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. Its body was then checked as a switch branch, so a
    type error there said "But this switch is expected to return" and
    added the hint about switch branches, although the source has no
    switch. The synthesized match is now marked like the one for optional
    argument defaults and gets function-style diagnostics.

Diagnostic modified (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>

Author: cknitt

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
+}