[ruby-core:111918] [Ruby master Feature#19245] Strict mode for Array#pack that doesn't silently truncate numbers that are too large for the given directive

Issue #19245 has been updated by mame (Yusuke Endoh).


Discussed at the dev meeting. We need to determine some detailed behaviors:

* Should `[-1].pack("C", strict: true)` raise an exception because 'C' represents unsigned char? If what you want is "wrap around", it should raise an error because `[-1].pack("C").unpack1("C")` returns 255, not -1.
* Should `[255].pack("c", strict: true)` raise an exception?
* Should `["foo"].pack("a2", strict: true)` raise an exception?
* Should `["f"].pack("a2", strict: true)` raise an exception?
* Perhaps we need to consider what is out of range for each format.

And just for the record: @nobu proposed a new modifier to check out-of-range value, such as `[256].pack("c^") #=> error`. This allows users to determine out-of-range policy per element in one format. However, @matz disliked it.

----------------------------------------
Feature #19245: Strict mode for Array#pack that doesn't silently truncate numbers that are too large for the given directive
https://bugs.ruby-lang.org/issues/19245#change-101341

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
```ruby
>> [256].pack("C").unpack1("C")
=> 0
>> [257].pack("C").unpack1("C")
=> 1
```

This is specified:

```ruby
  it "encodes the least significant 32 bits of a negative number" do
    [ [[-0x0000_0021], "\xdf\xff\xff\xff"],
      [[-0x0000_4321], "\xdf\xbc\xff\xff"],
      [[-0x0065_4321], "\xdf\xbc\x9a\xff"],
      [[-0x7865_4321], "\xdf\xbc\x9a\x87"]
    ].should be_computed_by(:pack, pack_format())
  end
```

But not documented in `Array#pack`.

I think that in many case this may lead to silent bugs.

### Possible solutions

We could have a strict version of `pack`, either `pack(template, strict: true)` or `pack!(template)`.

Or alternatively if we think this is never a desired behavior, we could change `pack` to first warn on truncation and later raise.





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