fix: avoid assertion error if typevar resolution fails when setting i…

…nference location

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)

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 -- 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

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