From aac2c745fe93bebbf5b19e8181712401eb1b5e7a Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 02:54:50 +0300
Subject: [PATCH] cgen: fix os.File.seek() not generating correct C code for
 mode parameter (fixes #12003)

---
 vlib/v/gen/c/fn.v                             |  7 ++-
 .../struct_init_field_call_or_block_test.v    | 62 +++++++++++++++++++
 2 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/tests/struct_init_field_call_or_block_test.v

diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index 01bd1bbb8..a53497fef 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -1996,7 +1996,12 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 					&& unwrapped_styp.starts_with('_v_') {
 					unwrapped_styp = unwrapped_styp[3..]
 				}
-				if node.is_return_used {
+				// Synthesized/transformed call nodes may lose `is_return_used`
+				// even though the enclosing C generation path is still
+				// emitting a value expression (struct fields, call args, etc).
+				// In those cases `is_gen_or_and_assign_rhs` still tells us that
+				// the unwrapped result is needed in the current expression.
+				if node.is_return_used || is_gen_or_and_assign_rhs {
 					is_fn := g.table.final_sym(unwrapped_typ).kind == .function
 					// return value is used, so we need to write the unwrapped temporary var
 					if is_fn && unwrapped_typ.nr_muls() > 0 {
diff --git a/vlib/v/tests/struct_init_field_call_or_block_test.v b/vlib/v/tests/struct_init_field_call_or_block_test.v
new file mode 100644
index 000000000..cc8fb2db7
--- /dev/null
+++ b/vlib/v/tests/struct_init_field_call_or_block_test.v
@@ -0,0 +1,62 @@
+module main
+
+enum SeekMode {
+	start
+	current
+	end
+}
+
+struct FileLike {}
+
+fn (mut f FileLike) seek(pos int, mode SeekMode) !int {
+	_ = mode
+	return pos
+}
+
+struct Token {
+	pos int
+}
+
+fn new_token(mut file FileLike, pos int) Token {
+	return Token{
+		pos: file.seek(pos, .start) or { return Token{} }
+	}
+}
+
+struct HeapToken {
+	kind   int
+	offset int
+	value  string
+	pos    int
+}
+
+struct Lexer {
+mut:
+	file FileLike
+}
+
+fn new_heap_token(mut file FileLike, pos int) &HeapToken {
+	return &HeapToken{0, 0, '', file.seek(pos, .start) or { return &HeapToken{} }}
+}
+
+fn (mut l Lexer) new_selector_heap_token(pos int) &HeapToken {
+	return &HeapToken{0, 0, '', l.file.seek(pos, .start) or { return &HeapToken{} }}
+}
+
+fn test_struct_init_field_call_with_or_block() {
+	mut file := FileLike{}
+	token := new_token(mut file, 123)
+	assert token.pos == 123
+}
+
+fn test_struct_init_field_call_with_or_block_in_heap_init() {
+	mut file := FileLike{}
+	token := new_heap_token(mut file, 456)
+	assert token.pos == 456
+}
+
+fn test_struct_init_field_call_with_or_block_on_selector_receiver() {
+	mut lexer := Lexer{}
+	token := lexer.new_selector_heap_token(789)
+	assert token.pos == 789
+}
-- 
2.39.5