[spec] Use fractionDigits/significantDigits in convertTo, allow Number values

Aligns Amount.prototype.convertTo with the README updates from #101:
replaces the min/max fraction/significant digit options and roundingPriority
with the constructor-style fractionDigits and significantDigits (mutually
exclusive, throwing RangeError if both are set). When neither precision
option is supplied — or the converted value is non-finite — the resulting
Amount is now Number-backed rather than always String-backed.

Author: jessealama

spec.emu

@@ -106,41 +106,29 @@ location: https://github.com/tc39/proposal-amount/
       <emu-clause id="sec-amount-getamountconverttooptions" type="abstract operation">
         <h1>GetAmountConvertToOptions (
           _opts_: an Object
-        ): either a normal completion containing a Record with fields [[MinimumFractionDigits]] (a non-negative integer or *undefined*), [[MaximumFractionDigits]] (a non-negative integer or *undefined*), [[RoundingMode]] (a <emu-xref href="#dfn-amount-rounding-mode">rounding mode</emu-xref>), [[RoundingPriority]] (a String), [[MinimumSignificantDigits]] (a positive integer or *undefined*), [[MaximumSignificantDigits]] (a positive integer or *undefined*), [[Locale]] (a String or *undefined*), [[Usage]] (a String or *undefined*), and [[Unit]] (a String or *undefined*) or a throw completion
+        ): either a normal completion containing a Record with fields [[FractionDigits]] (a non-negative integer or *undefined*), [[RoundingMode]] (a <emu-xref href="#dfn-amount-rounding-mode">rounding mode</emu-xref>), [[SignificantDigits]] (a positive integer or *undefined*), and [[Unit]] (a String or *undefined*) or a throw completion
         </h1>
         <dl class="header">
           <dt>description</dt>
           <dd>It validates the given _options_ (an ECMAScript object) for converting an Amount to another Amount and returns a Record with slots set to appropriate marthematical values (or *undefined*).</dd>
         </dl>
         <emu-alg>
           1. Let _opts_ be ? GetOptionsObject(_opts_).
-          1. Let _minFractionDigits_ be ? GetOption(_opts_, *"minimumFractionDigits"*, ~number~, ~empty~, *undefined*).
-          1. Let _maxFractionDigits_ be ? GetOption(_opts_, *"maximumFractionDigits"*, ~number~, ~empty~, *undefined*).
+          1. Let _fractionDigits_ be ? GetOption(_opts_, *"fractionDigits"*, ~number~, ~empty~, *undefined*).
           1. Let _roundingMode_ be ? GetOption(_opts_, *"roundingMode"*, ~string~, « *"ceil"*, *"floor"*, *"expand"*, *"trunc"*, *"halfCeil"*, *"halfFloor"*, *"halfExpand"*, *"halfTrunc"*, *"halfEven"* », *"halfEven"*).
-          1. Let _roundingPriority_ be ? GetOption(_opts_, *"roundingPriority"*, ~string~, ~empty~, *undefined*).
-          1. Let _minSignificantDigits_ be ? GetOption(_opts_, *"minimumSignificantDigits"*, ~number~, ~empty~, *undefined*).
-          1. Let _maxSignificantDigits_ be ? GetOption(_opts_, *"maximumSignificantDigits"*, ~number~, ~empty~, *undefined*).
+          1. Let _significantDigits_ be ? GetOption(_opts_, *"significantDigits"*, ~number~, ~empty~, *undefined*).
           1. Let _unit_ be ? GetOption(_opts_, *"unit"*, ~string~, ~empty~, *undefined*).
-          1. If _minFractionDigits_ is not *undefined*, then
-            1. If _minFractionDigits_ is not an integral Number, throw a *RangeError* exception.
-            1. Set _minFractionDigits_ to ℝ(_minFractionDigits_).
-            1. If _minFractionDigits_ is not in the inclusive interval from 0 to 100, throw a *RangeError* exception.
-          1. If _maxFractionDigits_ is not *undefined*, then
-            1. If _maxFractionDigits_ is not an integral Number, throw a *RangeError* exception.
-            1. Set _maxFractionDigits_ to ℝ(_maxFractionDigits_).
-            1. If _maxFractionDigits_ is not in the inclusive interval from 0 to 100, throw a *RangeError* exception.
-          1. If _minFractionDigits_ is not *undefined* and _maxFractionDigits_ is not *undefined* and _minFractionDigits_ > _maxFractionDigits_, throw a *RangeError* exception.
-          1. If _minSignificantDigits_ is not *undefined*, then
-            1. If _minSignificantDigits_ is not an integral Number, throw a *RangeError* exception.
-            1. Set _minSignificantDigits_ to ℝ(_minSignificantDigits_).
-            1. If _minSignificantDigits_ is not in the inclusive interval from 1 to 21, throw a *RangeError* exception.
-          1. If _maxSignificantDigits_ is not *undefined*, then
-            1. If _maxSignificantDigits_ is not an integral Number, throw a *RangeError* exception.
-            1. Set _maxSignificantDigits_ to ℝ(_maxSignificantDigits_).
-            1. If _maxSignificantDigits_ is not in the inclusive interval from 1 to 21, throw a *RangeError* exception.
-          1. If _minSignificantDigits_ is not *undefined* and _maxSignificantDigits_ is not *undefined* and _minSignificantDigits_ > _maxSignificantDigits_, throw a *RangeError* exception.
+          1. If _fractionDigits_ is not *undefined*, then
+            1. If _significantDigits_ is not *undefined*, throw a *RangeError* exception.
+            1. If _fractionDigits_ is not an integral Number, throw a *RangeError* exception.
+            1. Set _fractionDigits_ to ℝ(_fractionDigits_).
+            1. If _fractionDigits_ is not in the inclusive interval from 0 to 100, throw a *RangeError* exception.
+          1. Else if _significantDigits_ is not *undefined*, then
+            1. If _significantDigits_ is not an integral Number, throw a *RangeError* exception.
+            1. Set _significantDigits_ to ℝ(_significantDigits_).
+            1. If _significantDigits_ is not in the inclusive interval from 1 to 21, throw a *RangeError* exception.
           1. If _unit_ is the empty String, throw a *RangeError* exception.
-          1. Return the Record { [[MinimumFractionDigits]]: _minFractionDigits_, [[MaximumFractionDigits]]: _maxFractionDigits_, [[MinimumSignificantDigits]]: _minSignificantDigits_, [[MaximumSignificantDigits]]: _maxSignificantDigits_, [[RoundingMode]]: _roundingMode_, [[RoundingPriority]]: _roundingPriority_, [[Unit]]: _unit_ }.
+          1. Return the Record { [[FractionDigits]]: _fractionDigits_, [[RoundingMode]]: _roundingMode_, [[SignificantDigits]]: _significantDigits_, [[Unit]]: _unit_ }.
         </emu-alg>
       </emu-clause>
 
