dotnet/msbuild

Conditions on item metadata are evaluated even for empty item group

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#4,056 opened on Jan 9, 2019

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Area: Enginehelp wantedtriaged

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Description

Steps to reproduce

Project file

<Project>
  <Target Name="Build">
    <ItemGroup>
      <X Include="@(Empty)">
        <M Condition="!%(P)">not P</M>
      </X>
    </ItemGroup>
  </Target>
</Project>

I'd expect the condition to not be evaluated since @(Empty) is empty, so there are no items being added to X.

Expected behavior

No error reported.

Actual behavior

error MSB4113: Specified condition "!%(P)" evaluates to "!" instead of a boolean.

Environment data

msbuild /version output:

Microsoft (R) Build Engine version 15.9.14-preview+gae560092f5 for .NET Framework Copyright (C) Microsoft Corporation. All rights reserved.

15.9.14.22190

Contributor guide

Research direction
Investigate the condition evaluation logic for item groups when the item source is empty. Check if the engine prematurely evaluates metadata conditions before checking if any items exist.
Tech stack
csharp
Domain
build system
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
MSBuild basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75