diff --git a/vlib/os/process_windows.c.v b/vlib/os/process_windows.c.v
index 8b7193b992a150..4894feb5e6f4ad 100644
--- a/vlib/os/process_windows.c.v
+++ b/vlib/os/process_windows.c.v
@@ -14,7 +14,7 @@ type FN_NTSuspendResume = fn (voidptr) u64
 fn ntdll_fn(name &char) FN_NTSuspendResume {
 	ntdll := C.GetModuleHandleA(c'NTDLL')
 	if ntdll == 0 {
-		return FN_NTSuspendResume(0)
+		return unsafe { FN_NTSuspendResume(0) }
 	}
 	the_fn := FN_NTSuspendResume(C.GetProcAddress(ntdll, voidptr(name)))
 	return the_fn
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 4a8f7fa212de8f..2925cd7035130e 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3591,6 +3591,25 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		|| c.file.is_translated) && !c.check_matching_function_symbols(final_from_sym, final_to_sym) {
 		c.error('casting a function value from one function signature, to another function signature, should be done inside `unsafe{}` blocks',
 			node.pos)
+	} else if final_to_sym.kind == .function && final_from_sym.kind != .function {
+		if to_type.has_flag(.option) && node.expr !is ast.None {
+			c.error('casting number to Option function is not allowed, only compatible function or `none`',
+				node.pos)
+		} else if !(c.inside_unsafe || c.file.is_translated) {
+			if node.expr is ast.IntegerLiteral {
+				c.warn('casting number to function value should be done inside `unsafe{}` blocks',
+					node.pos)
+			} else if node.expr is ast.Nil {
+				c.warn('casting `nil` to function value should be done inside `unsafe{}` blocks',
+					node.pos)
+			} else if node.expr is ast.None {
+				if from_type.has_flag(.option) {
+					c.warn('cannot pass `none` to a non Option function type', node.pos)
+				}
+			} else if final_from_sym.kind != .voidptr {
+				c.error('invalid casting value to function', node.pos)
+			}
+		}
 	}
 	if to_type.is_ptr() && to_sym.kind == .alias && from_sym.kind == .map {
 		c.error('cannot cast to alias pointer `${c.table.type_to_str(to_type)}` because `${c.table.type_to_str(from_type)}` is a value',
diff --git a/vlib/v/checker/tests/cast_fn_err.out b/vlib/v/checker/tests/cast_fn_err.out
new file mode 100644
index 00000000000000..2e690ffd58629b
--- /dev/null
+++ b/vlib/v/checker/tests/cast_fn_err.out
@@ -0,0 +1,69 @@
+vlib/v/checker/tests/cast_fn_err.vv:24:7: warning: casting `nil` to function value should be done inside `unsafe{}` blocks
+   22 |     // wrong ones
+   23 |     _ := FnType(foo)
+   24 |     _ := FnType(nil)
+      |          ~~~~~~~~~~~
+   25 |     _ := FnType(0)
+   26 |     _ := FnType('foo')
+vlib/v/checker/tests/cast_fn_err.vv:25:7: warning: casting number to function value should be done inside `unsafe{}` blocks
+   23 |     _ := FnType(foo)
+   24 |     _ := FnType(nil)
+   25 |     _ := FnType(0)
+      |          ~~~~~~~~~
+   26 |     _ := FnType('foo')
+   27 |     _ := FnType(none)
+vlib/v/checker/tests/cast_fn_err.vv:23:7: error: casting a function value from one function signature, to another function signature, should be done inside `unsafe{}` blocks
+   21 | 
+   22 |     // wrong ones
+   23 |     _ := FnType(foo)
+      |          ~~~~~~~~~~~
+   24 |     _ := FnType(nil)
+   25 |     _ := FnType(0)
