Allow INTERFACE_SOURCES to specify $<TARGET_OBJECTS> (#14970)

Fix cmTarget::GetSourceFiles to set EvaluateForBuildsystem on the
$<TARGET_PROPERTY:...,INTERFACE_SOURCES> generator expression so that
the $<TARGET_OBJECTS> generator expression is allowed within an
INTERFACE_SOURCES value.

Extend the InterfaceLibrary test to cover this case.  Extend the
RunCMake.TargetObjects test to cover failure of $<TARGET_OBJECTS>
when used through $<TARGET_PROPERTY:...,INTERFACE_SOURCES> in a
non-buildsystem context.

2 files changed, 15 insertions, 2 deletions

diff --git a/Tests/RunCMake/TargetObjects/BadContext-stderr.txt b/Tests/RunCMake/TargetObjects/BadContext-stderr.txt
index 92f2c91..b78189e 100644
--- a/Tests/RunCMake/TargetObjects/BadContext-stderr.txt
+++ b/Tests/RunCMake/TargetObjects/BadContext-stderr.txt
@@ -1,4 +1,4 @@
-CMake Error at BadContext.cmake:2 \(file\):
+(CMake Error at BadContext.cmake:4 \(file\):
   Error evaluating generator expression:
 
     \$<TARGET_OBJECTS:NoTarget>
@@ -7,7 +7,17 @@ CMake Error at BadContext.cmake:2 \(file\):
   for consumption by CMake.  It is not suitable for writing out elsewhere.
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)
-+
+*)+
+(CMake Error at BadContext.cmake:5 \(file\):
+  Error evaluating generator expression:
+
+    \$<TARGET_OBJECTS:NoTarget>
+
+  The evaluation of the TARGET_OBJECTS generator expression is only suitable
+  for consumption by CMake.  It is not suitable for writing out elsewhere.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
+*)+
 CMake Error:
   Error evaluating generator expression:
 
diff --git a/Tests/RunCMake/TargetObjects/BadContext.cmake b/Tests/RunCMake/TargetObjects/BadContext.cmake
index 67962a4..5d7e33e 100644
--- a/Tests/RunCMake/TargetObjects/BadContext.cmake
+++ b/Tests/RunCMake/TargetObjects/BadContext.cmake
@@ -1,4 +1,7 @@
+add_library(iface INTERFACE)
+target_sources(iface INTERFACE $<TARGET_OBJECTS:NoTarget>)
 
 file(GENERATE OUTPUT test_output CONTENT $<TARGET_OBJECTS:NoTarget>)
+file(GENERATE OUTPUT test_output2 CONTENT $<TARGET_PROPERTY:iface,INTERFACE_SOURCES>)
 
 install(FILES $<TARGET_OBJECTS:NoTarget> DESTINATION objects)