Project dependencies ignored when compiling custom source set

When the demo source set is created, a number of configurations are also created (demoImplementation, demoRuntimeOnly, see Java Plugin docs for full list). While you are setting demo's compile and runtime classpaths to include the compiled output of the main source set, you are not configuring the compile or runtime classpath dependencies using the corresponding demo* configurations. If you want demo's configurations to mirror the main source set’s, then use the extendsFrom method of the configurations. For example, configurations.demoImplementation.extendsFrom(configurations.implementation).