ECMA-402 layering is incorrect

It will be necessary for ECMA-402 operations to supersede those in ECMA-262, but the current spec text handles that incorrectly—ECMA-402 requires GetAmountConvertToOptions to provide locale and usage by reading properties from options [at the right time], while ECMA-262 requires absence of such [[Get]] operations. This proposal currently defines a GetAmountConvertToOptions that is documented to return a Record with fields [[Locale]] and [[Usage]], but fails to do so.

There's also what looks to me like an uncomfortable about of redundancy between the ECMA-262 Amount.prototype.convertTo and the ECMA-402 Amount.prototype.convertTo which makes it difficult to observe what is actually different.

I think things would be well-structured like so:

• There is a single common GetAmountConvertToOptions, with the reading and validation of locale, localeMatcher, and usage required by ECMA-402 but absent in ECMA-262 being handled as either normative optional steps (analogous to ToBoolean) or as calls to operations with no-op default implementations and superseding definitions in ECMA-402 (analogous to ECMA-262 Array.prototype.toLocaleString and its ECMA-402 replacement).
• The majority of Amount.prototype.convertTo is likewise common to both specs, delegating to an operation like Let targetUnit be ? SelectUnitConversionTarget(sourceValue, sourceUnit, validatedOpts) with definitions like the following:
  • ECMA-262:

    1. Let _unit_ be _validatedOpts_.[[Unit]].
    1. If _unit_ is *undefined*, throw a *TypeError* exception.
    1. Return _unit_.
    

  • ECMA-402:

    1. Let _unit_ be _validatedOpts_.[[Unit]].
    1. Let _requestedLocales_ be _validatedOpts_.[[RequestedLocales]].
    1. Let _usage_ be _validatedOpts_.[[Usage]].
    1. If _unit_ is not *undefined*, then
      1. If _requestedLocales_ is not *undefined* or _usage_ is not *undefined*, throw a *TypeError* exception.
      1. Return _unit_.
    1. If _requestedLocales_ is *undefined* and _usage_ is *undefined*, throw a *TypeError* exception.
    1. NOTE: The following steps do not yet provide or utilize data associated with Unicode locale extension sequence keys <code>rg</code> (Region Override), <code>ms</code> (Unicode Measurement System Identifier), or <code>mu</code> (Measurement Unit Preference Override), despite their relevance to the algorithms of <a href="https://unicode.org/reports/tr35/tr35-info.html#Unit_Preferences">UTS #35 Part 6 Supplemental, Unit Preferences</a>.
    1. Let _availableLocales_ be an Available Locales List which includes all language tags corresponding with locales for which CLDR file <a href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/units.xml"><code>units.xml</code></a> defines unit preferences in <code><unitPreferenceData></code>. If the implementation supports additional unit preferences, _availableLocales_ should also include their corresponding language tags.
    1. Let _localeMatcher_ be _validatedOpts_.[[LocaleMatcher]].
    1. Let _relevantExtensionKeys_ be a new empty List.
    1. Let _localeResolution_ be ResolveLocale(_availableLocales_, _requestedLocales_, the Record { [[localeMatcher]]: _localeMatcher_ }, _relevantExtensionKeys_).
    1. Let _resolvedLocale_ be _localeResolution_.[[Locale]].
    1. Let _region_ be the result of running the <a href="https://unicode.org/reports/tr35/tr35-info.html#compute--regions">UTS #35 Part 6 Supplemental, Compute regions</a> algorithm with input locale identifier _resolvedLocale_.
    1. Let _sourceConversionRecord_ be ? GetUnitConversionFactor(_sourceUnit_).
    1. Let _category_ be _sourceConversionRecord_.[[Category]].
    1. If _usage_ is *undefined*, set _usage_ to *"default"*.
    1. Set _unit_ to the result of running the <a href="https://unicode.org/reports/tr35/tr35-info.html#compute-the-preferred-output-unit">UTS #35 Part 6 Supplemental, Compute the preferred output unit</a> and <a href="https://unicode.org/reports/tr35/tr35-info.html#search-the-ranked-units">UTS #35 Part 6 Supplemental, Search the ranked units</a> algorithms with inputs _category_, _usage_, and _region_ for an input measure corresponding with _sourceValue_ and _sourceUnit_.
    1. Return _unit_.
    

• To better accommodate the above, ResolveLocale parameter localeData is made optional:

   1. Let _foundLocale_ be _r_.[[locale]].
  +1. Let _result_ be a new Record.
  +1. If _localeData_ is not present, then
  +  1. Assert: _relevantExtensionKeys_ is an empty List.
  +  1. Set _result_.[[LocaleData]] to *undefined*.
  +1. Else,
  -1. Let _foundLocaleData_ be _localeData_.[[<_foundLocale_>]].
  -1. Assert: _foundLocaleData_ is a Record.
  -1. Let _result_ be a new Record.
  -1. Set _result_.[[LocaleData]] to _foundLocaleData_.
  +  1. Let _foundLocaleData_ be _localeData_.[[<_foundLocale_>]].
  +  1. Assert: _foundLocaleData_ is a Record.
  +  1. Set _result_.[[LocaleData]] to _foundLocaleData_.
  

Start with GetAmountConvertToOptions and the ECMA-262 and ECMA-402 Amount.prototype.convertTo sections, then review the ResolveLocale algorithm and the proposed layering examples. Compare the shared and superseding steps, and consider the issue resolved when the ECMA-262/ECMA-402 interaction and optional locale-data handling are specified consistently.