Hah! No wonder you don't grok my perspective. If you derive a C++ compiler that accepts file A as valid C++. And I derive a C++ compiler that rejects file A as

Hah! No wonder you don't grok my perspective. If you derive a C++ compiler that accepts file A as valid C++. And I derive a C++ compiler that rejects file A as valid C++, then from the lens of type theory the two compilers have different type-signatures! They are not the same formal language. One, or both compilers implement a language that is not C++.