Bigdecimal and coerce failing on subtraction operation

[seandmccarthy]

Environment

$ jruby -v jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 Java HotSpot(TM) 64-Bit Server VM 25.161-b12 on 1.8.0_161-b12 +jit [darwin-x86_64]

Also can be replicated on 9.1.17.0 and 1.7.27

$ uname -a Darwin [redacted] 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64

Issue also present on Windows 7

C:\Users\Sean Mccarthy>jruby -v jruby 9.1.17.0 (2.3.3) 2018-04-20 d8b1ff9 Java HotSpot(TM) 64-Bit Server VM 25.181-b13 on 1.8.0_181-b13 +jit [mswin32-x86_64]

Expected Behavior

Here's a test case:

require 'bigdecimal'

class NullValue
  def nil?
    true
  end

  def coerce(other)
    case other
    when Numeric
      [other, BigDecimal(0)]
    end
  end

  def method_missing(method_name, *arguments, &block)
    BigDecimal(0).send(method_name, *arguments, &block)
  end
end

puts "Trying addition:"
puts BigDecimal(100) + NullValue.new
puts "Trying multiplication:"
puts BigDecimal(100) * NullValue.new
puts "Trying division:"
puts BigDecimal(100) / NullValue.new
puts "Trying subtraction:"
puts BigDecimal(100) - NullValue.new

Output from MRI/CRuby 2.3.x

$ ruby -v
ruby 2.3.7p456 (2018-03-28 revision 63024) [x86_64-darwin17]
$ ruby null_value.rb
Trying addition:
0.1E3
Trying multiplication:
0.0
Trying division:
Infinity
Trying subtraction:
0.1E3

Actual Behavior

$ ruby -v
jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 Java HotSpot(TM) 64-Bit Server VM 25.161-b12 on 1.8.0_161-b12 +jit [darwin-x86_64]
$ ruby null_value.rb
Trying addition:
0.1e3
Trying multiplication:
0.0
Trying division:
Infinity
Trying subtraction:
TypeError: NullValue can't be coerced into BigDecimal
       - at org/jruby/ext/bigdecimal/RubyBigDecimal.java:1134
  <main> at null_value.rb:27

[seandmccarthy]

I think the issue is here:

jruby/core/src/main/java/org/jruby/ext/bigdecimal/RubyBigDecimal.java

Line 1134 in 4e8bb26

return subInternal(context, getVpValueWithPrec(context, b, true), b, 0);

The 3rd parameter to getVpValueWithPrec (the must value) is true, but in similar contexts, e.g.

jruby/core/src/main/java/org/jruby/ext/bigdecimal/RubyBigDecimal.java

Line 1039 in 4e8bb26

return addInternal(context, getVpValueWithPrec(context, b, false), b, vpPrecLimit(context.runtime));

the must value is false.

This means that the cannotBeCoerced method immediately throws an exception rather than returning null and letting the coercion behaviour occur later.

jruby/core/src/main/java/org/jruby/ext/bigdecimal/RubyBigDecimal.java

Lines 421 to 428 in 4e8bb26

	private static RubyBigDecimal cannotBeCoerced(ThreadContext context, IRubyObject value, boolean must) {
	if (must) {
	throw context.runtime.newTypeError(
	errMessageType(context, value) + " can't be coerced into BigDecimal"
	);
	}
	return null;
	}