Represent a let that destructures a GADT, and other synthesized matches, explicitly in the typed tree #9126

This is Claude, filing on behalf of @cristianoc after a look at #9081.

The type checker types a let whose pattern contains a GADT constructor by rewriting it into a one-case match (Typecore.type_expect_, the Pexp_let … when contains_gadt env spat case):

type rec any = Any('a, 'a => string): any

let get = value => {
  let Any(x, f) = value
  f(x)
}

expression
  attribute "#let_gadt_pattern"
    []
  Texp_match
  expression
    Texp_ident "value/1004"
  [
    <case>
      pattern
        Tpat_construct "Any"
      …

The typed tree records where this match came from only as a string attribute, which #9081 adds. The same goes for the match for an optional argument default (#optional_arg_default) and for let? (let.unwrap). Each consumer that needs to know the origin, such as call_context, which selects the diagnostic wording, has to match on those strings. A match that lacks the marker, or a new consumer that doesn't check for it, falls back to switch behaviour. That fallback is how #9069 happened.

Related observations:

• The rewrite applies only when the binding has no attributes (pvb_attributes = [], with a TODO: allow non-empty attributes?). A GADT let that carries an attribute takes the ordinary let path.
• bsc -dtypedtree prints neither pattern types nor a constructor's existentials, so the GADT information this rewrite exists for doesn't appear in the typed-tree dump.

Goal: a more principled representation, where the typed tree states the origin of a synthesized match, or represents the GADT let directly, so that consumers dispatch on a constructor rather than on attribute names. Two options:

• A typed origin field on Texp_match, for example source switch, GADT let, optional default or let?, set where the match is synthesized.
• A typed let form that scopes the existentials over the body.

Which one fits best, including how the printer and analysis read it back, is open.

🤖 Generated with Claude Code