Enforcing one declaration / one definition rule and their matching

[divanchykhin]

Multiple declarations are, at the very least, hard to maintain, which may lead to mismatches and all hardly detectable errors.
See also examples at https://en.wikipedia.org/wiki/One_Definition_Rule
Comments are welcome!

[marcosbracco]

I will take a look to see what can be done.
But ODR at its core is an issue that can't be detected by looking at one translation unit at the time. Many issues require you to see all translation units at the same time to find them. And C++ builds one TU at the time.

[nobugs-hare]

@marcosbracco Sure it requires analysis of multiple TUs; however, in principle nothing prevents us from performing cross-TU analysis (say, by writing all definitions into a temporary file while we're going over all the TUs).