From 39d51fc26d236a2138bafda1e53a0aec21972d7e Mon Sep 17 00:00:00 2001
From: agra <alex@swipelab.co>
Date: Sun, 31 May 2026 13:56:45 +0300
Subject: [PATCH] lower: diagnose passing a by-ref loop capture where a value
 is expected

`for xs: (*m)` binds `m` to a `*T`. Passing it directly to a parameter
that wants `T` produced invalid IR that only LLVM's verifier caught, with
the opaque 'Call parameter type does not match function signature'. Detect
it at the call site and emit a clear error with a fix-it suggesting `m.*`.

Add example 215 + expected output as a regression test.
---
 .../215-ref-capture-value-arg-diagnostic.sx    | 18 ++++++++++++++++++
 src/ir/lower.zig                               | 17 +++++++++++++++++
 .../215-ref-capture-value-arg-diagnostic.exit  |  1 +
 .../215-ref-capture-value-arg-diagnostic.txt   | 10 ++++++++++
 4 files changed, 46 insertions(+)
 create mode 100644 examples/215-ref-capture-value-arg-diagnostic.sx
 create mode 100644 tests/expected/215-ref-capture-value-arg-diagnostic.exit
 create mode 100644 tests/expected/215-ref-capture-value-arg-diagnostic.txt

diff --git a/examples/215-ref-capture-value-arg-diagnostic.sx b/examples/215-ref-capture-value-arg-diagnostic.sx
new file mode 100644
index 0000000..3ffd40f
--- /dev/null
+++ b/examples/215-ref-capture-value-arg-diagnostic.sx
@@ -0,0 +1,18 @@
+// A by-reference loop capture (`for xs: (*m)`) binds `m` to a `*T`.
+// Passing it where a `T` value is expected used to slip through to the
+// LLVM verifier ("Call parameter type does not match function signature").
+// The compiler now reports it at the call site with a fix-it: write `m.*`.
+
+#import "modules/std.sx";
+
+Move :: struct { flag: s64; }
+
+take :: (m: Move) -> s64 { return m.flag; }
+
+main :: () -> s32 {
+    moves : [2]Move = .[ Move.{ flag = 1 }, Move.{ flag = 2 } ];
+    for moves: (*m) {
+        take(m);
+    }
+    return 0;
+}
diff --git a/src/ir/lower.zig b/src/ir/lower.zig
index 7f89f09..99e54a4 100644
--- a/src/ir/lower.zig
+++ b/src/ir/lower.zig
@@ -6569,6 +6569,23 @@ pub const Lowering = struct {
                     }
                 }
             }
+            // A by-reference loop capture (`for xs: (*m)`) binds `m` to a `*T`.
+            // Passing it where a `T` value is expected used to slip through as a
+            // type-mismatched call that only LLVM's verifier rejected; report it
+            // here with a fix-it instead.
+            if (ai < param_types.len and arg.data == .identifier) {
+                if (self.refCapturePointee(arg)) |pointee| {
+                    if (pointee == param_types[ai]) {
+                        if (self.diagnostics) |d| {
+                            const nm = arg.data.identifier.name;
+                            const tn = self.formatTypeName(pointee);
+                            const fix = std.fmt.allocPrint(self.alloc, "{s}.*", .{nm}) catch nm;
+                            const pid = d.addFmtId(.err, arg.span, "by-reference loop capture '{s}' has type '*{s}', but '{s}' is expected here", .{ nm, tn, tn });
+                            d.addHelpFmt(pid, arg.span, fix, "dereference it to pass the value: `{s}`", .{fix});
+                        }
+                    }
+                }
+            }
             const val = self.lowerExpr(arg);
             self.target_type = saved_target;
             args.append(self.alloc, val) catch unreachable;
diff --git a/tests/expected/215-ref-capture-value-arg-diagnostic.exit b/tests/expected/215-ref-capture-value-arg-diagnostic.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/expected/215-ref-capture-value-arg-diagnostic.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/expected/215-ref-capture-value-arg-diagnostic.txt b/tests/expected/215-ref-capture-value-arg-diagnostic.txt
new file mode 100644
index 0000000..6aa3ad8
--- /dev/null
+++ b/tests/expected/215-ref-capture-value-arg-diagnostic.txt
@@ -0,0 +1,10 @@
+error: by-reference loop capture 'm' has type '*Move', but 'Move' is expected here
+  --> /Users/agra/projects/sx/examples/215-ref-capture-value-arg-diagnostic.sx:15:14
+   |
+15 |         take(m);
+   |              ^
+
+help: dereference it to pass the value: `m.*`
+   |
+15 | m.*
+   |              ^