From 288ce180560f3a715e74c3fdf14fcfbbbe1c953a Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 23 Apr 2026 19:38:44 +0300
Subject: [PATCH] checker: fix Generic method unable to implement an interface
 with a different interface passed as argument (fixes #20670)

---
 vlib/v/checker/if.v                           |  3 +-
 vlib/v/checker/infix.v                        |  3 +-
 .../tests/interface_generic_implements_test.v | 41 +++++++++++++++++++
 3 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/vlib/v/checker/if.v b/vlib/v/checker/if.v
index 4ff196a6a..06e2bbf0c 100644
--- a/vlib/v/checker/if.v
+++ b/vlib/v/checker/if.v
@@ -699,7 +699,8 @@ fn (mut c Checker) smartcast_if_conds(mut node ast.Expr, mut scope ast.Scope, co
 			}
 
 			if right_type != ast.no_type {
-				right_sym := c.table.sym(right_type)
+				resolved_right_type := c.unwrap_generic(right_type)
+				right_sym := c.table.final_sym(resolved_right_type)
 				mut expr_type := c.unwrap_generic(node.left_type)
 				left_sym := c.table.sym(expr_type)
 				left_final_sym := c.table.final_sym(expr_type)
diff --git a/vlib/v/checker/infix.v b/vlib/v/checker/infix.v
index 6d887391d..861547128 100644
--- a/vlib/v/checker/infix.v
+++ b/vlib/v/checker/infix.v
@@ -965,7 +965,8 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 							node.pos.line_nr + 1}')
 					}
 				}
-				typ_sym := c.table.sym(typ)
+				resolved_typ := c.unwrap_generic(typ)
+				typ_sym := c.table.final_sym(resolved_typ)
 				op := node.op.str()
 				if left_type.has_flag(.option) && !c.inside_sql {
 					c.error('${node.left} is an Optional, it needs to be unwrapped first',
diff --git a/vlib/v/tests/interface_generic_implements_test.v b/vlib/v/tests/interface_generic_implements_test.v
index 2a4994fcc..f3c10e0a8 100644
--- a/vlib/v/tests/interface_generic_implements_test.v
+++ b/vlib/v/tests/interface_generic_implements_test.v
@@ -62,3 +62,44 @@ fn test_generic_interface_value_can_be_returned_as_interface_ref() {
 	assert d.a == 200
 	assert e.a == 200
 }
+
+interface FilterModel {
+	model string
+}
+
+interface FilterCar {
+	name string
+}
+
+struct FilterCarRecord {
+	name  string
+	model string
+}
+
+fn collect_matching[T](mut cars []FilterCar) []T {
+	mut matches := []T{}
+	for mut car in cars {
+		if mut car is T {
+			matches << car
+		}
+	}
+	return matches
+}
+
+fn test_generic_interface_to_interface_smartcast() {
+	mut cars := []FilterCar{}
+	cars << FilterCarRecord{
+		name:  'Roadster'
+		model: 'Tesla'
+	}
+	cars << FilterCarRecord{
+		name:  'Corolla'
+		model: 'Toyota'
+	}
+
+	models := collect_matching[FilterModel](mut cars)
+
+	assert models.len == 2
+	assert models[0].model == 'Tesla'
+	assert models[1].model == 'Toyota'
+}
-- 
2.39.5