aggregate not accepted

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Yesterday I tried something similar :

(s2, s1) <= (std_logic'('0'), std_logic'('0'));

which unfortunately returned the same error. Do you have explanation why ?

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because the signed and unsigned types are also arrays of std_logic, although you are saying the zeros are std_logic and not chars or bits, it still doesnt know whether you mean std_logic_vector, signed or unsigned.