diff --git a/Source/cmListFileCache.cxx b/Source/cmListFileCache.cxx
index c499b6fb5..c02866cb5 100644
--- a/Source/cmListFileCache.cxx
+++ b/Source/cmListFileCache.cxx
@@ -343,6 +343,27 @@ void cmListFileParser::AddArgument(cmListFileLexer_Token* token,
 {
   cmListFileArgument a(token->text, delim, this->FileName, token->line);
   this->Function.Arguments.push_back(a);
+  if(delim == cmListFileArgument::Unquoted)
+    {
+    // Warn about a future behavior change.
+    const char* c = a.Value.c_str();
+    if(*c++ == '[')
+      {
+      while(*c == '=')
+        { ++c; }
+      if(*c == '[')
+        {
+        cmOStringStream m;
+        m << "Syntax Warning in cmake code at\n"
+          << "  " << this->FileName << ":" << token->line << ":"
+          << token->column << "\n"
+          << "A future version of CMake may treat unquoted argument:\n"
+          << "  " << a.Value << "\n"
+          << "as an opening long bracket.  Double-quote the argument.";
+        this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, m.str().c_str());
+        }
+      }
+    }
   if(this->Separation == SeparationOkay)
     {
     return;
diff --git a/Tests/RunCMake/Syntax/BracketWarn-stderr.txt b/Tests/RunCMake/Syntax/BracketWarn-stderr.txt
new file mode 100644
index 000000000..4a9cca641
--- /dev/null
+++ b/Tests/RunCMake/Syntax/BracketWarn-stderr.txt
@@ -0,0 +1,35 @@
+CMake Warning \(dev\) at CMakeLists.txt:3 \(include\):
+  Syntax Warning in cmake code at
+
+    .*/Tests/RunCMake/Syntax/BracketWarn.cmake:1:16
+
+  A future version of CMake may treat unquoted argument:
+
+    \[\[
+
+  as an opening long bracket.  Double-quote the argument.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Warning \(dev\) at CMakeLists.txt:3 \(include\):
+  Syntax Warning in cmake code at
+
+    .*/Tests/RunCMake/Syntax/BracketWarn.cmake:1:19
+
+  A future version of CMake may treat unquoted argument:
+
+    \[=\[
+
+  as an opening long bracket.  Double-quote the argument.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Warning \(dev\) at CMakeLists.txt:3 \(include\):
+  Syntax Warning in cmake code at
+
+    .*/Tests/RunCMake/Syntax/BracketWarn.cmake:1:27
+
+  A future version of CMake may treat unquoted argument:
+
+    \[==\[x
+
+  as an opening long bracket.  Double-quote the argument.
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/Syntax/BracketWarn-stdout.txt b/Tests/RunCMake/Syntax/BracketWarn-stdout.txt
new file mode 100644
index 000000000..01b2caaa4
--- /dev/null
+++ b/Tests/RunCMake/Syntax/BracketWarn-stdout.txt
@@ -0,0 +1 @@
+-- \[\[\[=\[\[=x\[==\[x
diff --git a/Tests/RunCMake/Syntax/BracketWarn.cmake b/Tests/RunCMake/Syntax/BracketWarn.cmake
new file mode 100644
index 000000000..8f33946f0
--- /dev/null
+++ b/Tests/RunCMake/Syntax/BracketWarn.cmake
@@ -0,0 +1 @@
+message(STATUS [[ [=[ [=x [==[x)
diff --git a/Tests/RunCMake/Syntax/RunCMakeTest.cmake b/Tests/RunCMake/Syntax/RunCMakeTest.cmake
index b1eeff94b..8975b256d 100644
--- a/Tests/RunCMake/Syntax/RunCMakeTest.cmake
+++ b/Tests/RunCMake/Syntax/RunCMakeTest.cmake
@@ -14,3 +14,4 @@ run_cmake(Unquoted1)
 run_cmake(UnterminatedCall1)
 run_cmake(UnterminatedCall2)
 run_cmake(UnterminatedString)
+run_cmake(BracketWarn)