Check cast safety in base invariant_fallback
#1821
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Closes #1782.
In the problematic test, the changed line was casting
4294967295UL(same after division bysizeof(char)) to the type of the lvaluen_update, which isint. This cast is not safe and caused an incorrect -1 bound to be used.This PR adds a check with
VD.is_dynamically_safe_castbefore doing the cast.This is the very base case of
invariant_fallback, so the change could have quite an impact. Although lvalues are also refined by HC4'sinv_expat the non-fallback level. So I'm not sure how often the fallback case actually gets used.