Type a let destructuring a GADT without switch diagnostics

Why:
    A `let` whose pattern contains a GADT constructor is typed as a
    one-case match, and that match took the diagnostic context of a
    `switch` in the source: a type error in the rest of the block said
    "But this switch is expected to return" and added the hint about
    switch branches, although the source has no switch. The match now
    carries `#let_gadt_pattern`, and its cases, like those of the match
    for an optional argument default, get function-style diagnostics.

Typedtree modified (let destructuring a GADT constructor):
Example:
    type rec any = Any('a, 'a => string): any
    let get = value => {
      let Any(x, f) = value
      f(x)
    }
Before (typed):
    expression
      Texp_match
      expression
        Texp_ident "value/1004"
      [
        <case>
          pattern
            Tpat_construct "Any"
            …
          expression
            Texp_apply
            …
      ]
After (typed):
    expression
      attribute "#let_gadt_pattern"
        []
      Texp_match
      expression
        Texp_ident "value/1004"
      [
        <case>
          pattern
            Tpat_construct "Any"
            …
          expression
            Texp_apply
            …
      ]

Decision modified (diagnostic context of a match's cases):
Before (text): `call_context` in `Typecore.type_expect_`
    match with @let.unwrap                    ↦ LetUnwrap
    match with #optional_arg_default          ↦ Function
    match typed for a let with a GADT pattern ↦ Switch
    any other match                           ↦ Switch
After (text):
    match with @let.unwrap                    ↦ LetUnwrap
    match with #optional_arg_default          ↦ Function
    match with #let_gadt_pattern              ↦ Function
    any other match                           ↦ Switch

Diagnostic modified (type error in the body of a let destructuring a GADT):
Example:
    type rec any = Any('a, 'a => string): any

    let get = value => {
      let Any(x, _) = value
      x
    }
Before (diagnostic):
    This has type: $a
    But this switch is expected to return: 'a
    $a is an existential type bound by the constructor Any.
    It is only known in the scope of the pattern that matches Any.

    All branches in a switch must return the same type.
    To fix this, change your branch to return the expected type.
After (diagnostic):
    This has type: $a
    But it's expected to have type: 'a
    $a is an existential type bound by the constructor Any.
    It is only known in the scope of the pattern that matches Any.

Fixes: #9069
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kbf3gLMawVgtmiKy3TMBc3
Claude-Session: https://claude.ai/code/session_01Kr4cjoUi2f5VpMvE6wT9bo
Signed-off-by: Christoph Knittel <ck@cca.io>
Signed-off-by: Cristiano Calcagno <cristianoc@users.noreply.github.com>

Author: 2 people

changes/existential-let-context.polish.md

@@ -0,0 +1 @@
+- A type error in the body after a `let` that destructures a GADT constructor no longer talks about switch branches.

compiler/ml/typecore.ml

@@ -2644,6 +2644,8 @@ and type_expect_ ?deprecated_context ~context ?(recarg = Rejected) env sexp
       {
         sexp with
         pexp_desc = Pexp_match (sval, [Ast_helper.Exp.case spat sbody]);
+        pexp_attributes =
+          (mknoloc "#let_gadt_pattern", PStr []) :: sexp.pexp_attributes;
       }
       ty_expected
   | Pexp_let (rec_flag, spat_sexp_list, sbody) ->
@@ -2832,10 +2834,13 @@ and type_expect_ ?deprecated_context ~context ?(recarg = Rejected) env sexp
     in
     let call_context =
       (* Optional-default lowering synthesizes a match expression, but from the
-         user's point of view this is still part of a function definition. Use
-         function-style diagnostics instead of reporting a phantom switch. *)
+         user's point of view this is still part of a function definition. A
+         [let] destructuring a GADT constructor is typed as a one-case match,
+         whose body is the rest of the block. Use function-style diagnostics
+         for both instead of reporting a phantom switch. *)
       if has_attr "let.unwrap" then `LetUnwrap
-      else if has_attr "#optional_arg_default" then `Function
+      else if has_attr "#optional_arg_default" || has_attr "#let_gadt_pattern"
+      then `Function
       else `Switch
     in
     let val_cases, partial =

tests/build_tests/super_errors/expected/gadt_existential_let.res.expected

@@ -0,0 +1,14 @@
+
+  We've found a bug for you!
+  /.../fixtures/gadt_existential_let.res:5:3
+
+  3 │ let get = value => {
+  4 │   let Any(x, _) = value
+  5 │   x
+  6 │ }
+  7 │ 
+
+  This has type: $a
+  But it's expected to have type: 'a
+  $a is an existential type bound by the constructor Any.
+  It is only known in the scope of the pattern that matches Any.

tests/build_tests/super_errors/fixtures/gadt_existential_let.res

@@ -0,0 +1,6 @@
+type rec any = Any('a, 'a => string): any
+
+let get = value => {
+  let Any(x, _) = value
+  x
+}