"valgrind and msan will give you an immediate and false positive message"
Why a false positive? Are you one of those people that 'knows' that their code is correct and always blames the tool? What you describe sounds like a real error. Sort of. The error is not triggered by reading the value, it gets triggered when it is used in some conditional context and the undefinedness has an observable impact on the execution of the program.
This is a change from undefined/erroneous behaviour to something else - defined but maybe not what you wanted.
I agree that this can make error analysis more difficult.
Why a false positive? Are you one of those people that 'knows' that their code is correct and always blames the tool? What you describe sounds like a real error. Sort of. The error is not triggered by reading the value, it gets triggered when it is used in some conditional context and the undefinedness has an observable impact on the execution of the program.
This is a change from undefined/erroneous behaviour to something else - defined but maybe not what you wanted.
I agree that this can make error analysis more difficult.