From f6336fc0ecbd4ce3e91c24a67c01f207d16674d5 Mon Sep 17 00:00:00 2001
From: CreeperFace <165158232+dy-tea@users.noreply.github.com>
Date: Tue, 4 Nov 2025 17:53:53 +0000
Subject: [PATCH] cgen: fix passing sumtype child to generic function taking
 sumtype parent (fix #25660) (#25664)

---
 vlib/v/gen/c/fn.v                             | 34 +++++++++++++++++++
 .../generic_array_of_sumtype_push_test.v      | 28 +++++++++++++++
 2 files changed, 62 insertions(+)

diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index 36ca46614..3e74c6151 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -2411,6 +2411,40 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 			}
 		}
 	}
+	if node.is_method {
+		left_sym := g.table.sym(node.left_type)
+		if left_sym.info is ast.Struct && left_sym.info.generic_types.len > 0
+			&& left_sym.info.concrete_types.len > 0 {
+			base_type_idx := g.table.find_type_idx(left_sym.name.all_before('['))
+			mut func := ast.Fn{}
+			mut found := false
+			for {
+				if base_type_idx > 0 {
+					base_sym := g.table.sym(ast.idx_to_type(base_type_idx))
+					if base_func := g.table.find_method(base_sym, node.name) {
+						func = base_func
+						found = true
+						break
+					}
+				}
+				if base_func := g.table.find_method(left_sym, node.name) {
+					func = base_func
+					found = true
+				}
+				break
+			}
+			if found && func.generic_names.len > 0 {
+				for i in 0 .. expected_types.len {
+					mut muttable := unsafe { &ast.Table(g.table) }
+					if utyp := muttable.convert_generic_type(node.expected_arg_types[i],
+						func.generic_names, left_sym.info.concrete_types)
+					{
+						expected_types[i] = utyp
+					}
+				}
+			}
+		}
+	}
 	// only v variadic, C variadic args will be appended like normal args
 	is_variadic := node.language == .v && node.is_variadic && expected_types.len > 0
 		&& expected_types.last().has_flag(.variadic)
diff --git a/vlib/v/tests/generics/generic_array_of_sumtype_push_test.v b/vlib/v/tests/generics/generic_array_of_sumtype_push_test.v
index 1975ffaea..0402746b3 100644
--- a/vlib/v/tests/generics/generic_array_of_sumtype_push_test.v
+++ b/vlib/v/tests/generics/generic_array_of_sumtype_push_test.v
@@ -17,3 +17,31 @@ fn test_generic_array_of_sumtype_push() {
 	assert ret2.len == 1
 	assert ret2[0] == SumType('hello')
 }
+
+struct EventA {
+	a u32
+}
+
+struct EventB {
+	b u32
+}
+
+type Event = EventA | EventB
+
+struct Queue[T] {
+mut:
+	data []T
+}
+
+fn (mut q Queue[T]) push(val T) {
+	q.data << val
+}
+
+fn test_generic_queue_sumtype_structs() {
+	mut queue := Queue[Event]{}
+	queue.push(EventA{ a: 10 })
+	assert queue.data.len == 1
+	assert queue.data[0] == Event(EventA{
+		a: 10
+	})
+}
-- 
2.39.5