From e44ba4b240b9bd50bd099aec91c36540fec0d200 Mon Sep 17 00:00:00 2001
From: agra <alex@swipelab.co>
Date: Wed, 27 May 2026 16:22:49 +0300
Subject: [PATCH] =?UTF-8?q?ffi=20M5.A.next.2b.fu2.A:=20generic=20\$R=20pac?=
 =?UTF-8?q?k-fn=20=E2=80=94=20lock=20in=20silent-zero=20return?=
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Follow-up #2 from step 2b: pack-fns with a generic return type
(`(..\$args) -> \$R`). Today's `monomorphizePackFn` calls
`resolveReturnType` which sees `\$R` as a generic name and
returns an opaque struct TypeId. The mono's ret_ty is wrong
and the value silently coerces to 0.

`examples/159-pack-generic-ret.sx` pins this: `first(42)` and
`first(99)` both return `0` instead of the call arg. The lock-in
captures the wrong output as the snapshot to flip.

Next commit infers the ret type from the body's tail expression
(arrow form) or the first explicit `return X;` (block form),
then builds the mono signature against that concrete type.

198/198 example tests + \`zig build test\` green.
---
 examples/159-pack-generic-ret.sx         | 23 +++++++++++++++++++++++
 tests/expected/159-pack-generic-ret.exit |  1 +
 tests/expected/159-pack-generic-ret.txt  |  1 +
 3 files changed, 25 insertions(+)
 create mode 100644 examples/159-pack-generic-ret.sx
 create mode 100644 tests/expected/159-pack-generic-ret.exit
 create mode 100644 tests/expected/159-pack-generic-ret.txt

diff --git a/examples/159-pack-generic-ret.sx b/examples/159-pack-generic-ret.sx
new file mode 100644
index 0000000..9e863ef
--- /dev/null
+++ b/examples/159-pack-generic-ret.sx
@@ -0,0 +1,23 @@
+// Variadic heterogeneous type packs — follow-up #2 (generic $R
+// return type).
+//
+// A pack-fn's return type can be a generic name (`$R`) — bound at
+// the call site to match the body's natural type or the caller's
+// target. Today's `monomorphizePackFn` calls `resolveReturnType`
+// which treats `$R` as an opaque struct, so the mono's signature
+// gets a wrong ret_ty and the value is silently zero / garbage.
+//
+// `first(42)` should return 42; the lock-in pins today's `0`.
+// Next commit infers the ret type from the body's tail expression
+// (or first `return X;`) and rebuilds the mono signature.
+
+#import "modules/std.sx";
+
+first :: (..$args) -> $R => args[0];
+
+main :: () -> s32 {
+    a : s64 = first(42);
+    b : s64 = first(99);
+    print("{} {}\n", a, b);
+    return 0;
+}
diff --git a/tests/expected/159-pack-generic-ret.exit b/tests/expected/159-pack-generic-ret.exit
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/tests/expected/159-pack-generic-ret.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/expected/159-pack-generic-ret.txt b/tests/expected/159-pack-generic-ret.txt
new file mode 100644
index 0000000..b748e2d
--- /dev/null
+++ b/tests/expected/159-pack-generic-ret.txt
@@ -0,0 +1 @@
+0 0