+vlib/v/checker/tests/cast_fn_err.vv:24:14: error: `nil` is only allowed in `unsafe` code
+   22 |     // wrong ones
+   23 |     _ := FnType(foo)
+   24 |     _ := FnType(nil)
+      |                 ~~~
+   25 |     _ := FnType(0)
+   26 |     _ := FnType('foo')
+vlib/v/checker/tests/cast_fn_err.vv:26:7: error: invalid casting value to function
+   24 |     _ := FnType(nil)
+   25 |     _ := FnType(0)
+   26 |     _ := FnType('foo')
+      |          ~~~~~~~~~~~~~
+   27 |     _ := FnType(none)
+   28 |     _ := ?FnType(0)
+vlib/v/checker/tests/cast_fn_err.vv:27:7: error: cannot cast `none` to `fn () bool`
+   25 |     _ := FnType(0)
+   26 |     _ := FnType('foo')
+   27 |     _ := FnType(none)
+      |          ~~~~~~~~~~~~
+   28 |     _ := ?FnType(0)
+   29 |     _ := ?FnType(nil)
+vlib/v/checker/tests/cast_fn_err.vv:28:8: error: casting number to Option function is not allowed, only compatible function or `none`
+   26 |     _ := FnType('foo')
+   27 |     _ := FnType(none)
+   28 |     _ := ?FnType(0)
+      |           ~~~~~~~~~
+   29 |     _ := ?FnType(nil)
+   30 |     _ := ?FnType(foo)
+vlib/v/checker/tests/cast_fn_err.vv:29:15: error: `nil` is only allowed in `unsafe` code
+   27 |     _ := FnType(none)
+   28 |     _ := ?FnType(0)
+   29 |     _ := ?FnType(nil)
+      |                  ~~~
+   30 |     _ := ?FnType(foo)
+   31 | }
+vlib/v/checker/tests/cast_fn_err.vv:29:8: error: casting number to Option function is not allowed, only compatible function or `none`
+   27 |     _ := FnType(none)
+   28 |     _ := ?FnType(0)
+   29 |     _ := ?FnType(nil)
+      |           ~~~~~~~~~~~
+   30 |     _ := ?FnType(foo)
+   31 | }
+vlib/v/checker/tests/cast_fn_err.vv:30:8: error: casting a function value from one function signature, to another function signature, should be done inside `unsafe{}` blocks
+   28 |     _ := ?FnType(0)
+   29 |     _ := ?FnType(nil)
+   30 |     _ := ?FnType(foo)
+      |           ~~~~~~~~~~~
+   31 | }
diff --git a/vlib/v/checker/tests/cast_fn_err.vv b/vlib/v/checker/tests/cast_fn_err.vv
new file mode 100644
index 00000000000000..7188a82313d23d
--- /dev/null
+++ b/vlib/v/checker/tests/cast_fn_err.vv
@@ -0,0 +1,31 @@
+type FnType = fn () bool
+
+fn foo() int {
+	return 0
+}
+
+fn bar() bool {
+	return true
+}
+
+fn main() {
+	// acceptable ones
+	_ := unsafe { FnType(nil) }
+	_ := unsafe { FnType(0) }
+	_ := unsafe { FnType(foo) }
+	_ := unsafe { FnType(bar) }
+	_ := FnType(bar)
+	_ := ?FnType(none)
+	_ := ?FnType(bar)
+	_ := unsafe { ?FnType(foo) }
+
+	// wrong ones
+	_ := FnType(foo)
+	_ := FnType(nil)
+	_ := FnType(0)
+	_ := FnType('foo')
+	_ := FnType(none)
+	_ := ?FnType(0)
+	_ := ?FnType(nil)
+	_ := ?FnType(foo)
+}
diff --git a/vlib/v/checker/tests/mut_arg_different_muls_err.vv b/vlib/v/checker/tests/mut_arg_different_muls_err.vv
index b4af7c30301be4..d04f29ecf1dcd4 100644
--- a/vlib/v/checker/tests/mut_arg_different_muls_err.vv
+++ b/vlib/v/checker/tests/mut_arg_different_muls_err.vv
@@ -16,7 +16,7 @@ pub:
 fn window_resized(mut w &Window) {
 	window_width, window_height := 200, 100
 
-	if w.resize_fn != WindowResizeFn(0) {
+	if w.resize_fn != unsafe { WindowResizeFn(0) } {
 		println('fn present ${window_width} ${window_height}')
 		w.resize_fn(w, window_width, window_height)
 	}