[ruby-core:78247] [Ruby trunk Feature#12968] Allow default value via block for Integer(), Float() and Rational()

Issue #12968 has been reported by Stefan Schテシテ殕er.

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Feature #12968: Allow default value via block for Integer(), Float() and Rational()
https://bugs.ruby-lang.org/issues/12968

* Author: Stefan Schテシテ殕er
* Status: Open
* Priority: Normal
* Assignee: 
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`Kernel` provides the global conversion functions `Integer()`, `Float()` and `Rational()`. Each one convert its argument to the respective type.

Parsing errors are signaled by raising a `TypeError` or an `ArgumentError`.

I often use these methods to parse numeric string values, but I find the need for a `rescue ArgumentError` block quite annoying an distracting.

I therefore request a new feature: `Integer()`, `Float()` and `Rational()` should take an _optional block_ that serves as a callback when parsing failed.

Examples:

```ruby
Integer('foo')                #=> ArgumentError  (just as before)
Integer('foo') { 0 }          #=> 0              (same as 'foo'.to_i)
Integer('foo') { nil }        #=> nil
Integer('foo') { Float::NAN } #=> NaN
```

This resembles the way `Hash#fetch` or `Hash#fetch_values` work when a block is given.

Although not used in the examples above, any arguments should be passed along to the block, i.e. `Integer('foo') { |arg, base| ... }`



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