Language lawyer
The person who can tell you what the standard actually says, as distinct from what the compiler happens to do.
Jargon for someone with detailed knowledge of a language's formal specification, able to decide whether a program is correct by clause rather than by experience. The expression predates the web and appears in the Jargon File. On Stack Overflow, [language-lawyer] is a tag of its own, populated mainly by C and C++ questions.
The role is less academic than it sounds. The difference between "works" and "is defined" is decisive in languages with undefined behaviour, where an optimisation can turn code that ran fine for fifteen years into something else entirely. The term still carries a mild irony: it hints at someone who would rather quote the standard than solve the problem.