[Bug] Types that require conversion don't work in OnPlatform

[mattleibow]

Description

I can't use NamedSize as a font size:

<Label x:Name="label0">
    <Label.FontSize>
        <OnPlatform x:TypeArguments="NamedSize">
            <On Platform="Android" Value="Large" />
            <On Platform="UWP" Value="Large" />
            <On Platform="iOS" Value="Large" />
        </OnPlatform>
    </Label.FontSize>
</Label>

Steps to Reproduce

1. Use the snippet above
2. Observe the error

Expected Behavior

The values convert from my string into a double value for the property.

Actual Behavior

I get an error:

XLS0504	Property 'FontSize' does not support values of type 'OnPlatform`1 (T)'.

or

XamlC error XFC0009: No property, BindableProperty, or event found for "FontSize", or mismatching type between value and property.

Basic Information

• Version with issue:
• Last known good version:
• Platform Target Frameworks:
  • iOS:
  • Android:
  • UWP:
• Android Support Library / AndroidX Version:
• NuGet Packages:
• Affected Devices:

Environment

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Build Logs

Screenshots

Reproduction Link

Workaround

[Tommigun1980]

If you guys take a sprint on named sizes I’ve got two more here:
#13626
#13628