I have thought an additional idea.

If we really want a feature to disable optional parentheses for normal functions, we can add @function attribute to the language spec.

int foo();

@property int bar();

@function int baz();  // new!

int x1 = foo();  // ok

int x2 = foo;  // optional parentheses, allowed

int y1 = bar();  // disallowed, calling int is meaningless

int y2 = bar;  // ok

int z1 = baz();  // ok

int z2 = baz;  // *disallowed* by @function attribute