cmListCommand: make list(ACTION not_a_list) succeed when idempotent

The operations changed here all are no-ops on empty lists anyways, so
just have them succeed when given non-extant lists.

  - `list(REMOVE_ITEM)`
  - `list(REMOVE_DUPLICATES)`
  - `list(SORT)`
  - `list(FILTER)`
  - `list(REVERSE)`

17 files changed, 31 insertions, 36 deletions

diff --git a/Help/release/dev/better-empty-list-behavior.rst b/Help/release/dev/better-empty-list-behavior.rst
new file mode 100644
index 0000000..12a131f
--- /dev/null
+++ b/Help/release/dev/better-empty-list-behavior.rst
@@ -0,0 +1,6 @@
+better-empty-list-behavior
+--------------------------
+
+* The :command:`list` operations ``REMOVE_ITEM``, ``REMOVE_DUPLICATES``,
+  ``SORT``, ``REVERSE``, and ``FILTER`` all now accept a non-existent variable
+  as the list since these operations on empty lists is also the empty list.
diff --git a/Source/cmListCommand.cxx b/Source/cmListCommand.cxx
index b9c7ada..b46eb6d 100644
--- a/Source/cmListCommand.cxx
+++ b/Source/cmListCommand.cxx
@@ -346,8 +346,7 @@ bool cmListCommand::HandleRemoveItemCommand(
   // expand the variable
   std::vector<std::string> varArgsExpanded;
   if (!this->GetList(varArgsExpanded, listName)) {
-    this->SetError("sub-command REMOVE_ITEM requires list to be present.");
-    return false;
+    return true;
   }
 
   std::vector<std::string> remove(args.begin() + 2, args.end());
@@ -376,8 +375,7 @@ bool cmListCommand::HandleReverseCommand(std::vector<std::string> const& args)
   // expand the variable
   std::vector<std::string> varArgsExpanded;
   if (!this->GetList(varArgsExpanded, listName)) {
-    this->SetError("sub-command REVERSE requires list to be present.");
-    return false;
+    return true;
   }
 
   std::string value = cmJoin(cmReverseRange(varArgsExpanded), ";");
@@ -399,9 +397,7 @@ bool cmListCommand::HandleRemoveDuplicatesCommand(
   // expand the variable
   std::vector<std::string> varArgsExpanded;
   if (!this->GetList(varArgsExpanded, listName)) {
-    this->SetError(
-      "sub-command REMOVE_DUPLICATES requires list to be present.");
-    return false;
+    return true;
   }
 
   std::vector<std::string>::const_iterator argsEnd =
@@ -1152,8 +1148,7 @@ bool cmListCommand::HandleSortCommand(std::vector<std::string> const& args)
   // expand the variable
   std::vector<std::string> varArgsExpanded;
   if (!this->GetList(varArgsExpanded, listName)) {
-    this->SetError("sub-command SORT requires list to be present.");
-    return false;
+    return true;
   }
 
   if ((sortCompare == cmStringSorter::Compare::STRING) &&
@@ -1304,8 +1299,7 @@ bool cmListCommand::HandleFilterCommand(std::vector<std::string> const& args)
   // expand the variable
   std::vector<std::string> varArgsExpanded;
   if (!this->GetList(varArgsExpanded, listName)) {
-    this->SetError("sub-command FILTER requires list to be present.");
-    return false;
+    return true;
   }
 
