Ruby

I also found 1.coerce(2) returns integers and
1.coerce(2**100) returns floats.

% ./ruby -e 'p 1.coerce(2)' [2, 1] % ./ruby -e 'p 1.coerce(2**100)' [1.2676506002282294e+30, 1.0] % ./ruby -v ruby 2.4.0dev (2016-05-01 trunk 54866) [x86_64-linux] 

I think it should return integers because
Fixnum and Bignum should work seamlessly.

I had a question about Bignum#coerce in http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/75176.

I think it has limited value to have Bignum.coerce(Fixnum) or Fixnum.coerce(Bignum) return [Bignum, Bignum] instead of just the operands,
as it creates a Bignum in the Fixnum range, which is inconsistent with all other methods and might cause performance problems.
With #12005 this would be naturally the result with additionally no class distinction.

But indeed, Fixnum.coerce(Bignum) => [Float, Float] as currently is even worse probably (and shows how rarely {Bignum,Fixnum}#coerce are used in practice).

Bignum.coerce(Fixnum) is used to implement fixnum binop bignum.
(binop is binary operator such as +, -, etc.)

x binop y is implemented as follows if x's class doesn't know y's class.

u, v = y.coerce(x) u binop v 

This is the extension mechanism that we can implement new class
which can be used with numeric objects.
For example, 3 * Vector[4,5] works because Fixnum#* invokes
Vector#coerce which returns [Matrix::Scalar, Vector] and
invokes Matrix::Scalar#* which calculates actual scalar-vector multiplication.

Bignum#coerce is just an instance of this extension mechanism.
If bignum.coerce(fixnum) returns [fixnum, bignum] as you said,
"u binop v" can cause infinite recursion.

This mechanism is cleary useful for classes implemented in Ruby
script such as matrix.rb.
However matz also use it for builtin numeric classes: Fixnum, Bignum and Float.

Akira Tanaka wrote:

Bignum.coerce(Fixnum) is used to implement fixnum binop bignum.
(binop is binary operator such as +, -, etc.)

x binop y is implemented as follows if x's class doesn't know y's class.
[...]
However matz also use it for builtin numeric classes: Fixnum, Bignum and Float.

Thanks for the clarification.
In the specific case of Fixnum binop Bignum, I expect all Fixnum methods already know about Bignum and do not call coerce, isn't it?

I tried locally to raise an error when Fixnum.coerce(Bignum) is called and got only 1 failure in test-all
(due to rb_num_coerce_cmp called by ruby_num_interval_step_size(#size) on an Enumerator from Numerc#step with a Bignum step).

That said, I think your patch is an improvement until #12005 is merged.