Fix BigDecimal#inspect to properly case non-finite values.

Author: nirvdrum

CHANGELOG.md

@@ -3,6 +3,7 @@
 Bug fixes:
 
 * Fixed `BigDecimal#dup` so it now just returns the receiver, per Ruby 2.5+ semantics (#1680).
+* Fixed `BigDecimal#inspect` output for non-finite values (e.g, NaN or -Infinity) (#1683).
 * Implemented `rb_eval_string_protect`.
 * Fixed `rb_get_kwargs` to correctly handle optional and rest arguments.
 

lib/truffle/bigdecimal.rb

@@ -245,7 +245,7 @@ def to_s(format = 'E')
   end
 
   def inspect
-    to_s.downcase
+    finite? ? to_s.downcase : to_s
   end
 
   def _dump(level=nil)

spec/ruby/library/bigdecimal/inspect_spec.rb

@@ -14,4 +14,11 @@
   it "looks like this" do
     @bigdec.inspect.should == "0.12345678e4"
   end
+
+  it "properly cases non-finite values" do
+    BigDecimal("NaN").inspect.should == "NaN"
+    BigDecimal("Infinity").inspect.should == "Infinity"
+    BigDecimal("+Infinity").inspect.should == "Infinity"
+    BigDecimal("-Infinity").inspect.should == "-Infinity"
+  end
 end