buildtype not derived from debug and optimization options, with surprising results

[dnicolodi]

The documentation states that the buildtype option, when not explicitly provided, is calculated from the debug and buildtype options:

For setting optimization levels and toggling debug, you can either set the buildtype option, or you can set the optimization and debug options which give finer control over the same. Whichever you decide to use, the other will be deduced from it. For example, -Dbuildtype=debugoptimized is the same as -Ddebug=true -Doptimization=2 and vice-versa.

However, this is not the case.

To reproduce, configure any Meson project withe meson setup -Ddebug=false and run meson configure to observe that buildtype is still reported as debug.

If Meson behavior would be based only on the debug and optimization options, this would be maybe only a cosmetic issue. However, the b_vscrt option default value is static_from_buildtype, which, as the name suggests, looks at buildtype for deriving which runtime library to link with. This results in debug=false builds to be linked with the debug version of the runtime library, unless the buildype is also explicitly changed. Another possibly confusing option is b_ndebug when set to if-release. It results in -DNDEBUG being passed to the compiler only when buildtype is explicitly set to release however I've seen multiple instances where it was believed that if-release is synonym with debug=false. I think there are other places where buildtype may be used for behavior decisions.

This would not be too bad if Meson would not steer users away from using the buildtype and either of debug and optimization options at the same time with a warning: there are instances where just setting debug or optimization is not enough to obtain the desired behavior.

I think it is already recognized that the buildtype concept is flawed and it is kept around for backward compatibility (correct me if I'm wrong) see for example #10808 (I also remember a blog post from Jussi, but I could be making this up) but we are stuck with it.

I wish that Meson could base all its decision based only on the debug and optimization flags, converting the buildtype option to values for these two when parsing options and then forgetting about it. I don't know if this is feasible. Failing that, the documentation needs some updates (see above the missing translation from debug and optimization to buildtype advertised in the documentation) and needs some more flashy warnings for the foot guns above.