parser: check generic fntype declaration without type name (#18033)

vlib/v/checker/tests/generics_fn_called_fntype_arg_mismatch.vv

@@ -1,9 +1,9 @@
-pub type FnArrayInit = fn (idx int) T
+pub type FnArrayInit[T] = fn (idx int) T
 
 pub fn new_array[T](len int, initfn FnArrayInit[T]) []T {
 	mut res := []T{len: len}
 	for idx in 0 .. res.len {
-		res[idx] = initfn(idx)
+		res[idx] = initfn[T](idx)
 	}
 	return res
 }

vlib/v/parser/parse_type.v

@@ -230,6 +230,7 @@ pub fn (mut p Parser) parse_multi_return_type() ast.Type {
 
 // given anon name based off signature when `name` is blank
 pub fn (mut p Parser) parse_fn_type(name string, generic_types []ast.Type) ast.Type {
+	fn_type_pos := p.peek_token(-2).pos()
 	p.check(.key_fn)
 
 	for attr in p.attrs {
@@ -277,6 +278,10 @@ pub fn (mut p Parser) parse_fn_type(name string, generic_types []ast.Type) ast.T
 		is_method: false
 		attrs: p.attrs
 	}
+	if has_generic && generic_types.len == 0 && name.len > 0 {
+		p.error_with_pos('`${name}` type is generic fntype, must specify the generic type names, e.g. ${name}[T]',
+			fn_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/generic_fn_type_decl_err.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/generic_fn_type_decl_err.vv:1:6: error: `Fn` type is generic fntype, must specify the generic type names, e.g. Fn[T]
+    1 | type Fn = fn (I) !(O, R)
+      |      ~~
+    2 |
+    3 | fn main() {

vlib/v/parser/tests/generic_fn_type_decl_err.vv

@@ -0,0 +1,4 @@
+type Fn = fn (I) !(O, R)
+
+fn main() {
+}