patch 8.2.1722: Vim9: cannot assign a lambda to a variable of type function

Problem:    Vim9: cannot assign a lambda to a variable of type function.
Solution:   Allow for assigning a partial to a variable of type function.
            (Naruhiko Nishino, closes #6996)

src/testdir/test_vim9_expr.vim

@@ -1687,6 +1687,18 @@ def Test_expr7_lambda_vim9script()
 	->map({_, v -> synIDattr(v, 'name')})->len()})
   END
   CheckScriptSuccess(lines)
+
+  # check if assign a lambda to a variable which type is func or any.
+  lines =<< trim END
+      vim9script
+      let FuncRef = {->123}
+      assert_equal(123, FuncRef())
+      let FuncRef_Func: func = {->123}
+      assert_equal(123, FuncRef_Func())
+      let FuncRef_Any: any = {->123}
+      assert_equal(123, FuncRef_Any())
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def Test_epxr7_funcref()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1722,
 /**/
     1721,
 /**/

src/vim9type.c

@@ -463,7 +463,11 @@ check_type(type_T *expected, type_T *actual, int give_msg, int argidx)
 	    && !(expected->tt_type == VAR_ANY && actual->tt_type != VAR_VOID))
 
     {
-	if (expected->tt_type != actual->tt_type)
+	// tt_type should match, except that a "partial" can be assigned to a
+	// variable with type "func".
+	if (!(expected->tt_type == actual->tt_type
+		    || (expected->tt_type == VAR_FUNC
+					   && actual->tt_type == VAR_PARTIAL)))
 	{
 	    if (expected->tt_type == VAR_BOOL
 					&& (actual->tt_flags & TTFLAG_BOOL_OK))