From 567a7d9d675fd7da5c45b27cbea052296314ea94 Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Thu, 3 Apr 2025 10:54:40 -0300
Subject: [PATCH] checker: fix wrong type hint on predicate arg type mismatch
 (fix #24122) (#24123)

---
 vlib/v/checker/fn.v                           | 13 ++++++------
 .../tests/wrong_predicate_type_err.out        |  7 +++++++
 .../checker/tests/wrong_predicate_type_err.vv | 21 +++++++++++++++++++
 3 files changed, 34 insertions(+), 7 deletions(-)
 create mode 100644 vlib/v/checker/tests/wrong_predicate_type_err.out
 create mode 100644 vlib/v/checker/tests/wrong_predicate_type_err.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 81cb72c60..71cd6441b 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -2957,7 +2957,6 @@ fn (mut c Checker) check_predicate_param(is_map bool, elem_typ ast.Type, node as
 		// Finish early so that it doesn't fail later
 		return
 	}
-	elem_sym := c.table.sym(elem_typ)
 	arg_expr := node.args[0].expr
 	match arg_expr {
 		ast.AnonFn {
@@ -2969,11 +2968,11 @@ fn (mut c Checker) check_predicate_param(is_map bool, elem_typ ast.Type, node as
 				c.error('function needs exactly 1 argument', arg_expr.decl.pos)
 			} else if is_map && (arg_expr.decl.return_type == ast.void_type
 				|| arg_expr.decl.params[0].typ != elem_typ) {
-				c.error('type mismatch, should use `fn(a ${elem_sym.name}) T {...}`',
+				c.error('type mismatch, should use `fn(a ${c.table.type_to_str(elem_typ)}) T {...}`',
 					arg_expr.decl.pos)
 			} else if !is_map && (arg_expr.decl.return_type != ast.bool_type
 				|| arg_expr.decl.params[0].typ != elem_typ) {
-				c.error('type mismatch, should use `fn(a ${elem_sym.name}) bool {...}`',
+				c.error('type mismatch, should use `fn(a ${c.table.type_to_str(elem_typ)}) bool {...}`',
 					arg_expr.decl.pos)
 			}
 		}
@@ -2991,11 +2990,11 @@ fn (mut c Checker) check_predicate_param(is_map bool, elem_typ ast.Type, node as
 					c.error('function needs exactly 1 argument', node.pos)
 				} else if is_map
 					&& (func.return_type == ast.void_type || func.params[0].typ != elem_typ) {
-					c.error('type mismatch, should use `fn(a ${elem_sym.name}) T {...}`',
+					c.error('type mismatch, should use `fn(a ${c.table.type_to_str(elem_typ)}) T {...}`',
 						arg_expr.pos)
 				} else if !is_map
 					&& (func.return_type != ast.bool_type || func.params[0].typ != elem_typ) {
-					c.error('type mismatch, should use `fn(a ${elem_sym.name}) bool {...}`',
+					c.error('type mismatch, should use `fn(a ${c.table.type_to_str(elem_typ)}) bool {...}`',
 						arg_expr.pos)
 				}
 			} else if arg_expr.kind == .variable {
@@ -3011,11 +3010,11 @@ fn (mut c Checker) check_predicate_param(is_map bool, elem_typ ast.Type, node as
 							c.error('function needs exactly 1 argument', expr.decl.pos)
 						} else if is_map && (expr.decl.return_type == ast.void_type
 							|| expr.decl.params[0].typ != elem_typ) {
-							c.error('type mismatch, should use `fn(a ${elem_sym.name}) T {...}`',
+							c.error('type mismatch, should use `fn(a ${c.table.type_to_str(elem_typ)}) T {...}`',
 								expr.decl.pos)
 						} else if !is_map && (expr.decl.return_type != ast.bool_type
 							|| expr.decl.params[0].typ != elem_typ) {
-							c.error('type mismatch, should use `fn(a ${elem_sym.name}) bool {...}`',
+							c.error('type mismatch, should use `fn(a ${c.table.type_to_str(elem_typ)}) bool {...}`',
 								expr.decl.pos)
 						}
 						return
diff --git a/vlib/v/checker/tests/wrong_predicate_type_err.out b/vlib/v/checker/tests/wrong_predicate_type_err.out
new file mode 100644
index 000000000..f1f5fd9ab
--- /dev/null
+++ b/vlib/v/checker/tests/wrong_predicate_type_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/wrong_predicate_type_err.vv:18:13: error: type mismatch, should use `fn(a &Foo) bool {...}`
+   16 | 
+   17 | fn main() {
+   18 |     t().filter(fn (a Foo) bool {
+      |                ~~~~~~~~~~~~~~~~~
+   19 |         return true
+   20 |     })
diff --git a/vlib/v/checker/tests/wrong_predicate_type_err.vv b/vlib/v/checker/tests/wrong_predicate_type_err.vv
new file mode 100644
index 000000000..b14f70393
--- /dev/null
+++ b/vlib/v/checker/tests/wrong_predicate_type_err.vv
@@ -0,0 +1,21 @@
+@[heap]
+struct Foo {
+	field int
+}
+
+fn t() []&Foo {
+	mut a := []&Foo{}
+	a << &Foo{
+		field: 1
+	}
+	a << &Foo{
+		field: 2
+	}
+	return a
+}
+
+fn main() {
+	t().filter(fn (a Foo) bool {
+		return true
+	})
+}
-- 
2.39.5