Subtracting a decimal value from a Char?

I'm looking at Muller's Micro-Max chess engine, trying to port it to z80 ASM just as a fun challenge. https://home.hccnet.nl/h.g.muller/umax4_8.c
While I consider myself well-versed in z80 as a hobby, I have little experience with C.

I'm looking at this line of code:

Code:

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b[16*L+K+8]=(K-4)*(K-4)+(L-3.5)*(L-3.5)

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If I'm correct, L was / has been declared as a variable of "char" type. So I'm confused about subtracting 3.5 from a "char" variable. Let's say that K = 7 and L = 7. What would the result be?

L will be promoted to double for that part of the calculation, which will result in the whole right hand side being promoted to double for evaluation.
The assignment will then truncate the whole thing back to char.

Thanks, that answers my question and fixes the problem.