Coverage: a tail-called body is still a body that ran

[revarbat]

Reported from the GUI overlay as "a ternary whose condition isn't covered, but whose branches are" — which should be impossible.

What was happening

recordTailCallHop never recorded the coverage hit that enterUserCall does. A tail call hops the frame instead of pushing one, so it never reaches enterUserCall — and a hop is the only way into a tail-called body. A function reached only in tail position reported its body as never run, while its own arms reported hits.

There is no span for a ternary's condition — the body span covers it. So an uncovered body paints the declaration line and the condition red while the covered arms stay green, which is exactly what it looked like on screen.

BOSL2's _str_split_recurse is the shape that shows it, because recursion through a ternary arm is a tail call every time:

L239 body    hits=0    function _str_split_recurse(str,sep,i,result) =
L240 branch  hits=1    concat(result,[str])
L241 branch  hits=2    let(...)

Now 3, 1, 2 — the body's count is the sum of its arms, as it must be.

Minimal repro

Same function, same work, only the call shape differs:

function inner(i)          = i == 0 ? "done" : inner(i-1);
function tail_caller(i)    = inner(i);            // inner's body: 0 hits
function nontail_caller(i) = str("", inner(i));   // inner's body: 1 hit

The fix

One coverHit(calleeDecl) in recordTailCallHop, beside the noteActiveDeclExit/noteActiveDeclEnter pair already there for the same reason: a hop swaps which declaration occupies the frame, so every piece of per-declaration bookkeeping has to follow it. Putting it there rather than at the Op::CallFnTail call site keeps it on the one path every hop takes.

Tests

Coverage.VmAndInterpreterAgree would have caught this years ago if anything in it recursed in tail position — nothing did. The new Coverage.TailCalledBodyIsCounted runs under both the VM and the interpreter and asserts the body count equals the sum of the arms. Verified it fails with the fix reverted.

1258 C++ tests and 42 binding tests pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JFTVMTH6yEktHDPF5csksb