5039 – Cannot use invariant() with auto methods

This code:
==========
module scratch;
class C {
	int x;
	
	invariant() {
		assert( x < int.max );
	}
	
	auto foo() {
		return x;
	}
}
=========
Reports this error:
$ dmd scratch.d
scratch.d(10): Error: function scratch.C.foo post conditions are not supported if the return type is inferred
Removing the invariant eliminates the error message. Converting the 'auto' to an explicit return type eliminates the error message.
Given that the compiler must eventually figure out the return type of the method, I have no idea why this error is occurring.

https://github.com/D-Programming-Language/dmd/pull/147 

Commit pushed to master at https://github.com/D-Programming-Language/dmd
https://github.com/D-Programming-Language/dmd/commit/007b936e0970666c7698ad7c3b2c2b3da2d8cbc0
Merge pull request #147 from 9rnsr/fix4785
Issue 4785 & 5039 Allow auto return function with contracts