Diagnostics

The parser validates the shape of a when and its completion clauses, rejecting malformed forms with three dedicated diagnostics instead of a generic "Expected expression" cascade.

ARIA-WHEN-001 — stray then / end

A then or end block must follow a when body. Writing one on its own is an error:

then {            // error: ARIA-WHEN-001
    // ...
}

error: ARIA-WHEN-001: 'then' block has no preceding 'when' —
       'then'/'end' clauses may only follow a 'when' loop body

Fix: attach the block to a when, or remove it if it was a stray paste.

ARIA-WHEN-002 — duplicate then / end

A when may carry at most one then and one end. A second of either is rejected:

when (i < 1i32) {
    i = i + 1i32;
} then {
    // ...
} then {          // error: ARIA-WHEN-002
    // ...
}

error: ARIA-WHEN-002: duplicate 'then' block — a 'when' may have at most
       one 'then' clause

Fix: merge the two blocks into one, or move the extra logic after the when.

ARIA-WHEN-003 — then after end

The clauses are ordered: then comes before end. Writing them in the wrong order is rejected:

when (i < 1i32) {
    i = i + 1i32;
} end {
    // ...
} then {          // error: ARIA-WHEN-003
    // ...
}

error: ARIA-WHEN-003: 'then' block must precede 'end' —
       write 'when (...) { } then { } end { }'

Fix: swap the blocks so then precedes end.

Why dedicated diagnostics

Before v0.39.5 these mistakes surfaced as a confusing chain of parser errors — the stray keyword was parsed as the start of an expression, producing "Expected expression" followed by a cascade of unrelated messages. The ARIA-WHEN-* codes point straight at the real problem and tell you the fix.