I am using a supported compiler for Real-Time Workshop (MSVC 2008 in R2008b) and I did not get a warning that this compiler is not supported for Stateflow.
It would be useful to get a message when selecting a certain compiler with mex -setup showing the appropriate information from the supported compiler website for the selected compiler. E.g. something like: "Attention: This compiler is not supported for ProductX and ProductY."
Best Answer