checker: fix inner functions are called before definition. (fix #15673) (#15719)

vlib/v/checker/fn.v

@@ -413,6 +413,12 @@ fn (mut c Checker) anon_fn(mut node ast.AnonFn) ast.Type {
 }
 
 pub fn (mut c Checker) call_expr(mut node ast.CallExpr) ast.Type {
+	// Check whether the inner function definition is before the call
+	if var := node.scope.find_var(node.name) {
+		if var.expr is ast.AnonFn && var.pos.pos > node.pos.pos {
+			c.error('unknown function: $node.name', node.pos)
+		}
+	}
 	// TODO merge logic from method_call and fn_call
 	// First check everything that applies to both fns and methods
 	old_inside_fn_arg := c.inside_fn_arg

vlib/v/checker/tests/inner_functions_call_before_define.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/inner_functions_call_before_define.vv:2:7: error: unknown function: f
+    1 | fn main() {
+    2 |     _ := f()
+      |          ~~~
+    3 |     go f()
+    4 |
+vlib/v/checker/tests/inner_functions_call_before_define.vv:3:5: error: unknown function: f
+    1 | fn main() {
+    2 |     _ := f()
+    3 |     go f()
+      |        ~~~
+    4 | 
+    5 |     f := fn () string { return "hello" }

vlib/v/checker/tests/inner_functions_call_before_define.vv

@@ -0,0 +1,7 @@
+fn main() {
+	_ := f()
+	go f()
+
+	f := fn () string { return "hello" }
+	_ := f() // avoid warnings
+}