checker: fix private type

vlib/builtin/cfns_wrapper.c.v

@@ -82,3 +82,5 @@ type FnSortCB = fn (const_a voidptr, const_b voidptr) int
 fn vqsort(base voidptr, nmemb usize, size usize, sort_cb FnSortCB) {
 	C.qsort(base, nmemb, size, voidptr(sort_cb))
 }
+
+type MyPrivateType = u32

vlib/v/checker/checker.v

@@ -5293,10 +5293,29 @@ fn (mut c Checker) ensure_type_exists(typ ast.Type, pos token.Pos) bool {
 		return false
 	}
 	sym := c.table.sym(typ)
-	if !c.is_builtin_mod && sym.kind == .struct && !sym.is_pub && sym.mod != c.mod {
-		c.error('struct `${sym.name}` was declared as private to module `${sym.mod}`, so it can not be used inside module `${c.mod}`',
-			pos)
-		return false
+	if !c.is_builtin_mod && !sym.is_pub && sym.mod != c.mod {
+		if sym.kind == .struct {
+			c.error('struct `${sym.name}` was declared as private to module `${sym.mod}`, so it can not be used inside module `${c.mod}`',
+				pos)
+			return false
+		}
+		if sym.kind == .alias || sym.kind == .sum_type {
+			c.error('module `${sym.mod}` type `${sym.name}` is private', pos)
+			return false
+		}
+		if sym.kind == .function {
+			fn_info := sym.info as ast.FnType
+			// hack: recover fn mod from func name
+			mut fn_mod := fn_info.func.name.all_before_last('.')
+			if fn_mod == fn_info.func.name {
+				fn_mod = 'builtin'
+			}
+			if fn_mod != '' && fn_mod != c.mod && fn_info.func.name != '' {
+				c.error('module `${fn_mod}` function type `${fn_info.func.name}` is private',
+					pos)
+				return false
+			}
+		}
 	}
 	match sym.kind {
 		.placeholder {

vlib/v/checker/tests/type_alias_decl_private_err.out

@@ -0,0 +1,15 @@
+type_alias_decl_private_err.vv:7:15: error: module `builtin` function type `FnSortCB` is private
+    5 | }
+    6 |
+    7 | fn hello(func FnSortCB) {
+      |               ~~~~~~~~
+    8 | }
+    9 |
+type_alias_decl_private_err.vv:11:11: error: module `cli` type `cli.Defaults` is private
+    9 |
+   10 | fn main() {
+   11 |     _ := cli.Defaults(true)
+      |              ~~~~~~~~~~~~~~
+   12 |     hello(my_sort)
+   13 | }
+

vlib/v/checker/tests/type_alias_decl_private_err.vv

@@ -0,0 +1,13 @@
+import cli
+
+fn my_sort(a voidptr, b voidptr) int {
+	return 1
+}
+
+fn hello(func FnSortCB) {
+}
+
+fn main() {
+	_ := cli.Defaults(true)
+	hello(my_sort)
+}