Why in Phobos is empty() sometimes const and sometimes not

[ag0aep6g]

On 29.07.19 21:35, H. S. Teoh wrote:
> Generally, the idiom is to let the compiler do attribute inference by
> templatizing your code and not writing any explicit attributes, then use
> unittests to ensure that instantiations of the range that ought to have
> certain attributes actually have those attributes.  For example, instead
> of writing:
> 
> 	struct MyRange(R) {
> 		...
> 		@property bool empty() const { ... }
> 		...
> 	}
> 
> write instead:
> 
> 	struct MyRange(R) {
> 		...
> 		// No attributes: compiler does inference
> 		@property bool empty() { ... }
> 		...
> 	}
> 
> 	// unittest ensures .empty is callable with const object.
> 	unittest {
> 		const rr = MyRange!(const(Range))(...);
> 		assert(rr.empty); // will fail compilation if .empty is non-const
> 	}
> 
> The unittest tests that a specific instantiation of MyRange has const
> .empty. It's still possible to use MyRange with a range that has
> non-const .empty, but this unittest ensures that the non-const-ness
> wasn't introduced by the implementation of MyRange itself, but only
> comes from the template argument.

But const isn't inferred.

----
struct MyRange()
{
     @property bool empty() { return true; }
}
void main()
{
     pragma(msg, typeof(&MyRange!().empty));
         /* Prints: "bool function() pure nothrow @nogc @property @safe"
         Note: no const. */
     const MyRange!() r;
     assert(r.empty); /* fails compilation */
}
----