[AlotOfReading]

C standard N3096, Section 4:

    A conforming program is one that is acceptable to a conforming implementation.

That definition goes all the way back to C89. The C++ standard drops it for the term "well-formed program", but adds enough clarifications in 1.4 to mean essentially the same thing.

[tialaramex]

Ah, no. Most C++ programs that compile are not well-formed programs. This functions as an escape hatch for Rice's Theorem. You see, C++ even more so than C has semantic requirements - but Rice says all non-trivial semantic requiremnts are Undecidable. So, if you want what C++ says it wants it appears that compilers would be entirely impossible and that's awkward. To "fix" that C++ says it's fine if the compiler will compile your program even though it is not well-formed, the program doesn't have any meaning of course, only well-formed programs have meaning, but it did compile, so as a programmer you're happy...

C++ has a recurring phrase in its standard document "Ill-formed No Diagnostic Required" or IFNDR which carries this intent. The compiler can't tell you made a mistake, but you didn't actually write a valid C++ program so -shrug-

Because there's no way to tell for sure without exhaustive human examination we don't know for sure how many C++ programs aren't actually well-formed but experts who've thought about it tend to think the answer for large C++ software projects is all or most of them.