cgen: fix vweb using generic method (fix #15888) (#15963)

vlib/v/gen/c/comptime.v

@@ -467,12 +467,19 @@ fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
 		if methods.len > 0 {
 			g.writeln('FunctionData $node.val_var = {0};')
 		}
-		for method in methods { // sym.methods {
-			/*
-			if method.return_type != vweb_result_type { // ast.void_type {
-				continue
+		typ_vweb_result := g.table.find_type_idx('vweb.Result')
+		for method in methods {
+			// filter vweb route methods (non-generic method)
+			if method.receiver_type != 0 && method.return_type == typ_vweb_result {
+				rec_sym := g.table.sym(method.receiver_type)
+				if rec_sym.kind == .struct_ {
+					if _ := g.table.find_field_with_embeds(rec_sym, 'Context') {
+						if method.generic_names.len > 0 {
+							continue
+						}
+					}
+				}
 			}
-			*/
 			g.comptime_for_method = method.name
 			g.writeln('/* method $i */ {')
 			g.writeln('\t${node.val_var}.name = _SLIT("$method.name");')

vlib/vweb/vweb_app_test.v

@@ -64,3 +64,31 @@ fn (mut app App) time_json_pretty() {
 		'time': time.now().format()
 	})
 }
+
+struct ApiSuccessResponse<T> {
+	success bool
+	result  T
+}
+
+fn (mut app App) json_success<T>(result T) vweb.Result {
+	response := ApiSuccessResponse<T>{
+		success: true
+		result: result
+	}
+
+	return app.json(response)
+}
+
+// should compile, this is a helper method, not exposed as a route
+fn (mut app App) some_helper<T>(result T) ApiSuccessResponse<T> {
+	response := ApiSuccessResponse<T>{
+		success: true
+		result: result
+	}
+	return response
+}
+
+// should compile, the route method itself is not generic
+fn (mut app App) ok() vweb.Result {
+	return app.json(app.some_helper(123))
+}