fix: method on array-index/deref receiver mutates the live place (issue 0145)

A *self method called directly on arr[i] (or a deref place) fell through to an
alloca+store-of-value, so the callee mutated a throwaway copy and the live slot
was never written. fixupMethodReceiver now takes the real address of
.index_expr/.deref_expr receivers via lowerExprAsPtr (normalized to *T),
mirroring the explicit-argument path. A comptime-pack index (xs[i] where xs is
a pack) is excluded -- a pack has no runtime storage to address -- so it keeps
flowing through the general copy path.

Regression: examples/0188-types-method-array-index-receiver.sx

examples/0188-types-method-array-index-receiver.sx

@@ -0,0 +1,26 @@
+// A `*self` method called directly on an array-index place (`arr[i].method()`)
+// must mutate the LIVE array slot, not a throwaway copy. `fixupMethodReceiver`
+// now takes the real address of `.index_expr`/`.deref_expr` receivers (via
+// `lowerExprAsPtr`), mirroring the explicit-argument path — instead of falling
+// through to an alloca+store-of-value that the callee then mutates in vain.
+//
+// Regression (issue 0145).
+#import "modules/std.sx";
+
+S :: struct {
+    flag: bool;
+    set :: (self: *S) { self.flag = true; }
+}
+
+A :: struct { items: [4]S; }
+
+main :: () -> i32 {
+    a : A = .{};
+    a.items[1].set();                          // direct: must mutate the slot
+    print("direct = {}\n", a.items[1].flag);   // true
+
+    p := @a.items[1];
+    p.set();                                   // via explicit pointer
+    print("ptr    = {}\n", a.items[1].flag);   // true
+    0
+}

examples/expected/0188-types-method-array-index-receiver.exit

@@ -0,0 +1 @@
+0

examples/expected/0188-types-method-array-index-receiver.stderr

@@ -0,0 +1 @@
+

examples/expected/0188-types-method-array-index-receiver.stdout

@@ -0,0 +1,2 @@
+direct = true
+ptr    = true

issues/0145-method-on-array-index-receiver-copies-element.md

@@ -0,0 +1,78 @@
+# 0145 — method with `*self` called directly on an array-index expression operates on a COPY
+
+> **RESOLVED.** `fixupMethodReceiver` (src/ir/lower/expr.zig) now takes the
+> real address of `.index_expr` and `.deref_expr` receivers via
+> `lowerExprAsPtr` (normalizing to `*T`), mirroring the explicit-argument path
+> in `call.zig` — so `arr[i].method()` mutates the live slot instead of a
+> throwaway copy. A comptime-pack index (`xs[i]` where `xs` is a pack) is
+> explicitly excluded: a pack has no runtime storage to address, so it keeps
+> flowing through the general alloca+store-of-value path. Regression test:
+> `examples/0188-types-method-array-index-receiver.sx`.
+
+## Summary
+
+Calling a method whose receiver is `*self` (mutating) directly on a
+fixed-array element expression — `arr[i].method(...)` — mutates a temporary
+COPY of the element, not the live array slot. The mutation is silently lost.
+Binding the element to a pointer first (`p := @arr[i]; p.method(...)`) works
+correctly.
+
+## Repro
+
+```
+S :: struct {
+    flag: bool;
+    set :: (self: *S) { self.flag = true; }
+}
+
+A :: struct { items: [4]S; }
+
+main :: () -> i32 {
+    a : A = .{};
+    a.items[1].set();                 // BUG: mutates a copy
+    print("direct = {}\n", a.items[1].flag);   // prints false
+
+    p := @a.items[1];
+    p.set();                          // OK: mutates the live slot
+    print("ptr    = {}\n", a.items[1].flag);   // prints true
+    0
+}
+```
+
+Observed:
+```
+direct = false
+ptr    = true
+```
+
+Expected: both print `true` — `a.items[1].set()` takes `*self` and should bind
+the receiver to the address of `a.items[1]`, exactly as the explicit-pointer
+form does.
+
+The same surfaced with a non-trivial method (`Slider.handle_event(self: *Slider,
+...)`): the direct call returned `true` (so the method body ran) yet left the
+element's `pressed`/`value` fields unchanged, while `@arr[i]` bound to a local
+and called on that pointer mutated the element as expected.
+
+## Impact
+
+Any struct that holds a fixed array of widgets/records and dispatches `*self`
+methods per element (a layers panel with one `Slider` per row, an entity table,
+etc.) silently no-ops the mutation. It is easy to miss because the method's
+return value is correct — only the in-place writes vanish.
+
+## Workaround
+
+Bind the element to a pointer before the call:
+
+```
+p := @arr[i];
+p.method(...);
+```
+
+Field stores through the index (`arr[i].field = v`) and value assignment
+(`arr[i] = v`) appear unaffected; only the implicit `&arr[i]` receiver of a
+`*self` method call is materialized as a copy.
+
+Found while implementing `ui/layers_panel.sx` in the photo editor (one opacity
+`Slider` per layer row).

src/ir/lower/expr.zig

@@ -282,15 +282,33 @@ pub fn fixupMethodReceiver(self: *Lowering, method_args: *std.ArrayList(Ref), fu
                     }
                 }
             }
-            // Field access: obj.field.method() → GEP to field, pass pointer directly.
+            // Compound lvalue receiver: obj.field.method() / arr[i].method() /
-            // This avoids copying the struct value (mutations through *T must be visible).
+            // (*p).method() → take the lvalue's real address so mutations
-            if (obj_node.data == .field_access) {
+            // through *T are visible on the original storage (not a throwaway
-                const gep_ref = self.lowerExprAsPtr(obj_node);
+            // copy). Mirrors the explicit-arg path in call.zig.
-                // GEP returns a pointer in LLVM but its IR type is the field value type.
+            //
-                // Wrap with addr_of (no-op in LLVM) to set the IR type to *T,
+            // Exclude a comptime-pack index (`xs[i]` where `xs` is a pack): a
+            // pack has no runtime storage to address — its element is materialized
+            // at comptime and can't be mutated in place — so it must keep flowing
+            // through the general alloca+store-of-value path below.
+            const is_pack_index = obj_node.data == .index_expr and
+                obj_node.data.index_expr.object.data == .identifier and
+                self.isPackName(obj_node.data.index_expr.object.data.identifier.name);
+            if (!is_pack_index and (obj_node.data == .field_access or obj_node.data == .index_expr or obj_node.data == .deref_expr)) {
+                // `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`: if it's already the pointer
+                // type, pass it directly; if it's the pointee value type, wrap
+                // with addr_of (a no-op in LLVM) to set the IR type to *T,
                 // preventing coerceCallArgs from doing a spurious alloca+store.
                 const ptr_ty = self.module.types.ptrTo(obj_ty);
-                method_args.items[0] = self.builder.emit(.{ .addr_of = .{ .operand = gep_ref } }, ptr_ty);
+                const place = self.lowerExprAsPtr(obj_node);
+                const place_ty = self.builder.getRefType(place);
+                if (place_ty == ptr_ty) {
+                    method_args.items[0] = place;
+                } else {
+                    method_args.items[0] = self.builder.emit(.{ .addr_of = .{ .operand = place } }, ptr_ty);
+                }
                 return;
             }
             // General case: alloca+store the value and pass the alloca pointer