ENH: Create CMP0013 to disallow duplicate dirs

In CMake 2.6.3 and below we silently accepted duplicate build
directories whose build files would then conflict.  At first this was
considured purely a bug that confused beginners but would not be used in
a real project.  In CMake 2.6.4 we explicitly made it an error.

However, some real projects took advantage of this as a "feature" and
got lucky that the subtle build errors it can cause did not occur.
Therefore we need a policy to deal with the case more gracefully.
See issue #9173.

1 files changed, 17 insertions, 0 deletions

diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index 5e80e2f..1f8bb89 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -369,6 +369,23 @@ cmPolicies::cmPolicies()
     "The NEW behavior for this policy is to treat strings like true as a "
     "boolean constant.",
     2,6,5, cmPolicies::WARN);
+
+    this->DefinePolicy(
+    CMP0013, "CMP0013",
+    "Duplicate binary directories are not allowed.",
+    "CMake 2.6.3 and below silently permitted add_subdirectory() calls "
+    "to create the same binary directory multiple times.  "
+    "During build system generation files would be written and then "
+    "overwritten in the build tree and could lead to strange behavior.  "
+    "CMake 2.6.4 and above explicitly detect duplicate binary directories.  "
+    "CMake 2.6.4 always considers this case an error.  "
+    "In CMake 2.6.5 and above this policy determines whether or not "
+    "the case is an error.  "
+    "The OLD behavior for this policy is to allow duplicate binary "
+    "directories.  "
+    "The NEW behavior for this policy is to disallow duplicate binary "
+    "directories with an error.",
+    2,6,5, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()