From 9f143f79ecfcc901af8a80062f58d0b19ea8472e Mon Sep 17 00:00:00 2001
From: Richard Feldman <oss@rtfeldman.com>
Date: Tue, 16 Dec 2025 07:53:38 -0500
Subject: [PATCH] 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       | 10 ++++++++--
 src/cli/test/fx_test_specs.zig |  5 +++++
 test/fx/issue8689.roc          | 19 +++++++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)
 create mode 100644 test/fx/issue8689.roc

diff --git a/src/canonicalize/Can.zig b/src/canonicalize/Can.zig
index 631f9fd297..fcc8376880 100644
--- a/src/canonicalize/Can.zig
+++ b/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,
         => {},
     }
diff --git a/src/cli/test/fx_test_specs.zig b/src/cli/test/fx_test_specs.zig
index b15013697c..079cca245b 100644
--- a/src/cli/test/fx_test_specs.zig
+++ b/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
diff --git a/test/fx/issue8689.roc b/test/fx/issue8689.roc
new file mode 100644
index 0000000000..5dc3275f8c
--- /dev/null
+++ b/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))
+}