Make it an error for INSTALL_PREFIX to be evaluated.

An empty string is not likely going to produce expected results
in any evaluation context. It is replaced by preprocessing
currently.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -621,14 +621,17 @@ static const struct InstallPrefixNode : public cmGeneratorExpressionNode
 {
   InstallPrefixNode() {}
 
-  virtual bool GeneratesContent() const { return false; }
+  virtual bool GeneratesContent() const { return true; }
   virtual int NumExpectedParameters() const { return 0; }
 
   std::string Evaluate(const std::vector<std::string> &,
-                       cmGeneratorExpressionContext *,
-                       const GeneratorExpressionContent *,
+                       cmGeneratorExpressionContext *context,
+                       const GeneratorExpressionContent *content,
                        cmGeneratorExpressionDAGChecker *) const
   {
+    reportError(context, content->GetOriginalExpression(),
+                "INSTALL_PREFIX is a marker for install(EXPORT) only.  It "
+                "should never be evaluated.");
     return std::string();
   }
 

Tests/GeneratorExpression/CMakeLists.txt

@@ -89,7 +89,6 @@ add_custom_target(check-part2 ALL
     -Dtest_install_interface=$<INSTALL_INTERFACE:install>
     -Dtest_target_name_1=$<TARGET_NAME:tgt,ok>
     -Dtest_target_name_2=$<TARGET_NAME:tgt:ok>
-    -Dtest_install_prefix=$<INSTALL_PREFIX>
     -P ${CMAKE_CURRENT_SOURCE_DIR}/check-part2.cmake
   COMMAND ${CMAKE_COMMAND} -E echo "check done (part 2 of 2)"
   VERBATIM

Tests/GeneratorExpression/check-part2.cmake

@@ -26,4 +26,3 @@ check(test_build_interface "build")
 check(test_install_interface "")
 check(test_target_name_1 "tgt,ok")
 check(test_target_name_2 "tgt:ok")
-check(test_install_prefix "")

Tests/RunCMake/GeneratorExpression/BadInstallPrefix-result.txt

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

Tests/RunCMake/GeneratorExpression/BadInstallPrefix-stderr.txt

@@ -0,0 +1,9 @@
+CMake Error at BadInstallPrefix.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<INSTALL_PREFIX>
+
+  INSTALL_PREFIX is a marker for install\(EXPORT\) only.  It should never be
+  evaluated.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)

Tests/RunCMake/GeneratorExpression/BadInstallPrefix.cmake

@@ -0,0 +1,3 @@
+add_custom_target(check ALL COMMAND check
+  $<INSTALL_PREFIX>/include
+  VERBATIM)

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

@@ -7,3 +7,4 @@ run_cmake(BadNOT)
 run_cmake(BadStrEqual)
 run_cmake(BadZero)
 run_cmake(BadTargetName)
+run_cmake(BadInstallPrefix)