Missing quotes in target Outputs -> expression results in batching, broken incrementality

[KirillOsenkov]

<Project DefaultTargets="Build">
  <PropertyGroup>
    <Destination>dest</Destination>
  </PropertyGroup>

  <ItemGroup>
    <File Include="1.proj" />
    <File Include="2.proj" />
  </ItemGroup>

  <Target Name="Delete">
    <!--<RemoveDir Directories="$(Destination)\Contents" />-->
  </Target>

  <Target Name="Build" DependsOnTargets="Delete" Inputs="@(File)" Outputs="@(File->$(Destination)\Contents\%(Filename)%(Extension))">
    <Copy SourceFiles="@(File)" DestinationFolder="$(Destination)\Contents\" SkipUnchangedFiles="true" />
  </Target>
</Project>

This behaves not in the expected way. Wondering if we should show a warning? (Probably an error would be a breaking change??)

[KirillOsenkov]

[KirillOsenkov]

Broken incrementality:

[rainersigwald]

I definitely find it surprising that a very slightly malformed item transformation expression gets interpreted as "oh you must have meant the literal string @(ItemName-> concatenated with some bare-metadata references that will trip us into batching concatenated with a literal )".

Probably an error would be a breaking change??

Yeah, I think so 😔. At least in the "your build is completing without errors even if it's not doing the thing you intended" sense.