On OS X, compiling introjucer off current tip, saving a project raises an assertion:
Loading project: /Users/adam/NewProject/NewProject.jucer
JUCE Assertion failure in jucer_Project.cpp:849
Printing description of compiledFile.fullPath:
(juce::String) fullPath = {
text = (data = "/Users/adam/JUCE/modules/juce_audio_plugin_client/VST3/juce_VST3_Wrapper.mm")
}
I noticed that just_VST3_Wrapper.mm was <a href="https://github.com/julianstorer/JUCE/commit/c63bf83bd90b03ec97eb459f31c210a0fb7e8ab0">removed</a> from github.
It seems the problem is that { "file": "VST3/juce_VST3_Wrapper.mm", "target": "xcode" } exists in modules/audio_plugin_client/juce_module_info, but juce_VST3_Wrapper.mm was removed, leading to the assertion mentioned in the OP.
I guess I shouldn't be running The Introjucer from XCode, but that's just how I've been doing it for years.