What it means. The transformer reached a pattern AST node it does not recognize. This is a defensive check: the grammar is supposed to prevent any pattern shape the transformer cannot handle from parsing in the first place. Seeing this code means either a new grammar rule for patterns was added without matching transformer support, or the grammar accepts a shape that has never been supported.
This is a transpiler bug, not an error in your program.
The header reads error[GALA-E0009]: unrecognized pattern syntax (internal type *grammar.…Context), followed by a framed snippet of the offending pattern and a hint asking you to report it.
No triggering program is quoted here on purpose: this code fires only when the grammar and the transformer disagree, which no well-formed source can force. The internal type in the message names the grammar context the transformer could not handle — copy it verbatim into your report.
File an issue with:
internal type *grammar.…Context string from the message,gala version).As a temporary workaround, rewrite the pattern in a simpler form — a plain binding plus a guard, or a nested match — and the transpiler will take a supported path.
This site previously produced an untracked error with no source span, so users could not tell “transpiler bug” from “my syntax is wrong”. The coded error makes the bug class explicit and greppable, so issues can be filed against a stable tag.