[antlr-interest] Why is this ambiguous..

[Peter Kooiman]

Hello, 

While working on a grammar I had something that boils down to 

grammar T; 
start: (y | y C)+ EOF; 
y: B (A)? | A; 

A : 'a'; 
B : 'b'; 
C : 'c'; 

This gives an expected warning on rule y, that's fine, ANTLR does the right thing and matches A greedily. 
However it also gives a warning on rule start: 

T.g:2:19: Decision can match input such as "B {EOF, B..A}" using multiple alternatives: 1, 2 
As a result, alternative(s) 2 were disabled for that input 

I quite understand the need to left factor this, I was just wondering why ANTLR gives this warning. It must be staring me in the face but I don't see it... 
How is this non-deterministic on input B EOF? Is ANTLR unable to see past rule y for some reason? 

Thanks 
Peter