From 90f0c9322778fffc06545dd0b3d5afee6e7db98e Mon Sep 17 00:00:00 2001
From: Viktor Hofer <viktor.hofer@microsoft.com>
Date: Wed, 29 Jul 2020 23:39:02 +0200
Subject: [PATCH] Fix CoreLib build when building libraries solutions (#40097)

If a project that is part of solution has a ProjectReference to CoreLib, the Platform property was set to AnyCPU and CoreLib was built with the wrong Platform/Platform_Target.
---
 src/libraries/Directory.Build.targets | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/libraries/Directory.Build.targets b/src/libraries/Directory.Build.targets
index 95dae2669d8e9..cc196f26392a7 100644
--- a/src/libraries/Directory.Build.targets
+++ b/src/libraries/Directory.Build.targets
@@ -176,7 +176,8 @@
         <SkipGetTargetFrameworkProperties>true</SkipGetTargetFrameworkProperties>
       </ProjectReference>
       <ProjectReference Condition="'%(Filename)' == 'System.Private.CoreLib'">
-        <UndefineProperties>$(UndefineProperties);TargetFramework</UndefineProperties>
+        <!-- Don't flow TargetFramework and Platform to use same inputs and outputs as the CoreLib's build as part of the runtime. -->
+        <UndefineProperties>$(UndefineProperties);TargetFramework;Platform</UndefineProperties>
         <SetConfiguration Condition="'$(RuntimeFlavor)' == 'CoreCLR' and
                                      '$(Configuration)' != '$(CoreCLRConfiguration)'">Configuration=$(CoreCLRConfiguration)</SetConfiguration>
         <SetConfiguration Condition="'$(RuntimeFlavor)' == 'Mono' and