Handle generic function in Signature

src/coro.zig

@@ -266,26 +266,29 @@ const CoroT = struct {
         YieldT: type = void,
         InjectT: type = void,
         ArgsT: type,
+        _ReturnT: type,
 
         // If the function this signature represents is compile-time known,
         // it can be held here.
         func_ptr: ?type = null,
 
         fn init(comptime Func: anytype, comptime options: CoroT.Options) @This() {
             const FuncT = if (@TypeOf(Func) == type) Func else @TypeOf(Func);
+            const ArgsT = options.ArgsT orelse ArgsTuple(FuncT);
             return .{
                 .Func = FuncT,
                 .YieldT = options.YieldT,
                 .InjectT = options.InjectT,
-                .ArgsT = options.ArgsT orelse ArgsTuple(FuncT),
+                .ArgsT = ArgsT,
                 .func_ptr = if (@TypeOf(Func) == type) null else struct {
                     const val = Func;
                 },
+                ._ReturnT = @TypeOf(@call(.auto, Func, @as(ArgsT, undefined))),
             };
         }
 
         pub fn ReturnT(comptime self: @This()) type {
-            return @typeInfo(self.Func).Fn.return_type.?;
+            return self._ReturnT;
         }
     };