Show warning when referencing an Item as a Property or viceversa

[cdmihai]

If you have an item foo but you forget it's an item and you reference it as a property ($(foo)), then MSBuild will just evaluate it to an empty string. This adds to the time it takes to debug the build.

To make debugging easier, MSBuild should emit a warning: "You are referencing the item foo as a property".

[rainersigwald]

This would potentially save a lot of grief. We may be able to learn from Perl, which has vaguely similar behavior between scalar and array values.

[sarajoiner]

For what it's worth, MSBuild already has a related feature, to warn when an uninitialized property is used. (Can be enabled through setting MSBUILDWARNONUNINITIALIZEDPROPERTY=1 or BuildParameters.WarnOnUninitializedProperty = true)

However, when we introduced this feature back in (IIRC) VS 2010, we quickly discovered that all existing targets files used that pattern so frequently (usually when setting up defaults for commonly known properties) that it was, essentially, useless.

So I'd suggest that this also only be available under a flag, at least at first -- I wouldn't be surprised if the existing targets also used this pattern intentionally at times.

[SamB]

@sarajoiner: Couldn't used-uninitialized tracking in MSBUILDWARNONUNINITIALIZEDPROPERTY easily be amended to ignore the use of a property in the Condition for a Property element of the same name? (Just tucking away the "used uninitialized" state for the property while evaluating its condition should work, shouldn't it?)

[stan-sz]

This is a good idea for #1777