checker,gen: fix generic type inference from a callback with Result return type

vlib/v/checker/check_types.v

@@ -1819,7 +1819,7 @@ fn (mut c Checker) infer_fn_generic_types(func &ast.Fn, mut node ast.CallExpr) {
 						}
 						if param_sym.info.func.return_type.has_flag(.generic)
 							&& c.table.sym(param_sym.info.func.return_type).name == gt_name {
-							typ = arg_sym.info.func.return_type
+							typ = arg_sym.info.func.return_type.clear_option_and_result()
 							if param_sym.info.func.return_type.nr_muls() > 0 && typ.nr_muls() > 0 {
 								typ = typ.set_nr_muls(0)
 							}

vlib/v/gen/c/fn.v

@@ -3890,7 +3890,7 @@ fn (mut g Gen) update_generic_call_concrete_types_from_fn_types(generic_names []
 	}
 	if param_fn.func.return_type.has_flag(.generic) {
 		gt_name := g.table.sym(param_fn.func.return_type).name
-		mut return_type := arg_fn.func.return_type
+		mut return_type := arg_fn.func.return_type.clear_option_and_result()
 		if arg.expr is ast.LambdaExpr && return_type.has_flag(.generic) {
 			return_type = g.type_resolver.unwrap_generic_expr(arg.expr.expr, return_type)
 			if return_type.has_flag(.generic) {

vlib/v/tests/generics/generic_fn_callback_result_return_test.v

@@ -0,0 +1,13 @@
+fn run[T](cb fn () !T) !T {
+	mut r := T{}
+	r = cb()!
+	return r
+}
+
+fn test_generic_fn_callback_result_return() {
+	cb := fn () !int {
+		return 42
+	}
+
+	assert run(cb)! == 42
+}