Is there a "safe" static_cast alternative?

There's gsl::narrow

narrow // narrow<T>(x) is static_cast<T>(x) if static_cast<T>(x) == x or it throws narrowing_error

You've got the use-case reversed.

The intended use of static_cast (and the other c++-style casts) is to indicate programmer intentions. When you write auto value = static_cast<int32_t>(value_64);, you're saying "Yes, I very much *intend* to downcast this value, possibly truncating it, when I perform this assignment". As a result, a compiler, which might have been inclined to complain about this conversion under normal circumstances (like if you'd have written int32_t value = value_64;) instead observes "well, the programmer has told me that this is what they intended; why would they lie to me?" and will silently compile the code.

If you want your C++ code to warn or throw an error on unsafe conversions, you need to explicitly not use static_cast, const_cast, reinterpret_cast, and let the compiler do its job. Compilers have flags that change how warnings are treated (downcasting int64_t to int32_t usually only results in a Warning), so make sure you're using the correct flags to force warnings to be treated as errors.