patch 8.2.3844: Vim9: no type error if assigning func(number) to func(string)

Problem:    Vim9: no type error if assigning a value with type func(number) to
            a variable of type func(string).
Solution:   Use check_type_maybe(): return MAYBE if a runtime type check is
            useful.  (issue #8492)

src/proto/vim9type.pro

@@ -16,6 +16,7 @@ void type_mismatch(type_T *expected, type_T *actual);
 void arg_type_mismatch(type_T *expected, type_T *actual, int arg_idx);
 void type_mismatch_where(type_T *expected, type_T *actual, where_T where);
 int check_type(type_T *expected, type_T *actual, int give_msg, where_T where);
+int check_type_maybe(type_T *expected, type_T *actual, int give_msg, where_T where);
 int check_argument_types(type_T *type, typval_T *argvars, int argcount, char_u *name);
 char_u *skip_type(char_u *start, int optional);
 type_T *parse_type(char_u **arg, garray_T *type_gap, int give_error);

src/testdir/test_vim9_assign.vim

@@ -2146,6 +2146,23 @@ def Test_script_funcref_case()
   CheckScriptFailure(lines, 'E704:')
 enddef
 
+def Test_script_funcref_runtime_type_check()
+  var lines =<< trim END
+      vim9script
+      def FuncWithNumberArg(n: number)
+      enddef
+      def Test()
+        var Ref: func(string) = function(FuncWithNumberArg)
+      enddef
+      defcompile
+  END
+  # OK at compile time
+  CheckScriptSuccess(lines)
+
+  # Type check fails at runtime
+  CheckScriptFailure(lines + ['Test()'], 'E1012: Type mismatch; expected func(string) but got func(number)')
+enddef
+
 def Test_inc_dec()
   var lines =<< trim END
       var nr = 7

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3844,
 /**/
     3843,
 /**/

src/vim9compile.c

@@ -1061,6 +1061,8 @@ need_type_where(
 	int	silent,
 	int	actual_is_const)
 {
+    int ret;
+
     if (expected == &t_bool && actual != &t_bool
 					&& (actual->tt_flags & TTFLAG_BOOL_OK))
     {
@@ -1070,13 +1072,14 @@ need_type_where(
 	return OK;
     }
 
-    if (check_type(expected, actual, FALSE, where) == OK)
+    ret = check_type_maybe(expected, actual, FALSE, where);
+    if (ret == OK)
 	return OK;
 
     // If the actual type can be the expected type add a runtime check.
     // If it's a constant a runtime check makes no sense.
-    if ((!actual_is_const || actual == &t_any)
-					    && use_typecheck(actual, expected))
+    if (ret == MAYBE || ((!actual_is_const || actual == &t_any)
+					   && use_typecheck(actual, expected)))
     {
 	generate_TYPECHECK(cctx, expected, offset, where.wt_index);
 	return OK;

src/vim9type.c

@@ -531,9 +531,31 @@ type_mismatch_where(type_T *expected, type_T *actual, where_T where)
  * Check if the expected and actual types match.
  * Does not allow for assigning "any" to a specific type.
  * When "argidx" > 0 it is included in the error message.
+ * Return OK if types match.
+ * Return FAIL if types do not match.
  */
     int
-check_type(type_T *expected, type_T *actual, int give_msg, where_T where)
+check_type(
+	type_T	*expected,
+	type_T	*actual,
+	int	give_msg,
+	where_T where)
+{
+    int ret = check_type_maybe(expected, actual, give_msg, where);
+
+    return ret == MAYBE ? OK : ret;
+}
+
+/*
+ * As check_type() but return MAYBE when a runtime type check should be used
+ * when compiling.
+ */
+    int
+check_type_maybe(
+	type_T	*expected,
+	type_T	*actual,
+	int	give_msg,
+	where_T where)
 {
     int ret = OK;
 
@@ -568,17 +590,21 @@ check_type(type_T *expected, type_T *actual, int give_msg, where_T where)
 	{
 	    // If the return type is unknown it can be anything, including
 	    // nothing, thus there is no point in checking.
-	    if (expected->tt_member != &t_unknown
-					    && actual->tt_member != &t_unknown)
+	    if (expected->tt_member != &t_unknown)
+	    {
+		if (actual->tt_member != &t_unknown)
 		    ret = check_type(expected->tt_member, actual->tt_member,
 								 FALSE, where);
-	    if (ret == OK && expected->tt_argcount != -1
+		else
+		    ret = MAYBE;
+	    }
+	    if (ret != FAIL && expected->tt_argcount != -1
 		    && actual->tt_min_argcount != -1
 		    && (actual->tt_argcount == -1
 			|| (actual->tt_argcount < expected->tt_min_argcount
 			    || actual->tt_argcount > expected->tt_argcount)))
 		ret = FAIL;
-	    if (ret == OK && expected->tt_args != NULL
+	    if (ret != FAIL && expected->tt_args != NULL
 						    && actual->tt_args != NULL)
 	    {
 		int i;
@@ -593,10 +619,21 @@ check_type(type_T *expected, type_T *actual, int give_msg, where_T where)
 			break;
 		    }
 	    }
+	    if (ret == OK && expected->tt_argcount >= 0
+						  && actual->tt_argcount == -1)
+		// check the argument count at runtime
+		ret = MAYBE;
 	}
 	if (ret == FAIL && give_msg)
 	    type_mismatch_where(expected, actual, where);
     }
+
+    if (ret == OK && expected->tt_type != VAR_UNKNOWN
+	    && expected->tt_type != VAR_ANY
+	    && (actual->tt_type == VAR_UNKNOWN || actual->tt_type == VAR_ANY))
+	// check the type at runtime
+	ret = MAYBE;
+
     return ret;
 }