checker: add a check for casting to an unknown type (#8968)

vlib/v/checker/checker.v

@@ -3802,6 +3802,9 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) table.Type {
 	node.expr_type = c.expr(node.expr) // type to be casted
 	from_type_sym := c.table.get_type_symbol(node.expr_type)
 	to_type_sym := c.table.get_type_symbol(node.typ) // type to be used as cast
+	if to_type_sym.kind == .placeholder && to_type_sym.language != .c {
+		c.error('unknown type `$to_type_sym.name`', node.pos)
+	}
 	expr_is_ptr := node.expr_type.is_ptr() || node.expr_type.idx() in table.pointer_type_idxs
 	if expr_is_ptr && to_type_sym.kind == .string && !node.in_prexpr {
 		if node.has_arg {

vlib/v/checker/tests/cast_err.out

@@ -17,4 +17,11 @@ vlib/v/checker/tests/cast_err.vv:5:6: error: cannot cast to bool - use e.g. `som
     4 |     _ = bool(&v)
     5 |     _ = bool([2])
       |         ~~~~~~~~~
-    6 | }
+    6 | }
+    7 |
+vlib/v/checker/tests/cast_err.vv:9:6: error: unknown type `Foo`
+    7 | 
+    8 | fn unknown() {
+    9 |     _ = Foo(3)
+      |         ~~~~~~
+   10 | }

vlib/v/checker/tests/cast_err.vv

@@ -4,3 +4,7 @@ fn test_bool_cast() {
 	_ = bool(&v)
 	_ = bool([2])
 }
+
+fn unknown() {
+	_ = Foo(3)
+}