fix: function: do not mistake optional argument for vararg

Fixes #826.

spec/lang/subtyping/function_spec.lua

@@ -0,0 +1,29 @@
+local util = require("spec.util")
+
+describe("subtyping of functions:", function()
+   it("f(a, ?b) <: f(a) (regression test 1 for #826)", util.check([[
+      local type shared = string
+      local interface test1
+          a: function(a:string,b:shared)
+      end
+      local record test2 is test1
+          a: function(a:string, b:shared, c?:string) -- Error when shared != string
+      end
+      local record test3 is test1
+          a: function(a:string, b:shared, c?:number) -- Error when shared != number
+      end
+   ]]))
+
+   it("f(a, ?b) <: f(a) (regression test 2 for #826)", util.check([[
+      local type shared = number
+      local interface test1
+          a: function(a:string,b:shared)
+      end
+      local record test2 is test1
+          a: function(a:string, b:shared, c?:string) -- Error when shared != string
+      end
+      local record test3 is test1
+          a: function(a:string, b:shared, c?:number) -- Error when shared != number
+      end
+   ]]))
+end)

tl.lua

@@ -8942,7 +8942,7 @@ a.types[i], b.types[i]), }
             else
                for i = ((a.is_method or b.is_method) and 2 or 1), #aa do
                   local ai = aa[i]
-                  local bi = ba[i] or ba[#ba]
+                  local bi = ba[i] or (b.args.is_va and ba[#ba])
                   if bi then
                      self:arg_check(nil, errs, ai, bi, "bivariant", "argument", i)
                   end

tl.tl

@@ -8942,7 +8942,7 @@ do
             else
                for i = ((a.is_method or b.is_method) and 2 or 1), #aa do
                   local ai = aa[i]
-                  local bi = ba[i] or ba[#ba]
+                  local bi = ba[i] or (b.args.is_va and ba[#ba])
                   if bi then
                      self:arg_check(nil, errs, ai, bi, "bivariant", "argument", i)
                   end