The problem of the chained implicit rule?

[Peng Yu]

I have the following makefile. I thought that 'a.y' and 'b.y' should
be printed. But it is not. What is the problem?

$ ls
a.x   b.x  Makefile
$ cat Makefile
.PHONY: all

Y_FILES=$(patsubst %x,%y,$(wildcard *.x))

.PHONY: $(Y_FILES)

all: $(Y_FILES)

%.y: %.x
        echo $@
$ make
make: Nothing to be done for `all'.