Move tuple heuristic on patterns to the typer

Motivation and Context

Xavier reported a bug on the following JsLIGO snippet:

const f = ([] : unit) : int => 42

due to the heuristics in pattern matching which are not great:

Invalid type(s)
Cannot unify "list (^a)" with "unit".
Hint: "^a" represent placeholder type(s).

Description

This MR proposes to move the heuristics for pattern matching to the typer.

Changes:

• rename P_tuple_with_ellipsis to P_array to be consistent with expressions, and re-use Array_repr.
• use a decorator on patterns (which is identity on ast-typed, and kept a bool in ast-core).
• add a P_typed on Pattern.t: it is only inhabited on ast-core, for ast-typed/ast-aggregated is the empty type.
• removed List ... pattern for using directly Nil, as it didn't work in the typer before anyway.
• implement the heuristic logic in infer_tuple_pattern, it has basically four cases:
  • the tuple is [] and it is a single clause pattern: it is understood as a pattern on unit.
  • the tuple is [] and it is a non-single clause pattern: it is understood as a pattern on empty list.
  • the tuple has the form [x1, ..., xn, ...xs]: it is understood as a pattern on a list.
  • in other cases: it is understood as a pattern on a tuple.

Example:

const f = ([] : unit) : unit => [];
const g = ([]) : unit => []; // unit => unit
const h = (l) => match (l) { when([]): 42; } // unit => int
const i = (l : unit) => match (l) { when([]): 42; }
const j = (l) => match (l) { when([]): 42; default: 0; } // list<a> => int
const k = (l : list<int>) => match (l) { when([]): 42; default: 0; }

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Checklist:

• If a new syntax has been introduced, put a message on slack ligo-lsp
• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).