chenBright opened a new pull request, #3547:
URL: https://github.com/apache/brpc/pull/3547
### What problem does this PR solve?
Issue Number: resolve #3546
Problem Summary:
`std::enable_if<cond>` is a class type whatever `cond` is, only
`std::enable_if<cond>::type`
goes away when the condition fails. Three places write the former, so their
constraints check
nothing.
- `bvar/reducer.h`: the babylon backed partial specializations of
`Adder`/`Maxer`/`Miner` are
keyed on `std::enable_if<cond>` while `Adder<T>` means `Adder<T, void>`,
so they never
match and the babylon counters are dead code even when built with
`--define with_babylon_counter=true`. Adding the missing `::type` is not
enough: babylon
constrains its counters with a `static_assert` in the class body, so
probing them with
`std::is_constructible` is a hard error instead of a substitution
failure, and the specializations
then expose three latent defects. `Adder::reset()` hits an inverted
`CHECK` and never resets,
`Maxer`/`Miner` return 0 instead of the identity of their operator for
an empty sampling period,
and `get_value()` carries no `CHECK` so `Window<Maxer<> >` loses its
warning.
- `butil/memory/scope_guard.h`: the default argument of the primary
template and the argument
list of the partial specialization share the same wrong expression, so
they always agree and a
callback returning non void is silently accepted.
- `butil/containers/optional.h`: the in place constructor is constrained by
`std::enable_if<cond>*`,
a valid pointer type whatever `cond` is, so `optional<int>` is reported
constructible from a string
literal. The `initializer_list` in place constructor puts its
constrained parameter after a parameter
pack without a default argument, so it can neither be deduced nor
specified and is never viable.
### What is changed and the side effects?
Changed:
`bvar/reducer.h`
- Key the babylon backed specializations on `typename
std::enable_if<cond>::type`.
- Replace the `std::is_constructible` probe with
`IsBabylonCounterSupported`, a two step trait
mirroring the constraint of babylon counters. It cannot be one expression
because `sizeof` does
not apply to void or to an incomplete type.
`butil/memory/scope_guard.h`
- Resolve the constraint to `void` and specialize on `ScopeGuard<Callback,
void>`.
`butil/containers/optional.h`
- Constrain both in place constructors with `typename std::enable_if<cond,
bool>::type = false`,
the form already used elsewhere in the file.
Side effects:
- Performance effects:
- Breaking backward compatibility:
---
### Check List:
- Please make sure your changes are compilable.
- When providing us with a new feature, it is best to add related tests.
- Please follow [Contributor Covenant Code of
Conduct](https://github.com/apache/brpc/blob/master/CODE_OF_CONDUCT.md).
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