From 24eac4d74d43a4c14048ac2146979b3879b4594c Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Sun, 22 Dec 2024 08:58:45 -0300
Subject: [PATCH] cgen: fix lambda inferring generic type on method call (fix
 #23221) (#23234)

---
 vlib/v/checker/check_types.v                  |  6 +++-
 vlib/v/checker/fn.v                           | 25 +++++++++-----
 .../generics/generic_method_lambda_arg_test.v | 34 +++++++++++++++++++
 3 files changed, 55 insertions(+), 10 deletions(-)
 create mode 100644 vlib/v/tests/generics/generic_method_lambda_arg_test.v

diff --git a/vlib/v/checker/check_types.v b/vlib/v/checker/check_types.v
index 16796e835e..ca456270ea 100644
--- a/vlib/v/checker/check_types.v
+++ b/vlib/v/checker/check_types.v
@@ -1125,7 +1125,11 @@ fn (mut c Checker) infer_fn_generic_types(func &ast.Fn, mut node ast.CallExpr) {
 								if typ.has_flag(.generic) {
 									lambda_ret_gt_name := c.table.type_to_str(typ)
 									idx := func.generic_names.index(lambda_ret_gt_name)
-									typ = node.concrete_types[idx]
+									if idx < node.concrete_types.len {
+										typ = node.concrete_types[idx]
+									} else {
+										typ = ast.void_type
+									}
 								}
 							}
 						}
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index af7f3ef8f3..353610cd3d 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1806,15 +1806,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 						}
 						if mut call_arg.expr is ast.LambdaExpr {
 							// Calling fn is generic and lambda arg also is generic
-							if node.concrete_types.len > 0 && call_arg.expr.func != unsafe { nil }
-								&& call_arg.expr.func.decl.generic_names.len > 0 {
-								call_arg.expr.call_ctx = unsafe { node }
-								if c.table.register_fn_concrete_types(call_arg.expr.func.decl.fkey(),
-									node.concrete_types)
-								{
-									call_arg.expr.func.decl.ninstances++
-								}
-							}
+							c.handle_generic_lambda_arg(node, mut call_arg.expr)
 							continue
 						}
 						c.error('${err.msg()} in argument ${i + 1} to `${fn_name}`', call_arg.pos)
@@ -2793,6 +2785,10 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 			c.error('${err.msg()} in argument ${i + 1} to `${left_sym.name}.${method_name}`',
 				arg.pos)
 		}
+		if mut arg.expr is ast.LambdaExpr {
+			// Calling fn is generic and lambda arg also is generic
+			c.handle_generic_lambda_arg(node, mut arg.expr)
+		}
 		param_typ_sym := c.table.sym(exp_arg_typ)
 		if param_typ_sym.kind == .struct && got_arg_typ !in [ast.voidptr_type, ast.nil_type]
 			&& !c.check_multiple_ptr_match(got_arg_typ, param.typ, param, arg) {
@@ -2871,6 +2867,17 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 	return node.return_type
 }
 
+fn (mut c Checker) handle_generic_lambda_arg(node &ast.CallExpr, mut lambda ast.LambdaExpr) {
+	// Calling fn is generic and lambda arg also is generic
+	if node.concrete_types.len > 0 && lambda.func != unsafe { nil }
+		&& lambda.func.decl.generic_names.len > 0 {
+		lambda.call_ctx = unsafe { node }
+		if c.table.register_fn_concrete_types(lambda.func.decl.fkey(), node.concrete_types) {
+			lambda.func.decl.ninstances++
+		}
+	}
+}
+
 fn (mut c Checker) spawn_expr(mut node ast.SpawnExpr) ast.Type {
 	ret_type := c.call_expr(mut node.call_expr)
 	if node.call_expr.or_block.kind != .absent {
diff --git a/vlib/v/tests/generics/generic_method_lambda_arg_test.v b/vlib/v/tests/generics/generic_method_lambda_arg_test.v
new file mode 100644
index 0000000000..7af47dbec1
--- /dev/null
+++ b/vlib/v/tests/generics/generic_method_lambda_arg_test.v
@@ -0,0 +1,34 @@
+module main
+
+fn test_main() {
+	my := MyError{
+		path: 'err msg'
+	}
+	p := my.to[string](|m| m.path)
+	p2 := my.to_str(|m| m.path)
+	println(p)
+	println(p2)
+
+	assert p == p2
+}
+
+struct MyError {
+pub:
+	path string
+}
+
+fn (e &MyError) msg() string {
+	return e.path
+}
+
+fn (e &MyError) code() int {
+	return 1
+}
+
+fn (e &MyError) to[T](func fn (MyError) T) T {
+	return func(e)
+}
+
+fn (e &MyError) to_str(func fn (MyError) string) string {
+	return func(e)
+}