Test case:

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int f6328a()(lazy int x) {
    return x;
}
@safe int f6328b()(lazy int x) {   // <-- adding a '@safe' is OK
    return x;
}
@safe int g6328() {
    return f6328a(1+1);   // <-- but it won't be inferred as '@safe'
}
------------------------------------
x.d(8): Error: safe function 'g6328' cannot call system function 'f6328a'
------------------------------------

I'm marking it as 'enhancement' because I don't know if this is intentional to avoid the lazy argument from calling unsafe code.