GumpacG opened a new pull request, #3556:
URL: https://github.com/apache/tinkerpop/pull/3556
## Summary
`subgraph()` produces an edge-induced subgraph and must be called at an edge
step.
When the traversal instead feeds it a non-Edge value, `SubgraphStep` passed
that
value straight into an `Edge`-typed method, producing a raw
`ClassCastException`
with no indication of what went wrong.
This change validates the traverser value at runtime and throws a descriptive
`IllegalStateException` naming the required `Edge` input, consistent with the
existing pattern in `AddPropertyStep` and `AddEdgeStep`. Valid queries are
unaffected; only the failure message for already-invalid queries changes.
This is a backward-compatible diagnostics fix (runtime validation only, no
API or
generics change).
## Before vs After
### Non-edge (primitive) input
```groovy
g.inject(1).subgraph('sg').cap('sg').iterate()
```
- Before: `ClassCastException: class java.lang.Integer cannot be cast to
class ...Edge`
- After: `IllegalStateException: subgraph() requires Edge input but
encountered Integer; use an edge step such as outE(), inE(), or bothE()`
### Vertex input
```groovy
g.addV().subgraph('sg').cap('sg').iterate()
```
- Before: `ClassCastException: class ...TinkerVertex cannot be cast to class
...Edge`
- After: `IllegalStateException: subgraph() requires Edge input but
encountered TinkerVertex; use an edge step such as outE(), inE(), or bothE()`
### Correct usage (unchanged)
```groovy
g.V().has('v',1).outE('X').subgraph('sg').cap('sg') // edges in -> builds
subgraph
```
- Before and after: works identically
## Changes
- `gremlin-core`: runtime `Edge` validation in `SubgraphStep.sideEffect(...)`
- `gremlin-test`: two negative tests added to the `SubgraphTest` suite
- `CHANGELOG.asciidoc`: entry under 3.7.7
Assisted-by: Kiro: Claude Opus 4.8
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