fix subtype error messages and add fixtures

Author: zth

jscomp/build_tests/super_errors/expected/variant_pattern_type_spreads_not_subtype.res.expected

@@ -0,0 +1,11 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_pattern_type_spreads_not_subtype.res:7:5-13
+
+  5 │ let lookup = (b: b) =>
+  6 │   switch b {
+  7 │   | ...c as c => Js.log(c)
+  8 │   | Four => Js.log("four")
+  9 │   | Five => Js.log("five")
+
+  Type c is not a subtype of b

jscomp/build_tests/super_errors/expected/variant_pattern_type_spreads_not_variant.res.expected

@@ -0,0 +1,12 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_pattern_type_spreads_not_variant.res:7:8
+
+  5 │ let lookup = (b: b) =>
+  6 │   switch b {
+  7 │   | ...c as c => Js.log(c)
+  8 │   | Four => Js.log("four")
+  9 │   | Five => Js.log("five")
+
+  The type c
+is not a variant type

jscomp/build_tests/super_errors/fixtures/variant_pattern_type_spreads_not_subtype.res

@@ -0,0 +1,10 @@
+type a = One | Two | Three
+type b = | ...a | Four | Five
+type c = Six | Seven
+
+let lookup = (b: b) =>
+  switch b {
+  | ...c as c => Js.log(c)
+  | Four => Js.log("four")
+  | Five => Js.log("five")
+  }

jscomp/build_tests/super_errors/fixtures/variant_pattern_type_spreads_not_variant.res

@@ -0,0 +1,10 @@
+type a = One | Two | Three
+type b = | ...a | Four | Five
+type c = {name: string}
+
+let lookup = (b: b) =>
+  switch b {
+  | ...c as c => Js.log(c)
+  | Four => Js.log("four")
+  | Five => Js.log("five")
+  }

jscomp/ml/typecore.ml

@@ -1182,7 +1182,12 @@ and type_pat_aux ~constrs ~labels ~no_existentials ~mode ~explode ~env
       end
     | Ppat_alias({ppat_desc=Ppat_type lid; ppat_attributes}, name) when Variant_coercion.has_res_pat_variant_spread_attribute ppat_attributes ->
       let (_, p, ty) = build_or_pat_for_variant_spread !env loc lid expected_ty in
-      Ctype.subtype !env ty expected_ty ();
+      (try 
+        Ctype.subtype !env ty expected_ty () 
+      with 
+        Ctype.Subtype (tr1, tr2) -> 
+          raise(Error(loc, !env, Not_subtype(tr1, tr2)))
+      );
       assert (constrs = None);
 
       let id = enter_variable ~is_as_variable:true loc name ty in
@@ -1519,7 +1524,12 @@ and type_pat_aux ~constrs ~labels ~no_existentials ~mode ~explode ~env
         in k p)
   | Ppat_type lid when Variant_coercion.has_res_pat_variant_spread_attribute sp.ppat_attributes ->
     let (path, p, ty) = build_or_pat_for_variant_spread !env loc lid expected_ty in
-      Ctype.subtype !env ty expected_ty ();
+      (try 
+        Ctype.subtype !env ty expected_ty () 
+      with 
+        Ctype.Subtype (tr1, tr2) -> 
+          raise(Error(loc, !env, Not_subtype(tr1, tr2)))
+      );
     k { p with pat_extra =
       (Tpat_type (path, lid), loc, sp.ppat_attributes) :: p.pat_extra }
   | Ppat_type lid ->

jscomp/test/VariantPatternMatchingSpreads.res

@@ -1,5 +1,4 @@
 type a = One | Two | Three
-type a1 = One
 type b = | ...a | Four | Five
 
 let doWithA = (a: a) => {