Type a let destructuring a GADT without switch diagnostics

.agents/skills/records/SKILL.md

@@ -177,10 +177,32 @@ The table lists the common kinds.
 ## What a commit holds
 
 Every commit says why it is made. A commit that changes what the compiler
-or a tool does gets one object block for each object it changes, with the
-smallest example that shows the change. A refactor, a test, a script or a
-review fix needs no block, only its subject and a line or two of prose; a
-review fix names the finding it answers.
+or a tool does shows each representation, rule, transformation or decision
+it changes, Before and After, at the level where it operates, then the
+representative consequences that show why it matters: the blocks go from
+the changed operation to what it affects, and one block serves both when
+the operation directly changes the output shown. Each change the prose
+names, such as an attribute the compiler adds or a case a decision takes,
+is also shown in a block. Each block has the smallest example that shows
+its change.
+
+- A printed form shows the change when one exposes it. Otherwise a block
+  shows a schematic, captioned with the function that implements it: an
+  inference rule for a changed typing judgment, a clause matrix and its
+  split for the match compiler, a mapping from inputs to results for a
+  decision, keeping the premises, priorities and side conditions that
+  matter.
+- Printing each form for the example before and after the commit finds
+  the layers it changes; the record keeps the excerpts that show the
+  changed operation and its distinct consequences.
+- When an error stops later forms from being printed, the internal forms
+  come from a related example that compiles, the diagnostic from one that
+  fails, and the prose says how they relate.
+
+A commit whose changes are only organization, tests or documentation
+needs no block, only its subject and a line or two of prose; a script or
+a review fix that changes behaviour gets blocks like any other. A review
+fix names the finding it answers.
 
 ## Rendering
 

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
+}