export: Add policy CMP0024 to disallow include() of export files

Currently, export() is executed at configure-time.

One problem with this is that certain exported properties like
the link interface may not be complete at the point the export() is
encountered leading to an incorrect or incomplete exported
representation. Additionally, the generated IMPORTED_LOCATION
property may even be incorrect if commands following the export()
have an effect on it.

Another problem is that it requires the C++ implementation of cmake
to be capable of computing the exported information at configure time.
This is a limitation on the cleanup and maintenance of the code. At
some point in the future, this limitation will be dropped and more
implementation will be moved from cmTarget to cmGeneratorTarget.

1 files changed, 19 insertions, 0 deletions

diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index a823f05..45670b3 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -601,6 +601,25 @@ cmPolicies::cmPolicies()
     "The NEW behavior for this policy is to not to allow mixing of the "
     "keyword and plain signatures.",
     2,8,12,0, cmPolicies::WARN);
+
+  this->DefinePolicy(
+    CMP0024, "CMP0024",
+    "Disallow include export result.",
+    "CMake 2.8.12 and lower allowed use of the include() command with the "
+    "result of the export() command.  This relies on the assumption that "
+    "the export() command has an immediate effect at configure-time during a "
+    "cmake run.  Certain properties of targets are not fully determined "
+    "until later at generate-time, such as the link language and complete "
+    "list of link libraries.  Future refactoring will change the effect of "
+    "the export() command to be executed at generate-time.  Use ALIAS "
+    "targets instead in cases where the goal is to refer to targets by "
+    "another name"
+    "\n"
+    "The OLD behavior for this policy is to allow including the result "
+    "of an export() command.  "
+    "The NEW behavior for this policy is to not to allow including the "
+    "result of an export() command.",
+    2,8,13,0, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()