@@ -389,11 +377,8 @@ location: https://github.com/tc39/proposal-amount/
       1. Let _targetUnit_ be _validatedOpts_.[[Unit]].
       1. If _targetUnit_ is *undefined*, throw a *TypeError* exception.
       1. Let _roundingMode_ be _validatedOpts_.[[RoundingMode]].
-      1. Let _roundingPriority_ be _validatedOpts_.[[RoundingPriority]].
-      1. Let _minFractionDigits_ be _validatedOpts_.[[MinimumFractionDigits]].
-      1. Let _maxFractionDigits_ be _validatedOpts_.[[MaximumFractionDigits]].
-      1. Let _minSignificantDigits_ be _validatedOpts_.[[MinimumSignificantDigits]].
-      1. Let _maxSignificantDigits_ be _validatedOpts_.[[MaximumSignificantDigits]].
+      1. Let _fractionDigits_ be _validatedOpts_.[[FractionDigits]].
+      1. Let _significantDigits_ be _validatedOpts_.[[SignificantDigits]].
       1. Let _v_ be _O_.[[AmountValue]].
       1. If _v_ is a Number, then
         1. Let _sourceValue_ be _v_.
@@ -403,10 +388,13 @@ location: https://github.com/tc39/proposal-amount/
         1. Assert: _v_ is a String.
         1. Let _sourceValue_ be StringToNumber(_v_).
       1. Let _convertedValue_ be ? ConvertUnitValue(_sourceValue_, _sourceUnit_, _targetUnit_).
-      1. Let _formatter_ be CreateFormatterObject(_roundingMode_, _minFractionDigits_, _maxFractionDigits_, _minSignificantDigits_, _maxSignificantDigits_, _roundingPriority_).
-      1. Let _formatted_ be FormatNumericToString(_formatter_, ℝ(_convertedValue_), 0).
       1. Let _result_ be OrdinaryObjectCreate(%Amount.prototype%, « [[AmountValue]], [[Unit]] »).
-      1. Set _result_.[[AmountValue]] to _formatted_.[[FormattedString]].
+      1. If _convertedValue_ is finite and (_fractionDigits_ is not *undefined* or _significantDigits_ is not *undefined*), then
+        1. Let _formatter_ be CreateFormatterObject(_roundingMode_, _fractionDigits_, _fractionDigits_, _significantDigits_, _significantDigits_, *undefined*).
+        1. Let _formatted_ be FormatNumericToString(_formatter_, ℝ(_convertedValue_), 0).
+        1. Set _result_.[[AmountValue]] to _formatted_.[[FormattedString]].
+      1. Else,
+        1. Set _result_.[[AmountValue]] to _convertedValue_.
       1. Set _result_.[[Unit]] to _targetUnit_.
       1. Return _result_.
     </emu-alg>