   const std::string& mode = args[3];
diff --git a/Tests/RunCMake/list/FILTER-NotList-result.txt b/Tests/RunCMake/list/FILTER-NotList-result.txt
deleted file mode 100644
index d00491f..0000000
--- a/Tests/RunCMake/list/FILTER-NotList-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/list/FILTER-NotList-stderr.txt b/Tests/RunCMake/list/FILTER-NotList-stderr.txt
deleted file mode 100644
index 159c28d..0000000
--- a/Tests/RunCMake/list/FILTER-NotList-stderr.txt
+++ /dev/null
@@ -1,4 +0,0 @@
-^CMake Error at FILTER-NotList.cmake:2 \(list\):
-  list sub-command FILTER requires list to be present.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)$
diff --git a/Tests/RunCMake/list/FILTER-NotList.cmake b/Tests/RunCMake/list/FILTER-NotList.cmake
index 1e15635..bf09ec7 100644
--- a/Tests/RunCMake/list/FILTER-NotList.cmake
+++ b/Tests/RunCMake/list/FILTER-NotList.cmake
@@ -1,2 +1,6 @@
 unset(nosuchlist)
 list(FILTER nosuchlist EXCLUDE REGEX "^FILTER_THIS_.+")
+if (DEFINED nosuchlist)
+  message(FATAL_ERROR
+    "list(FILTER) created our list")
+endif ()
diff --git a/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList-result.txt b/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList-result.txt
deleted file mode 100644
index d00491f..0000000
--- a/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList-stderr.txt b/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList-stderr.txt
deleted file mode 100644
index 96f3446..0000000
--- a/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList-stderr.txt
+++ /dev/null
@@ -1,4 +0,0 @@
-^CMake Error at REMOVE_DUPLICATES-NotList.cmake:2 \(list\):
-  list sub-command REMOVE_DUPLICATES requires list to be present.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)$
diff --git a/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList.cmake b/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList.cmake
index 218f227..b9f3999 100644
--- a/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList.cmake
+++ b/Tests/RunCMake/list/REMOVE_DUPLICATES-NotList.cmake
@@ -1,2 +1,6 @@
 unset(nosuchlist)
 list(REMOVE_DUPLICATES nosuchlist)
+if (DEFINED nosuchlist)
+  message(FATAL_ERROR
+    "list(REMOVE_DUPLICATES) created our list")
+endif ()
diff --git a/Tests/RunCMake/list/REMOVE_ITEM-NotList-result.txt b/Tests/RunCMake/list/REMOVE_ITEM-NotList-result.txt
deleted file mode 100644
index d00491f..0000000
--- a/Tests/RunCMake/list/REMOVE_ITEM-NotList-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/list/REMOVE_ITEM-NotList-stderr.txt b/Tests/RunCMake/list/REMOVE_ITEM-NotList-stderr.txt
deleted file mode 100644
index c32a4c0..0000000
--- a/Tests/RunCMake/list/REMOVE_ITEM-NotList-stderr.txt
+++ /dev/null
@@ -1,4 +0,0 @@
-^CMake Error at REMOVE_ITEM-NotList.cmake:2 \(list\):
-  list sub-command REMOVE_ITEM requires list to be present.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)$
diff --git a/Tests/RunCMake/list/REMOVE_ITEM-NotList.cmake b/Tests/RunCMake/list/REMOVE_ITEM-NotList.cmake
index 079e7fb..0c66837 100644
--- a/Tests/RunCMake/list/REMOVE_ITEM-NotList.cmake
+++ b/Tests/RunCMake/list/REMOVE_ITEM-NotList.cmake
@@ -1,2 +1,6 @@
 unset(nosuchlist)
 list(REMOVE_ITEM nosuchlist alpha)
+if (DEFINED nosuchlist)
+  message(FATAL_ERROR
+    "list(REMOVE_ITEM) created our list")
+endif ()
diff --git a/Tests/RunCMake/list/REVERSE-NotList-result.txt b/Tests/RunCMake/list/REVERSE-NotList-result.txt
deleted file mode 100644
index d00491f..0000000
--- a/Tests/RunCMake/list/REVERSE-NotList-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/list/REVERSE-NotList-stderr.txt b/Tests/RunCMake/list/REVERSE-NotList-stderr.txt
deleted file mode 100644
index e9dcc06..0000000
--- a/Tests/RunCMake/list/REVERSE-NotList-stderr.txt
+++ /dev/null
@@ -1,4 +0,0 @@
-^CMake Error at REVERSE-NotList.cmake:2 \(list\):
-  list sub-command REVERSE requires list to be present.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)$
diff --git a/Tests/RunCMake/list/REVERSE-NotList.cmake b/Tests/RunCMake/list/REVERSE-NotList.cmake
index 977e2cc..7138329 100644
--- a/Tests/RunCMake/list/REVERSE-NotList.cmake
+++ b/Tests/RunCMake/list/REVERSE-NotList.cmake
@@ -1,2 +1,6 @@
 unset(nosuchlist)
 list(REVERSE nosuchlist)
+if (DEFINED nosuchlist)
+  message(FATAL_ERROR
+    "list(REVERSE) created our list")
+endif ()
diff --git a/Tests/RunCMake/list/SORT-NotList-result.txt b/Tests/RunCMake/list/SORT-NotList-result.txt
deleted file mode 100644
index d00491f..0000000
--- a/Tests/RunCMake/list/SORT-NotList-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/list/SORT-NotList-stderr.txt b/Tests/RunCMake/list/SORT-NotList-stderr.txt
deleted file mode 100644
index 396c5b5..0000000
--- a/Tests/RunCMake/list/SORT-NotList-stderr.txt
+++ /dev/null
@@ -1,4 +0,0 @@
-^CMake Error at SORT-NotList.cmake:2 \(list\):
-  list sub-command SORT requires list to be present.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)$
diff --git a/Tests/RunCMake/list/SORT-NotList.cmake b/Tests/RunCMake/list/SORT-NotList.cmake
index 8f48e10..6314f14 100644
--- a/Tests/RunCMake/list/SORT-NotList.cmake
+++ b/Tests/RunCMake/list/SORT-NotList.cmake
@@ -1,2 +1,6 @@
 unset(nosuchlist)
 list(SORT nosuchlist)
+if (DEFINED nosuchlist)
+  message(FATAL_ERROR
+    "list(SORT) created our list")
+endif ()