checker: check error for struct field init with nobody anon fn (#13777)

vlib/v/checker/struct.v

@@ -324,6 +324,15 @@ pub fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 							field.pos)
 					}
 				}
+				if field_type_sym.kind == .function && field_type_sym.language == .v {
+					pos := field.expr.pos()
+					if mut field.expr is ast.AnonFn {
+						if field.expr.decl.no_body {
+							c.error('cannot initialize the fn field with anonymous fn that does not have a body',
+								pos)
+						}
+					}
+				}
 				node.fields[i].typ = expr_type
 				node.fields[i].expected_type = field_info.typ
 

vlib/v/checker/tests/struct_field_init_with_nobody_anon_fn_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_field_init_with_nobody_anon_fn_err.vv:7:7: error: cannot initialize the fn field with anonymous fn that does not have a body
+    5 | fn main() {
+    6 |     _ = App{
+    7 |         cb: fn(x int)  // Note the missing `{}` (the function body) here
+      |             ~~~~~~~~~
+    8 |     }
+    9 | }

vlib/v/checker/tests/struct_field_init_with_nobody_anon_fn_err.vv

@@ -0,0 +1,9 @@
+struct App {
+	cb fn(x int) // the function signature doesn't make a difference
+}
+
+fn main() {
+	_ = App{
+		cb: fn(x int)  // Note the missing `{}` (the function body) here
+	}
+}