Msbuild: Log a warning when an undefined property is used

[KirillOsenkov]

When an undefined property is used, consider logging a warning or at least a message. In Strict Mode, fail the build.

[rainersigwald]

Such a warning would be _incredibly_ noisy.

It would also have to be special cased for the if-not-set-set-to-my-default case

<PropertyGroup>
  <FunProperty Condition="'$(FunProperty)' == ''">FunDefault</FunProperty>
</PropertyGroup>

I think we already have logic to detect that case somewhere, but I don't remember why . ..

[pakrym]

You can make a differentiation between when it's used directly like in '$(FunProperty)' or in expression '$(FunProperty)\foo'.
Better pattern to check is property was defined might also help

[KirillOsenkov]

yeah like we have Exists for files and directories, have Defined for properties.

<PropertyGroup>
  <FunProperty Condition="!Defined(FunProperty)">FunDefault</FunProperty>
</PropertyGroup>

[KirillOsenkov]

See related https://github.com/microsoft/msbuild/issues/5015

[rainersigwald]

This can be enabled with MSBUILDWARNONUNINITIALIZEDPROPERTY. I presume it wasn't made default when that was implemented in the Stone Age because of noisiness (but we can always reevaluate).