Don't autocorrect BigDecimal with float when precision is specified

The precision argument to `BigDecimal()` is ignored unless the value is
a float or rational. Converting the value from a float to a string when
a precision is specified changes the result.

    $ ruby -rbigdecimal -e "puts BigDecimal(3.14, 1)"
    0.3e1

    $ ruby -rbigdecimal -e "puts BigDecimal('3.14', 1)"
    0.314e1

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#150](https://github.com/rubocop-hq/rubocop-performance/pull/150): Fix an incorrect autocorrect for `Performance/BigDecimalWithNumericArgument` when a precision is specified. ([@eugeneius][])
+
 ## 1.7.0 (2020-07-07)
 
 ### New features

lib/rubocop/cop/performance/big_decimal_with_numeric_argument.rb

@@ -25,6 +25,8 @@ class BigDecimalWithNumericArgument < Cop
 
         def on_send(node)
           big_decimal_with_numeric_argument?(node) do |numeric|
+            next if numeric.float_type? && specifies_precision?(node)
+
             add_offense(node, location: numeric.source_range)
           end
         end
@@ -36,6 +38,12 @@ def autocorrect(node)
             end
           end
         end
+
+        private
+
+        def specifies_precision?(node)
+          node.arguments.size > 1 && !node.arguments[1].hash_type?
+        end
       end
     end
   end

spec/rubocop/cop/performance/big_decimal_with_numeric_argument_spec.rb

@@ -16,12 +16,31 @@
 
   it 'registers an offense and corrects when using `BigDecimal` with float' do
     expect_offense(<<~RUBY)
-      BigDecimal(1.5, 2, exception: true)
+      BigDecimal(1.5, exception: true)
                  ^^^ Convert numeric argument to string before passing to `BigDecimal`.
     RUBY
 
     expect_correction(<<~RUBY)
-      BigDecimal('1.5', 2, exception: true)
+      BigDecimal('1.5', exception: true)
+    RUBY
+  end
+
+  it 'does not register an offense when using `BigDecimal` with float and precision' do
+    expect_no_offenses(<<~RUBY)
+      BigDecimal(3.14, 1)
+    RUBY
+  end
+
+  it 'does not register an offense when using `BigDecimal` with float and non-literal precision' do
+    expect_no_offenses(<<~RUBY)
+      precision = 1
+      BigDecimal(3.14, precision)
+    RUBY
+  end
+
+  it 'does not register an offense when using `BigDecimal` with float, precision, and a keyword argument' do
+    expect_no_offenses(<<~RUBY)
+      BigDecimal(3.14, 1, exception: true)
     RUBY
   end