set: warn of extra arguments after ENV value.

Fixes: #18842

Help/command/set.rst

@@ -86,7 +86,7 @@ Set Environment Variable
 
 .. code-block:: cmake
 
-  set(ENV{<variable>} <value>...)
+  set(ENV{<variable>} [<value>])
 
 Sets an :manual:`Environment Variable <cmake-env-variables(7)>`
 to the given value.
@@ -95,3 +95,10 @@ Subsequent calls of ``$ENV{<variable>}`` will return this new value.
 This command affects only the current CMake process, not the process
 from which CMake was called, nor the system environment at large,
 nor the environment of subsequent build or test processes.
+
+If no argument is given after ``ENV{<variable>}`` or if ``<value>`` is
+an empty string, then this command will clear any existing value of the
+environment variable.
+
+Arguments after ``<value>`` are ignored. If extra arguments are found,
+then an author warning is issued.

Source/cmSetCommand.cxx

@@ -38,6 +38,14 @@ bool cmSetCommand::InitialPass(std::vector<std::string> const& args,
         putEnvArg += args[1];
         cmSystemTools::PutEnv(putEnvArg);
       }
+      // if there's extra arguments, warn user
+      // that they are ignored by this command.
+      if (args.size() > 2) {
+        std::string m = "Only the first value argument is used when setting "
+                        "an environment variable.  Argument '" +
+          args[2] + "' and later are unused.";
+        this->Makefile->IssueMessage(MessageType::AUTHOR_WARNING, m);
+      }
       return true;
     }
 

Tests/RunCMake/set/ExtraEnvValue-stderr.txt

@@ -0,0 +1,6 @@
+CMake Warning \(dev\) at ExtraEnvValue.cmake:1 \(set\):
+  Only the first value argument is used when setting an environment variable.
+  Argument 'value_2' and later are unused.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/set/ExtraEnvValue.cmake

@@ -0,0 +1 @@
+set (ENV{sample_key} value_1 value_2)

Tests/RunCMake/set/RunCMakeTest.cmake

@@ -4,3 +4,4 @@ run_cmake(ParentScope)
 run_cmake(ParentPulling)
 run_cmake(ParentPullingRecursive)
 run_cmake(UnknownCacheType)
+run_cmake(ExtraEnvValue)