From 49dc6222349344b7f42338672dd85954efaed64d Mon Sep 17 00:00:00 2001
From: agra <alex@swipelab.co>
Date: Mon, 1 Jun 2026 17:56:51 +0300
Subject: [PATCH] fix(lower): auto-ref compound lvalues passed to *T params
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The implicit address-of that gives `*T` params reference semantics only
fired for plain identifiers (`mut(v)`). For a field-access / index /
deref lvalue (`make_move(self.board, m)`, `mut(w.s)`), the branch was
skipped: the arg was loaded into a temporary and the callee mutated a
throwaway copy — silent data loss, with the type check satisfied through
the temp so no diagnostic fired.

Now compound lvalues auto-ref too: take the real lvalue address via
`lowerExprAsPtr`, normalizing the "place" ref to `*T` exactly as
`@field_access` does. Mutations through the pointer are now visible to
the caller, matching the identifier case.

Regression: examples/255-autoref-compound-lvalue.sx.
---
 examples/255-autoref-compound-lvalue.sx       | 20 ++++++++++++
 src/ir/lower.zig                              | 31 +++++++++++++++++++
 .../expected/255-autoref-compound-lvalue.exit |  1 +
 .../expected/255-autoref-compound-lvalue.txt  |  1 +
 4 files changed, 53 insertions(+)
 create mode 100644 examples/255-autoref-compound-lvalue.sx
 create mode 100644 tests/expected/255-autoref-compound-lvalue.exit
 create mode 100644 tests/expected/255-autoref-compound-lvalue.txt

diff --git a/examples/255-autoref-compound-lvalue.sx b/examples/255-autoref-compound-lvalue.sx
new file mode 100644
index 0000000..ee546a2
--- /dev/null
+++ b/examples/255-autoref-compound-lvalue.sx
@@ -0,0 +1,20 @@
+// Auto-ref of a COMPOUND lvalue (field access / index / deref) passed to a
+// `*T` parameter must reference the REAL lvalue, not a copy. Regression: a
+// field-access argument (e.g. `make_move(self.board, m)`) silently passed the
+// address of a temporary copy, so mutations through the pointer were lost with
+// no diagnostic. A plain local (`bump(x)`) already auto-ref'd; now compound
+// lvalues do too. Expected: w.s.v == 42 (the real field was mutated).
+
+#import "modules/std.sx";
+
+S :: struct { v: s32; }
+W :: struct { s: S; }
+
+bump :: (p: *S) { p.v = p.v + 41; }
+
+main :: () -> s32 {
+    w : W = .{ s = .{ v = 1 } };
+    bump(w.s);                       // field access, no `@` — auto-refs &w.s
+    print("w.s.v = {}\n", w.s.v);    // 42, not 1
+    return 0;
+}
diff --git a/src/ir/lower.zig b/src/ir/lower.zig
index d83da96..ff10617 100644
--- a/src/ir/lower.zig
+++ b/src/ir/lower.zig
@@ -6784,6 +6784,37 @@ pub const Lowering = struct {
                     }
                 }
             }
+            // Implicit address-of for compound lvalues (field access / index /
+            // deref): when the param expects `*T` and the arg is an addressable
+            // lvalue of type `T`, pass the lvalue's real address (GEP) — same
+            // reference semantics as the identifier case above. Without this the
+            // arg would be loaded into a temporary and the callee would mutate a
+            // throwaway copy (silent data loss — e.g. `make_move(self.board, m)`).
+            if (ai < param_types.len and (arg.data == .field_access or arg.data == .index_expr or arg.data == .deref_expr)) {
+                const pt = param_types[ai];
+                if (!pt.isBuiltin()) {
+                    const pti = self.module.types.get(pt);
+                    if (pti == .pointer and self.inferExprType(arg) == pti.pointer.pointee) {
+                        // `lowerExprAsPtr` yields the lvalue's address, typed
+                        // either as `*T` already (index/deref) or as the pointee
+                        // `T` (a field "place" ref); normalize to `*T` — exactly
+                        // what `@field_access` does.
+                        const place = self.lowerExprAsPtr(arg);
+                        const place_ty = self.builder.getRefType(place);
+                        const ref: ?Ref = if (place_ty == pt)
+                            place
+                        else if (place_ty == pti.pointer.pointee)
+                            self.builder.emit(.{ .addr_of = .{ .operand = place } }, pt)
+                        else
+                            null;
+                        if (ref) |r| {
+                            args.append(self.alloc, r) catch unreachable;
+                            self.target_type = saved_target;
+                            continue;
+                        }
+                    }
+                }
+            }
             const val = self.lowerExpr(arg);
             self.target_type = saved_target;
             // Passing a `*T` where a `T` value is expected — a by-reference loop
diff --git a/tests/expected/255-autoref-compound-lvalue.exit b/tests/expected/255-autoref-compound-lvalue.exit
new file mode 100644
index 0000000..c227083
--- /dev/null
+++ b/tests/expected/255-autoref-compound-lvalue.exit
@@ -0,0 +1 @@
+0
\ No newline at end of file
diff --git a/tests/expected/255-autoref-compound-lvalue.txt b/tests/expected/255-autoref-compound-lvalue.txt
new file mode 100644
index 0000000..01f9c06
--- /dev/null
+++ b/tests/expected/255-autoref-compound-lvalue.txt
@@ -0,0 +1 @@
+w.s.v = 42