From 67c53a94e6c29387653d83b571331247bfab3245 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:23 +0300
Subject: [PATCH] checker: fix veb closure middleware generic inference error
 (fixes #23868)

---
 vlib/v/checker/fn.v                           | 36 +++++++++++++++++++
 ...od_call_with_short_syntax_fn_field_err.out |  6 ++++
 ...hod_call_with_short_syntax_fn_field_err.vv | 34 ++++++++++++++++++
 3 files changed, 76 insertions(+)
 create mode 100644 vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.out
 create mode 100644 vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 24d04b7a0..0e1cf7232 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1887,6 +1887,8 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 				c.table.cur_concrete_types)
 			param = unwrapped
 		}
+		param.typ = c.resolve_short_syntax_call_arg_type(call_arg, param.typ, func.generic_names,
+			concrete_types)
 		// registers if the arg must be passed by ref to disable auto deref args
 		call_arg.should_be_ptr = param.typ.is_ptr() && !param.is_mut
 		if func.is_variadic && call_arg.expr is ast.ArrayDecompose {
@@ -3106,6 +3108,24 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 			param_is_mut = false
 			no_type_promotion = false
 		}
+		resolved_method_concrete_types := if method_generic_names_len == rec_concrete_types.len {
+			rec_concrete_types
+		} else {
+			concrete_types
+		}
+		mut resolved_method_generic_names := method.generic_names.clone()
+		if resolved_method_generic_names.len == 0 {
+			match rec_sym.info {
+				ast.Struct, ast.Interface, ast.SumType {
+					if rec_sym.info.generic_types.len == resolved_method_concrete_types.len {
+						resolved_method_generic_names = rec_sym.info.generic_types.map(c.table.sym(it).name)
+					}
+				}
+				else {}
+			}
+		}
+		exp_arg_typ = c.resolve_short_syntax_call_arg_type(arg, exp_arg_typ, resolved_method_generic_names,
+			resolved_method_concrete_types)
 		exp_arg_sym := c.table.sym(exp_arg_typ)
 		c.expected_type = exp_arg_typ
 
@@ -3388,6 +3408,22 @@ fn (mut c Checker) handle_generic_lambda_arg(node &ast.CallExpr, mut lambda ast.
 	}
 }
 
+fn (mut c Checker) resolve_short_syntax_call_arg_type(arg ast.CallArg, param_typ ast.Type, generic_names []string, concrete_types []ast.Type) ast.Type {
+	if !param_typ.has_flag(.generic) || generic_names.len == 0
+		|| generic_names.len != concrete_types.len {
+		return param_typ
+	}
+	if arg.expr is ast.StructInit {
+		expr := arg.expr as ast.StructInit
+		if expr.is_short_syntax && expr.typ == ast.void_type {
+			return c.table.convert_generic_type(param_typ, generic_names, concrete_types) or {
+				param_typ
+			}
+		}
+	}
+	return param_typ
+}
+
 fn (mut c Checker) check_unresolved_generic_param(node &ast.CallExpr, arg ast.CallArg) bool {
 	if node.raw_concrete_types.len == 0 && arg.expr is ast.ArrayInit
 		&& arg.expr.typ == ast.void_type {
diff --git a/vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.out b/vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.out
new file mode 100644
index 000000000..ecee47440
--- /dev/null
+++ b/vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.vv:33:10: error: cannot assign to field `handler`: expected `fn (mut Context) bool`, not `Options[Context]`
+   31 | fn main() {
+   32 |     mut app := &App{}
+   33 |     app.use(handler: create_options(mut app))
+      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   34 | }
diff --git a/vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.vv b/vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.vv
new file mode 100644
index 000000000..14bb4d2ed
--- /dev/null
+++ b/vlib/v/checker/tests/generic_method_call_with_short_syntax_fn_field_err.vv
@@ -0,0 +1,34 @@
+struct Context {
+	ok bool
+}
+
+type Handler[T] = fn (mut T) bool
+
+@[params]
+struct Options[T] {
+	handler Handler[T] @[required]
+}
+
+struct Middleware[T] {}
+
+fn (mut m Middleware[T]) use(options Options[T]) {}
+
+struct App {
+	Middleware[Context]
+mut:
+	hits int
+}
+
+fn create_options(mut app App) Options[Context] {
+	return Options[Context]{
+		handler: fn [mut app] (mut ctx Context) bool {
+			app.hits++
+			return ctx.ok
+		}
+	}
+}
+
+fn main() {
+	mut app := &App{}
+	app.use(handler: create_options(mut app))
+}
-- 
2.39.5