From 8466c6c03d77ae623ffc023272aacaa569c942ae Mon Sep 17 00:00:00 2001
From: Swastik Baranwal <swstkbaranwal@gmail.com>
Date: Wed, 26 Jun 2024 04:05:55 +0530
Subject: [PATCH] parser: disallow self referencing function alias types like
 `type FnType = fn(string) FnType` and `type FnType = fn (FnType) string`
 (#21733)

---
 vlib/v/parser/parse_type.v                            | 9 +++++++++
 vlib/v/parser/tests/fn_type_decl_same_return_type.out | 3 +++
 vlib/v/parser/tests/fn_type_decl_same_return_type.vv  | 1 +
 vlib/v/parser/tests/fn_type_del_same_param_type.out   | 3 +++
 vlib/v/parser/tests/fn_type_del_same_param_type.vv    | 1 +
 5 files changed, 17 insertions(+)
 create mode 100644 vlib/v/parser/tests/fn_type_decl_same_return_type.out
 create mode 100644 vlib/v/parser/tests/fn_type_decl_same_return_type.vv
 create mode 100644 vlib/v/parser/tests/fn_type_del_same_param_type.out
 create mode 100644 vlib/v/parser/tests/fn_type_del_same_param_type.vv

diff --git a/vlib/v/parser/parse_type.v b/vlib/v/parser/parse_type.v
index dfe432f08..1e324bb5c 100644
--- a/vlib/v/parser/parse_type.v
+++ b/vlib/v/parser/parse_type.v
@@ -315,6 +315,10 @@ fn (mut p Parser) parse_fn_type(name string, generic_types []ast.Type) ast.Type
 			has_generic = true
 			break
 		}
+		if p.table.sym(param.typ).name == name {
+			p.error_with_pos('`${name}` cannot be a parameter as it references the fntype',
+				param.type_pos)
+		}
 	}
 	mut return_type := ast.void_type
 	mut return_type_pos := token.Pos{}
@@ -345,6 +349,11 @@ fn (mut p Parser) parse_fn_type(name string, generic_types []ast.Type) ast.Type
 		p.error_with_pos('`${name}` type is generic fntype, must specify the generic type names, e.g. ${name}[T]',
 			fn_type_pos)
 	}
+
+	if p.table.sym(return_type).name == name {
+		p.error_with_pos('`${name}` cannot be a return type as it references the fntype',
+			return_type_pos)
+	}
 	// MapFooFn typedefs are manually added in cheaders.v
 	// because typedefs get generated after the map struct is generated
 	has_decl := p.builtin_mod && name.starts_with('Map') && name.ends_with('Fn')
diff --git a/vlib/v/parser/tests/fn_type_decl_same_return_type.out b/vlib/v/parser/tests/fn_type_decl_same_return_type.out
new file mode 100644
index 000000000..96bf1d6fe
--- /dev/null
+++ b/vlib/v/parser/tests/fn_type_decl_same_return_type.out
@@ -0,0 +1,3 @@
+vlib/v/parser/tests/fn_type_decl_same_return_type.vv:1:27: error: `FnType` cannot be a return type as it references the fntype
+    1 | type FnType = fn (string) FnType
+      |                           ~~~~~~
diff --git a/vlib/v/parser/tests/fn_type_decl_same_return_type.vv b/vlib/v/parser/tests/fn_type_decl_same_return_type.vv
new file mode 100644
index 000000000..cc0966c81
--- /dev/null
+++ b/vlib/v/parser/tests/fn_type_decl_same_return_type.vv
@@ -0,0 +1 @@
+type FnType = fn (string) FnType
diff --git a/vlib/v/parser/tests/fn_type_del_same_param_type.out b/vlib/v/parser/tests/fn_type_del_same_param_type.out
new file mode 100644
index 000000000..f0e5c17aa
--- /dev/null
+++ b/vlib/v/parser/tests/fn_type_del_same_param_type.out
@@ -0,0 +1,3 @@
+vlib/v/parser/tests/fn_type_del_same_param_type.vv:1:19: error: `FnType` cannot be a parameter as it references the fntype
+    1 | type FnType = fn (FnType) string
+      |                   ~~~~~~
diff --git a/vlib/v/parser/tests/fn_type_del_same_param_type.vv b/vlib/v/parser/tests/fn_type_del_same_param_type.vv
new file mode 100644
index 000000000..8244c0a73
--- /dev/null
+++ b/vlib/v/parser/tests/fn_type_del_same_param_type.vv
@@ -0,0 +1 @@
+type FnType = fn (FnType) string
-- 
2.39.5