Review fix legend pure compiler mismatch

[FeruzGS]

Purpose

Aligns Legend Java compiler's handling of single-element collection literals

([$x]) with Pure's own compiler. Previously, wrapping any non-[1] variable

in brackets — e.g. [$optional] where $optional : String[0..1] — was

rejected by ValueSpecificationBuilder.visit(Collection) with:

Collection element must have a multiplicity [1] ... multiplicity:[0..1]

Reproduction (before fix)

function test::f(opt:String[0..1], many:String[*]):String[*]

{

   concatenate([$opt], $many)

}

• Pure IDE: compiles

• Legend compiler: [Error] "Collection element must have a multiplicity [1] ... [0..1]"

Fix

In ValueSpecificationBuilder.visit(Collection):

1. Guard relaxed to multi-element case only. The [1]-only element rule

   now fires only when collection.values.size() > 1. Single-element literals

   [$x] are accepted regardless of the wrapped variable's multiplicity —

   they act as a semantically transparent wrapper (Pure has no nested

   collections; [$x] and $x produce the same flat shape).

2. Effective multiplicity propagated truthfully. For a 1-element literal

   wrapping a non-[1] variable, the resulting InstanceValue's multiplicity

   is set to the inner variable's multiplicity (not the parser-supplied count

   of 1). This ensures downstream consumers see the real runtime cardinality

   rather than being told "exactly 1" when the value could be [0..1] or

   [*].

3. Unwrap guarded by isExactlyOne. The value-compaction step (unwrap a

   single-element InstanceValue into its raw first value) now only fires

   when the element's multiplicity is exactly [1].

For every collection shape that compiled before this change, the resulting

M3 graph is byte-identical. Only newly-accepted shapes produce a new graph,

and that graph matches what Pure's own compiler produces from equivalent

.pure source.

Patterns unblocked

All five surface syntaxes that lower to visit(Collection):

1. Function arguments — concatenate([$opt], $many) (the ticket case).

2. if branches — if($cond, | [$opt], | []).

3. Class constructor slots — ^Foo(xs = [$opt]) where xs : String[*].

4. Pipeline steps — $people->filter(p | [$p.middleName]->isNotEmpty()).

Invariant preserved

Multi-element literals [a, b, ...] still require every element to be [1].

This is the invariant that parser-rewritten binary operators rely on:

$a + 'x'  →  plus([$a, 'x'])   -- 2-element, still strict-checked

Files changed

• ValueSpecificationBuilder.java — the fix (+ isExactlyOne helper).

• TestDomainCompilationFromGrammar.java — 5 regression tests.

[github-actions]

Test Results

  1 026 files    1 026 suites   3h 8m 8s ⏱️
16 866 tests 16 695 ✔️ 171 💤 0 ❌
36 976 runs  36 805 ✔️ 171 💤 0 ❌

Results for commit 46104b2.