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.

Source/cmMakefile.cxx

@@ -1545,17 +1545,8 @@ void cmMakefile::AddSubDirectory(const char* srcPath, const char *binPath,
     }
 
   // Make sure the binary directory is unique.
-  cmGlobalGenerator* gg = this->LocalGenerator->GetGlobalGenerator();
-  if(!gg->BinaryDirectoryIsNew(binPath))
+  if(!this->EnforceUniqueDir(srcPath, binPath))
     {
-    cmOStringStream e;
-    e << "The binary directory\n"
-      << "  " << binPath << "\n"
-      << "is already used to build another source directory, so it cannot "
-      << "be used to build source directory\n"
-      << "  " << srcPath << "\n"
-      << "Specify a unique binary directory name.";
-    this->IssueMessage(cmake::FATAL_ERROR, e.str());
     return;
     }
 
@@ -3719,6 +3710,54 @@ bool cmMakefile::EnforceUniqueName(std::string const& name, std::string& msg,
   return true;
 }
 
+//----------------------------------------------------------------------------
+bool cmMakefile::EnforceUniqueDir(const char* srcPath, const char* binPath)
+{
+  // Make sure the binary directory is unique.
+  cmGlobalGenerator* gg = this->LocalGenerator->GetGlobalGenerator();
+  if(gg->BinaryDirectoryIsNew(binPath))
+    {
+    return true;
+    }
+  cmOStringStream e;
+  switch (this->GetPolicyStatus(cmPolicies::CMP0013))
+    {
+    case cmPolicies::WARN:
+      // Print the warning.
+      e << this->GetPolicies()->GetPolicyWarning(cmPolicies::CMP0013)
+        << "\n"
+        << "The binary directory\n"
+        << "  " << binPath << "\n"
+        << "is already used to build a source directory.  "
+        << "This command uses it to build source directory\n"
+        << "  " << srcPath << "\n"
+        << "which can generate conflicting build files.  "
+        << "CMake does not support this use case but it used "
+        << "to work accidentally and is being allowed for "
+        << "compatibility.";
+      this->IssueMessage(cmake::AUTHOR_WARNING, e.str());
+    case cmPolicies::OLD:
+      // OLD behavior does not warn.
+      return true;
+    case cmPolicies::REQUIRED_IF_USED:
+    case cmPolicies::REQUIRED_ALWAYS:
+      e << this->GetPolicies()->GetRequiredPolicyError(cmPolicies::CMP0013)
+        << "\n";
+    case cmPolicies::NEW:
+      // NEW behavior prints the error.
+      e << "The binary directory\n"
+        << "  " << binPath << "\n"
+        << "is already used to build a source directory.  "
+        << "It cannot be used to build source directory\n"
+        << "  " << srcPath << "\n"
+        << "Specify a unique binary directory name.";
+      this->IssueMessage(cmake::FATAL_ERROR, e.str());
+      break;
+    }
+
+  return false;
+}
+
 //----------------------------------------------------------------------------
 cmPolicies::PolicyStatus
 cmMakefile::GetPolicyStatus(cmPolicies::PolicyID id)

Source/cmMakefile.h

@@ -900,6 +900,8 @@ private:
 
   bool ParseDefineFlag(std::string const& definition, bool remove);
 
+  bool EnforceUniqueDir(const char* srcPath, const char* binPath);
+
   void ReadSources(std::ifstream& fin, bool t);
   friend class cmMakeDepend;    // make depend needs direct access
                                 // to the Sources array 

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()

Source/cmPolicies.h

@@ -53,6 +53,7 @@ public:
     CMP0010, // Bad variable reference syntax is an error
     CMP0011, // Strong policy scope for include and find_package
     CMP0012, // Strong handling of boolean constants
+    CMP0013, // Duplicate binary directories not allowed
 
     // Always the last entry.  Useful mostly to avoid adding a comma
     // the last policy when adding a new one.