From bc640bb41103ebabf78237eed2a0b92088d1c2c4 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 05:22:52 +0300
Subject: [PATCH] cgen: fix comptime $for f in t.fields over generic struct
 with ?[]t / ?[n]t fields (fixes #26903)

---
 vlib/v/gen/c/assign.v                         |  6 +++-
 .../comptime_for_fixed_array_field_test.v     | 29 +++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 1d6309282..60c974ccf 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -1440,7 +1440,11 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 			} else if g.inside_for_c_stmt {
 				g.expr(val)
 			} else if var_type.has_flag(.option) {
-				g.expr_with_opt(val, val_type, var_type)
+				// Blank ident option types can be stale across generic instantiations.
+				// Use the actual RHS option type when it is available so `_ = val`
+				// does not rewrap the expression into a mismatched option payload.
+				blank_option_type := if val_type.has_flag(.option) { val_type } else { var_type }
+				g.expr_with_opt(val, val_type, blank_option_type)
 			} else {
 				if left_sym.kind == .function {
 					g.write('{void* _ = ')
diff --git a/vlib/v/tests/comptime/comptime_for_fixed_array_field_test.v b/vlib/v/tests/comptime/comptime_for_fixed_array_field_test.v
index 4c43aca65..440ed435b 100644
--- a/vlib/v/tests/comptime/comptime_for_fixed_array_field_test.v
+++ b/vlib/v/tests/comptime/comptime_for_fixed_array_field_test.v
@@ -84,3 +84,32 @@ fn test_fixed_array_comptime_iteration() {
 	assert field_values[0].contains('999')
 	assert field_values[3].contains('iteration')
 }
+
+struct GenericOptionFilter[T] {
+mut:
+	selector  ?T
+	selector2 ?[]T
+	selector3 ?[1]T
+}
+
+fn process_generic_option_field[T](val T) {
+	_ = val
+}
+
+fn count_generic_option_fields[T](val T) int {
+	mut count := 0
+	$for f in T.fields {
+		process_generic_option_field(val.$(f.name))
+		count++
+	}
+	return count
+}
+
+fn test_comptime_for_generic_option_array_fields() {
+	t := GenericOptionFilter[string]{
+		selector:  'aa'
+		selector2: ['a', 'b']
+		selector3: ['z']!
+	}
+	assert count_generic_option_fields(t) == 3
+}
-- 
2.39.5