Fix nominal pattern bound variable collection (issue #8689)

collectBoundVarsToScratch was not recursing into the backing pattern
for nominal and nominal_external patterns. This caused variables
bound in nominal patterns (e.g., Wrapper.Simple(s)) to not be tracked
as bound variables, leading to them incorrectly being included in the
free variables of match expression bodies.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/canonicalize/Can.zig

@@ -2534,6 +2534,14 @@ fn collectBoundVarsToScratch(self: *Self, pattern_idx: Pattern.Idx) !void {
                 }
             }
         },
+        .nominal => |n| {
+            // Recurse into the backing pattern to collect bound variables
+            try self.collectBoundVarsToScratch(n.backing_pattern);
+        },
+        .nominal_external => |n| {
+            // Recurse into the backing pattern to collect bound variables
+            try self.collectBoundVarsToScratch(n.backing_pattern);
+        },
         .num_literal,
         .small_dec_literal,
         .dec_literal,
@@ -2541,8 +2549,6 @@ fn collectBoundVarsToScratch(self: *Self, pattern_idx: Pattern.Idx) !void {
         .frac_f64_literal,
         .str_literal,
         .underscore,
-        .nominal,
-        .nominal_external,
         .runtime_error,
         => {},
     }

src/cli/test/fx_test_specs.zig

@@ -243,6 +243,11 @@ pub const io_spec_tests = [_]TestSpec{
         .io_spec = "1>is ok",
         .description = "Regression test: List.get with method syntax (issue #8662)",
     },
+    .{
+        .roc_file = "test/fx/issue8689.roc",
+        .io_spec = "1>hello",
+        .description = "Regression test: Field access on record payload in tag union wrapped by opaque type (issue #8689)",
+    },
 };
 
 /// Get the total number of IO spec tests

test/fx/issue8689.roc

@@ -0,0 +1,19 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+import pf.Stdout
+
+Wrapper := [
+    WithRecord({ name : Str, value : U64 }),
+    Simple(Str),
+]
+
+getName : Wrapper -> Str
+getName = |w| match w {
+    Wrapper.Simple(s) => s
+    Wrapper.WithRecord(payload) => payload.name
+}
+
+main! = || {
+    wrapper = Wrapper.WithRecord({ name: "hello", value: 42 })
+    Stdout.line!(getName(wrapper))
+}