From 8b02d8f406119a4ec25e22dc98b161727a367726 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 06:00:39 +0300
Subject: [PATCH] comptime: fix unknown type error when using a generic
 interface (fixes #20762)

---
 vlib/v/checker/comptime.v                     | 14 +++++++--
 .../comptime_if_is_generic_interface_test.v   | 31 +++++++++++++++++++
 2 files changed, 43 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/tests/comptime/comptime_if_is_generic_interface_test.v

diff --git a/vlib/v/checker/comptime.v b/vlib/v/checker/comptime.v
index e6f78502c..00386287a 100644
--- a/vlib/v/checker/comptime.v
+++ b/vlib/v/checker/comptime.v
@@ -1266,10 +1266,19 @@ fn (mut c Checker) check_compatible_types(left_type ast.Type, left_name string,
 	} else if expr is ast.TypeNode {
 		typ := c.get_expr_type(expr)
 		right_type := c.unwrap_generic(typ)
+		mut right_sym := c.table.sym(right_type)
+		if right_sym.kind == .generic_inst {
+			gi := right_sym.info as ast.GenericInst
+			if gi.parent_idx > 0 && gi.parent_idx < c.table.type_symbols.len
+				&& c.table.type_symbols[gi.parent_idx].kind == .interface
+				&& !gi.concrete_types.any(c.type_has_unresolved_generic_parts(it)) {
+				c.table.generic_insts_to_concrete()
+				right_sym = c.table.sym(right_type)
+			}
+		}
 		if c.type_resolver.bind_matching_generic_type(resolved_left_type, right_type) {
 			return true
 		}
-		right_sym := c.table.sym(right_type)
 		if right_sym.kind == .placeholder || right_type.has_flag(.generic) {
 			c.error('unknown type `${right_sym.name}`', expr.pos)
 		}
@@ -1278,7 +1287,8 @@ fn (mut c Checker) check_compatible_types(left_type ast.Type, left_name string,
 				&& c.table.does_type_implement_interface(resolved_left_type, right_type)
 		}
 		if right_sym.info is ast.FnType && c.comptime.comptime_for_method_var == left_name {
-			return c.table.fn_signature(right_sym.info.func,
+			right_fn_type := right_sym.info as ast.FnType
+			return c.table.fn_signature(right_fn_type.func,
 				skip_receiver: true
 				type_only:     true
 			) == c.table.fn_signature(c.comptime.comptime_for_method,
diff --git a/vlib/v/tests/comptime/comptime_if_is_generic_interface_test.v b/vlib/v/tests/comptime/comptime_if_is_generic_interface_test.v
new file mode 100644
index 000000000..f365815de
--- /dev/null
+++ b/vlib/v/tests/comptime/comptime_if_is_generic_interface_test.v
@@ -0,0 +1,31 @@
+struct Context {}
+
+struct App {
+mut:
+	called bool
+}
+
+fn (mut app App) before_request(mut ctx Context) {
+	app.called = true
+}
+
+interface HasBeforeRequest[X] {
+mut:
+	before_request(mut ctx X)
+}
+
+fn check_before_request[A, X](mut app A) bool {
+	mut ctx := Context{}
+	$if A is HasBeforeRequest[X] {
+		app.before_request(mut ctx)
+		return true
+	} $else {
+		return false
+	}
+}
+
+fn test_comptime_if_is_generic_interface_with_generic_type_param() {
+	mut app := App{}
+	assert check_before_request[App, Context](mut app)
+	assert app.called
+}
-- 
2.39.5