make compatibility of functions with optionals more lax

Fixes #827.

spec/lang/assignment/to_function_spec.lua

@@ -9,4 +9,45 @@ describe("assignment to function", function()
          chunk = my_load("")
       end
    ]]))
+
+   it("bivariant type checking for functions with optional arguments (regression test for #827)", util.check([[
+      local fcts: {string:function(val: any, opt?: string)}
+
+      fcts['foo'] = function (val: string)
+          print(val)
+      end
+
+      fcts['foo2'] = function (val: string, val2: string)  --  in assignment: incompatible number of arguments: got 2 (string, string), expected at least 1 and at most 2 (<any type>, string)
+          print(val, val2)
+      end
+
+      fcts['bar'] = function (val: number)
+          print(val)
+      end
+
+      fcts['bar2'] = function (val: number, val2: string)  -- in assignment: incompatible number of arguments: got 2 (number, string), expected at least 1 and at most 2 (<any type>, string)
+          print(val, val2)
+      end
+   ]]))
+
+   it("bivariant type checking for functions with missing arguments (regression test for #827)", util.check([[
+      local fcts2: {string:function(val: any, val2: string)}
+
+      fcts2['foo'] = function (val: string)
+          print(val)
+      end
+
+      fcts2['foo2'] = function (val: string, val2: string)
+          print(val, val2)
+      end
+
+      fcts2['bar'] = function (val: number)
+          print(val)
+      end
+
+      fcts2['bar2'] = function (val: number, val2: string)
+          print(val, val2)
+      end
+   ]]))
+
 end)

tl.lua

@@ -8937,7 +8937,7 @@ a.types[i], b.types[i]), }
             local errs = {}
 
             local aa, ba = a.args.tuple, b.args.tuple
-            if (not b.args.is_va) and (self.feat_arity and a.min_arity > b.min_arity) then
+            if (not b.args.is_va) and (self.feat_arity and (#aa > #ba and a.min_arity > b.min_arity)) then
                table.insert(errs, Err("incompatible number of arguments: got " .. show_arity(a) .. " %s, expected " .. show_arity(b) .. " %s", a.args, b.args))
             else
                for i = ((a.is_method or b.is_method) and 2 or 1), #aa do

tl.tl

@@ -8937,7 +8937,7 @@ do
             local errs = {}
 
             local aa, ba = a.args.tuple, b.args.tuple
-            if (not b.args.is_va) and (self.feat_arity and a.min_arity > b.min_arity) then
+            if (not b.args.is_va) and (self.feat_arity and (#aa > #ba and a.min_arity > b.min_arity)) then
                table.insert(errs, Err("incompatible number of arguments: got " .. show_arity(a) .. " %s, expected " .. show_arity(b) .. " %s", a.args, b.args))
             else
                for i = ((a.is_method or b.is_method) and 2 or 1), #aa do