Error 10327: Can't determine definition of operator "+"

Hello everyone, I think there's one more strange construction in the code:

--- Quote Start ---

WAIT UNTIL wipe_on1'EVENT AND wipe_on1 = '1';--indicates when sensor1 is blocked and gives a 'high signal'.

loop_t1 : WHILE wipe_on1 = '1' loop

WAIT UNTIL a2MHz_clk'EVENT AND a2MHz_clk = '1';

t1 <= t1 + 1;

END LOOP loop_t1;

--- Quote End ---

I can't imaging the RTL construction corresponding thise code.

First of all it would be better if you use sensitive list instead of wait and if ... then ...else construction to write your code. Because strings

--- Quote Start ---

loop_t1 : WHILE wipe_on1 = '1' loop

WAIT UNTIL a2MHz_clk'EVENT AND a2MHz_clk = '1';

t1 <= t1 + 1;

END LOOP loop_t1;

--- Quote End ---

don't seems to me synthesisable.

P.S. Sorry for my english. It's not very good.