevar resolution fails when
 setting inference location

---
 spec/statement/return_spec.lua | 19 +++++++++++++++++++
 tl.lua                         |  3 +++
 tl.tl                          |  3 +++
 3 files changed, 25 insertions(+)

diff --git a/spec/statement/return_spec.lua b/spec/statement/return_spec.lua
index 86727eb80..b52fc912a 100644
--- a/spec/statement/return_spec.lua
+++ b/spec/statement/return_spec.lua
@@ -189,4 +189,23 @@ describe("return", function()
       assert.same({}, result.type_errors)
    end)
 
+   it("does not crash when a return type inference causes an error", util.check_type_error([[
+      local type A = record
+      end
+
+      local type B = record
+      end
+
+      local function f(): A
+      end
+
+      local function fail(): A | B
+         return f()
+      end
+   ]], {
+      -- the duplicated error is not ideal, but it's harmless, and better than a crash
+      { y = 10, msg = "cannot discriminate a union between multiple table types" },
+      { y = 11, msg = "cannot discriminate a union between multiple table types" },
+   }))
+
 end)
diff --git a/tl.lua b/tl.lua
index 1288f5297..1d1a8a20f 100644
--- a/tl.lua
+++ b/tl.lua
@@ -6234,6 +6234,9 @@ tl.type_check = function(ast, opts)
 
    local function infer_at(where, t)
       local ret = resolve_typevars_at(where, t)
+      if ret.typename == "invalid" then
+         ret = t
+      end
       ret = (ret ~= t) and ret or shallow_copy_type(t)
       ret.inferred_at = where
       ret.inferred_at_file = filename
diff --git a/tl.tl b/tl.tl
index ecc7ab67e..9bf74695c 100644
--- a/tl.tl
+++ b/tl.tl
@@ -6234,6 +6234,9 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
 
    local function infer_at(where: Node, t: Type): Type
       local ret = resolve_typevars_at(where, t)
+      if ret.typename == "invalid" then
+         ret = t -- errors are produced by resolve_typevars_at
+      end
       ret = (ret ~= t) and ret or shallow_copy_type(t)
       ret.inferred_at = where
       ret.inferred_at_file = filename