fix: Register union operator in BinaryOperatorType

The FHIRPath union operator (|) was fully implemented in
UnionOperator but never registered in the operator resolution
chain. Parsing | threw UnsupportedFhirPathFeatureError, and the
DSL tests that cover it were silently skipped. Adding a UNION
enum constant wires the existing implementation into the parser.

Author: johngrimes

fhirpath/src/main/java/au/csiro/pathling/fhirpath/operator/BinaryOperatorType.java

@@ -66,7 +66,9 @@ public enum BinaryOperatorType {
   /** Arithmetic division operator. */
   DIVISION("/", new MathOperator(MathOperation.DIVISION)),
   /** Arithmetic modulus operator. */
-  MODULUS("mod", new MathOperator(MathOperation.MODULUS));
+  MODULUS("mod", new MathOperator(MathOperation.MODULUS)),
+  /** Union operator. */
+  UNION("|", new UnionOperator());
 
   private final String symbol;
 

openspec/changes/archive/2026-02-17-fix-union-operator-registration/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-02-17

openspec/changes/archive/2026-02-17-fix-union-operator-registration/design.md

@@ -0,0 +1,47 @@
+## Context
+
+The FHIRPath parser resolves binary operators in `Visitor.visitBinaryOperator()`
+through a two-stage lookup:
+
+1. `BINARY_OPERATORS` map (built from `@FhirPathOperator`-annotated methods in
+   `CollectionOperations`) — contains `in` and `contains`.
+2. `BinaryOperatorType.fromSymbol()` enum lookup — contains all arithmetic,
+   comparison, equality, and boolean operators.
+
+The union operator `|` is not registered in either location.
+`UnionOperator` exists with correct logic but is unreachable.
+
+## Goals / Non-goals
+
+**Goals:**
+
+- Make `|` resolve to `UnionOperator` during FHIRPath parsing.
+- Ensure the existing `CombiningOperatorsDslTest` tests execute and pass.
+
+**Non-goals:**
+
+- Changing the `UnionOperator` implementation itself.
+- Adding new test cases beyond what already exists.
+
+## Decisions
+
+### Register `|` in the `BinaryOperatorType` enum
+
+Add `UNION("|", new UnionOperator())` to the enum. This is consistent with how
+all other non-collection operators (arithmetic, comparison, equality, boolean)
+are registered.
+
+**Alternative considered:** Register via `CollectionOperations` with a
+`@FhirPathOperator(name = "|")` annotated method. Rejected because
+`CollectionOperations` uses `MethodDefinedOperator` (method-reflection-based
+dispatch), while `UnionOperator` is a class-based operator extending
+`SameTypeBinaryOperator`. Mixing the two patterns would be inconsistent.
+
+## Risks / Trade-offs
+
+- **Low risk.** The `UnionOperator` is already tested (tests exist, just
+  skipped). Wiring it in should cause those tests to pass without any other
+  changes.
+- The test framework's silent skipping of `UnsupportedFhirPathFeatureError`
+  masked this gap. No change to that framework behaviour is proposed here, but
+  it is worth noting as a broader concern.

openspec/changes/archive/2026-02-17-fix-union-operator-registration/proposal.md

@@ -0,0 +1,31 @@
+## Why
+
+The FHIRPath union operator (`|`) is fully implemented in `UnionOperator` but
+never registered in the operator resolution chain. Parsing `|` throws
+`UnsupportedFhirPathFeatureError`, and the DSL tests that cover it are silently
+skipped by the test framework (which catches that error and converts it to a
+JUnit `TestAbortedException`).
+
+## What changes
+
+- Register the `UnionOperator` in `BinaryOperatorType` so that parsing `|`
+  resolves to the existing implementation.
+- Ensure the DSL tests in `CombiningOperatorsDslTest` actually execute rather
+  than being silently skipped.
+
+## Capabilities
+
+### New capabilities
+
+(none)
+
+### Modified capabilities
+
+(none - this is a bug fix, not a requirement change)
+
+## Impact
+
+- `BinaryOperatorType.java` — add a new enum constant.
+- `CombiningOperatorsDslTest` — tests will transition from skipped to passing.
+- All contexts that parse FHIRPath (server, library API, fhirpath-lab-api) gain
+  working union operator support.

openspec/changes/archive/2026-02-17-fix-union-operator-registration/specs/.gitkeep

@@ -0,0 +1,2 @@
+No specification changes. This is a bug fix that wires existing implementation
+to existing parsing infrastructure.

openspec/changes/archive/2026-02-17-fix-union-operator-registration/specs/no-spec-changes.md

@@ -0,0 +1,8 @@
+## 1. Register the union operator
+
+- [x] 1.1 Add `UNION("|", new UnionOperator())` to the `BinaryOperatorType` enum in `fhirpath/src/main/java/au/csiro/pathling/fhirpath/operator/BinaryOperatorType.java`
+
+## 2. Verify
+
+- [x] 2.1 Run `CombiningOperatorsDslTest` and confirm all 19 tests execute and pass (not skipped)
+- [x] 2.2 Run the full fhirpath module test suite to check for regressions