Fix crash on $<TARGET_PROPERTY:...,LOCATION> genex (#16134)

Policy CMP0026 deprecated the LOCATION property, and we have long
provided a $<TARGET_FILE:...> generator expression.  However, if
a project tries to use $<TARGET_PROPERTY:...,LOCATION> we should
at least not crash.

The compatibility implementation of the LOCATION property uses
cmGlobalGenerator::CreateGenerationObjects to create the structures
needed to evaluate the property before generation starts.  The
implementation assumed that accessing the property could only be done
during configuration (via the typical get_property command use case).
The $<TARGET_PROPERTY:...,LOCATION> genex causes the LOCATION property
to be accessed during generation.  Calling CreateGenerationObjects
during generation blows away all the objects currently being used for
generation and is not safe.  Add a condition to call it only when
configuration is not finished.

1 files changed, 10 insertions, 0 deletions

diff --git a/Tests/RunCMake/GeneratorExpression/TARGET_PROPERTY-LOCATION-stderr.txt b/Tests/RunCMake/GeneratorExpression/TARGET_PROPERTY-LOCATION-stderr.txt
new file mode 100644
index 0000000..e4dbb71
--- /dev/null
+++ b/Tests/RunCMake/GeneratorExpression/TARGET_PROPERTY-LOCATION-stderr.txt
@@ -0,0 +1,10 @@
+CMake Warning \(dev\) in CMakeLists.txt:
+  Policy CMP0026 is not set: Disallow use of the LOCATION target property.
+  Run "cmake --help-policy CMP0026" for policy details.  Use the cmake_policy
+  command to set the policy and suppress this warning.
+
+  The LOCATION property should not be read from target "foo".  Use the target
+  name directly with add_custom_command, or use the generator expression
+  \$<TARGET_FILE>, as appropriate.
+
+This warning is for project developers.  Use -Wno-dev to suppress it.