LATCH primitive ... is permanently disabled"

i am a new user of quartus,now there is a warning when compilation.

:LATCH primitive ... is permanently disabled".

i do not understand the meaning of that.

though i've used the quartus help,but...ha~ha~...

the ex. code

reg [15:0] a,d;

reg [15:0] b=0;

reg [15:0] c=0;

reg [3:0] state=0;

always @ (posedge clk)

begin

if(state <6)

begin

state <=state+1;

end

else if(state ==6)

begin

state <= 0;

end

else begin end

end

always @(state)

begin

case(state)

1'd0:begin

a=b-c;

end

2'd1:begin

d=a+b;

end

default:begin end

endcase

end