parser: disallow type FnType = fn(string) FnType and `type FnType =…

… fn(FnType) string`

vlib/v/parser/parse_type.v

@@ -312,6 +312,10 @@ 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('`${name}` cannot be a parameter as it references the fntype',
+				param.type_pos)
+		}
 	}
 	mut return_type := ast.void_type
 	mut return_type_pos := token.Pos{}
@@ -342,6 +346,11 @@ fn (mut p Parser) parse_fn_type(name string, generic_types []ast.Type) ast.Type
 		p.error_with_pos('`${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')

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
+      |                           ~~~~~~

vlib/v/parser/tests/fn_type_decl_same_return_type.vv

@@ -0,0 +1 @@
+type FnType = fn (string) FnType

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 return type as it references the fntype
+    1 | type FnType = fn (FnType) string
+      |                   ~~~~~~

vlib/v/parser/tests/fn_type_del_same_param_type.vv

@@ -0,0 +1 @@
+type FnType = fn (FnType) string