In what contexts or disciplines does "One may assume X" imply "One may ignore the possibility of any statement contrary to X being true"?

You seem to assume that this assumption is absolute and that it allows the compilers to completely detach from reality.

I don't believe that this is the case and I am pretty sure that compiler design is as evidence-driven as all other human activities. That is, if such assumptions allow to create a compiler that, despite following the standards to the letter, will produce buggy, slow or otherwise worthless code, this compiler won't see much use and will be abandoned in favour of another one that makes more reasonable assumptions.

If my assumption is valid, this would make compiler design not particularly unique among various disciplines.

Example: Tolerance limits in engineering design.

Tolerance limits might be a good example.

For example, engineers are often asked to design

• shelves, that hold up to a certain weight;

• electrical wires, that carry up to a certain electrical current;

• web servers, that serve up to so many web clients;

• towers, that withstand up to so much wind;

and other things that must work up to a certain limit. By knowing a limitation, someone designing a book shelf can ignore, say, the possibility that 10 tonnes of books might be piled on-top.

The underlying logic is that the designer (compiler, etc.) isn't held accountable for its behavior in the excluded cases, so it doesn't need to worry about them.