genex: Add TARGET_NAME_IF_EXISTS expression

Define `$<TARGET_NAME_IF_EXISTS:tgt>` to mean `tgt` if the target
exists and otherwise an empty string.

Help/manual/cmake-generator-expressions.7.rst

@@ -274,6 +274,9 @@ Available output expressions are:
   Marks ``...`` as being the name of a target.  This is required if exporting
   targets to multiple dependent export sets.  The ``...`` must be a literal
   name of a target- it may not contain generator expressions.
+``$<TARGET_NAME_IF_EXISTS:...>``
+  Expands to the ``...`` if the given target exists, an empty string
+  otherwise.
 ``$<LINK_ONLY:...>``
   Content of ``...`` except when evaluated in a link interface while
   propagating :ref:`Target Usage Requirements`, in which case it is the

Help/release/dev/genex-TARGET_NAME_IF_EXISTS.rst

@@ -0,0 +1,6 @@
+genex-TARGET_NAME_IF_EXISTS
+---------------------------
+
+* A new ``$<TARGET_NAME_IF_EXISTS:...>``
+  :manual:`generator expression <cmake-generator-expressions(7)>`
+  has been added.

Source/cmGeneratorExpressionNode.cxx

@@ -331,6 +331,40 @@ static const struct TargetExistsNode : public cmGeneratorExpressionNode
   }
 } targetExistsNode;
 
+static const struct TargetNameIfExistsNode : public cmGeneratorExpressionNode
+{
+  TargetNameIfExistsNode() {}
+
+  int NumExpectedParameters() const override { return 1; }
+
+  std::string Evaluate(
+    const std::vector<std::string>& parameters,
+    cmGeneratorExpressionContext* context,
+    const GeneratorExpressionContent* content,
+    cmGeneratorExpressionDAGChecker* /*dagChecker*/) const override
+  {
+    if (parameters.size() != 1) {
+      reportError(context, content->GetOriginalExpression(),
+                  "$<TARGET_NAME_IF_EXISTS:...> expression requires one "
+                  "parameter");
+      return std::string();
+    }
+
+    std::string targetName = parameters.front();
+    if (targetName.empty() ||
+        !cmGeneratorExpression::IsValidTargetName(targetName)) {
+      reportError(context, content->GetOriginalExpression(),
+                  "$<TARGET_NAME_IF_EXISTS:tgt> expression requires a "
+                  "non-empty valid target name.");
+      return std::string();
+    }
+
+    return context->LG->GetMakefile()->FindTargetToUse(targetName)
+      ? targetName
+      : std::string();
+  }
+} targetNameIfExistsNode;
+
 static const struct LowerCaseNode : public cmGeneratorExpressionNode
 {
   LowerCaseNode() {}
@@ -1897,6 +1931,7 @@ const cmGeneratorExpressionNode* cmGeneratorExpressionNode::GetNode(
     nodeMap["TARGET_OBJECTS"] = &targetObjectsNode;
     nodeMap["TARGET_POLICY"] = &targetPolicyNode;
     nodeMap["TARGET_EXISTS"] = &targetExistsNode;
+    nodeMap["TARGET_NAME_IF_EXISTS"] = &targetNameIfExistsNode;
     nodeMap["BUILD_INTERFACE"] = &buildInterfaceNode;
     nodeMap["INSTALL_INTERFACE"] = &installInterfaceNode;
     nodeMap["INSTALL_PREFIX"] = &installPrefixNode;

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

@@ -38,6 +38,10 @@ run_cmake(TARGET_EXISTS-no-arg)
 run_cmake(TARGET_EXISTS-empty-arg)
 run_cmake(TARGET_EXISTS)
 run_cmake(TARGET_EXISTS-not-a-target)
+run_cmake(TARGET_NAME_IF_EXISTS-no-arg)
+run_cmake(TARGET_NAME_IF_EXISTS-empty-arg)
+run_cmake(TARGET_NAME_IF_EXISTS)
+run_cmake(TARGET_NAME_IF_EXISTS-not-a-target)
 
 run_cmake(ImportedTarget-TARGET_BUNDLE_DIR)
 run_cmake(ImportedTarget-TARGET_BUNDLE_CONTENT_DIR)

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-check.cmake

@@ -0,0 +1,6 @@
+file(READ "${RunCMake_TEST_BINARY_DIR}/TARGET_NAME_IF_EXISTS-generated.txt" content)
+
+set(expected "foo")
+if(NOT content STREQUAL expected)
+  set(RunCMake_TEST_FAILED "actual content:\n [[${content}]]\nbut expected:\n [[${expected}]]")
+endif()

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-empty-arg-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-empty-arg-stderr.txt

@@ -0,0 +1,9 @@
+CMake Error at TARGET_NAME_IF_EXISTS-empty-arg.cmake:2 \(file\):
+  Error evaluating generator expression:
+
+    \$<TARGET_NAME_IF_EXISTS:>
+
+  \$<TARGET_NAME_IF_EXISTS:tgt> expression requires a non-empty valid target
+  name.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-empty-arg.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0070 NEW)
+file(GENERATE OUTPUT TARGET_NAME_IF_EXISTS-generated.txt CONTENT "$<TARGET_NAME_IF_EXISTS:${empty}>")

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-no-arg-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-no-arg-stderr.txt

@@ -0,0 +1,8 @@
+CMake Error at TARGET_NAME_IF_EXISTS-no-arg.cmake:2 \(file\):
+  Error evaluating generator expression:
+
+    \$<TARGET_NAME_IF_EXISTS>
+
+  \$<TARGET_NAME_IF_EXISTS> expression requires exactly one parameter.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-no-arg.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0070 NEW)
+file(GENERATE OUTPUT TARGET_NAME_IF_EXISTS-generated.txt CONTENT "$<TARGET_NAME_IF_EXISTS>")

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-not-a-target-check.cmake

@@ -0,0 +1,5 @@
+file(READ "${RunCMake_TEST_BINARY_DIR}/TARGET_NAME_IF_EXISTS-not-a-target-generated.txt" content)
+
+if(content)
+  set(RunCMake_TEST_FAILED "actual content:\n [[${content}]]\nbut expected an empty string")
+endif()

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS-not-a-target.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0070 NEW)
+file(GENERATE OUTPUT TARGET_NAME_IF_EXISTS-not-a-target-generated.txt CONTENT "$<TARGET_NAME_IF_EXISTS:just-random-string>")

Tests/RunCMake/GeneratorExpression/TARGET_NAME_IF_EXISTS.cmake

@@ -0,0 +1,3 @@
+cmake_policy(SET CMP0070 NEW)
+add_custom_target(foo)
+file(GENERATE OUTPUT TARGET_NAME_IF_EXISTS-generated.txt CONTENT "$<TARGET_NAME_IF_EXISTS:foo>")