[XamlC] Produce warning when Binding inside of DataTemplate inherits x:DataType from outside of the template

[simonrozsival]

I think XamlC should produce a warning when there is a {Binding ...} inside of a DataTemplate which inherits x:DataType from outside of the template. Consider the following example:

<ScrollView x:DataType="local:PageViewModel">
    <StackLayout BindableLayout.ItemsSource="{Binding Items}">
        <BindableLayout.ItemTemplate>
            <DataTemplate>
                <Label Text="{Binding .}" />
            </DataTemplate>
        </BindableLayout.ItemTemplate>
    </StackLayout>
</ScrollView>

• XamlC won't report any warning
• The Text property will contain a TypedBinding<PageViewModel, PageViewModel> and when the item will have a different type, the label will be empty without producing any warning ([X] do not apply Bindings if DataType doesnt match #22056)

My suggestion is to report a warning something like

XC0024: Binding might be compiled incorrectly because it is using an x:DataType local:PageViewModel that is defined outside of the current DataTemplate scope. Consider adding x:DataType directly to the DataTemplate. See https://learn.microsoft.com/dotnet/maui/fundamentals/data-binding/compiled-bindings for more information.

... and add a paragraph describing this issue to the linked documentation page.

Related to #22056

/cc @StephaneDelcroix @ivanpovazan @PureWeen @mattleibow

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• XamlC resolves x:DataType incorrectly (#11956), similarity score: 0.71
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• XamlC XC0022 warning : Binding could be compiled if x:DataType is specified after upgrade to 8.0.7 (#21288), similarity score: 0.74
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[ivanpovazan]

For reference:
@PureWeen recently fixed an issue with the same root cause in: rachelkang/recipeSearch@b63c3a7

[ivanpovazan]

FWIW:
Another idea for the warning message could be that our recommendation is to always specify x:DataType for bindings in templates explicitly, as if it is omitted it could inherit x:DataType from a parent scope and cause runtime failures.

[simonrozsival]

The implementation was merged into the net9.0 branch