checker: check generics fn called outside of generic fn (#9984)

vlib/v/checker/checker.v

@@ -2135,6 +2135,10 @@ pub fn (mut c Checker) fn_call(mut call_expr ast.CallExpr) ast.Type {
 			concrete_types << concrete_type
 		}
 	}
+	if c.cur_fn.cur_concrete_types.len == 0 && has_generic {
+		c.error('generic fn using generic types cannot be called outside of generic fn',
+			call_expr.pos)
+	}
 	if has_generic {
 		mut no_exists := true
 		if c.mod != '' && !fn_name.contains('.') {

vlib/v/checker/tests/generics_fn_called_outside_of_generic_fn.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/generics_fn_called_outside_of_generic_fn.vv:4:2: error: generic fn using generic types cannot be called outside of generic fn
+    2 |
+    3 | fn main() {
+    4 |     foo<T>()
+      |     ~~~~~~~~
+    5 | }

vlib/v/checker/tests/generics_fn_called_outside_of_generic_fn.vv

@@ -0,0 +1,5 @@
+fn foo<T>() {}
+
+fn main() {
+	foo<T>()
+}