install,export: Maybe transform OBJECT libraries to INTERFACE libraries

Teach the `install` and `export` commands to support installing and
exporting `OBJECT` libraries without their object files.  Transform
them to `INTERFACE` libraries in such cases.

For `install(TARGETS)`, activate this when no destination for the object
files is specified.  For `export`, activate this only under Xcode with
multiple architectures when we have no well-defined object file
locations to give to clients.

Source/cmExportBuildAndroidMKGenerator.cxx

@@ -40,7 +40,8 @@ void cmExportBuildAndroidMKGenerator::GenerateExpectedTargetsCode(
 }
 
 void cmExportBuildAndroidMKGenerator::GenerateImportTargetCode(
-  std::ostream& os, const cmGeneratorTarget* target)
+  std::ostream& os, cmGeneratorTarget const* target,
+  cmStateEnums::TargetType /*targetType*/)
 {
   std::string targetName = this->Namespace;
   targetName += target->GetExportName();

Source/cmExportBuildAndroidMKGenerator.h

@@ -11,6 +11,7 @@
 
 #include "cmExportBuildFileGenerator.h"
 #include "cmExportFileGenerator.h"
+#include "cmStateTypes.h"
 
 class cmGeneratorTarget;
 
@@ -47,8 +48,9 @@ protected:
   void GenerateImportHeaderCode(std::ostream& os,
                                 const std::string& config = "") override;
   void GenerateImportFooterCode(std::ostream& os) override;
-  void GenerateImportTargetCode(std::ostream& os,
-                                const cmGeneratorTarget* target) override;
+  void GenerateImportTargetCode(
+    std::ostream& os, cmGeneratorTarget const* target,
+    cmStateEnums::TargetType /*targetType*/) override;
   void GenerateExpectedTargetsCode(
     std::ostream& os, const std::string& expectedTargets) override;
   void GenerateImportPropertyCode(

Source/cmExportBuildFileGenerator.cxx

@@ -59,7 +59,7 @@ bool cmExportBuildFileGenerator::GenerateMainFile(std::ostream& os)
           this->LG->GetMakefile()->GetBacktrace());
         return false;
       }
-      if (te->GetType() == cmStateEnums::INTERFACE_LIBRARY) {
+      if (this->GetExportTargetType(te) == cmStateEnums::INTERFACE_LIBRARY) {
         this->GenerateRequiredCMakeVersion(os, "3.0.0");
       }
     }
@@ -71,7 +71,7 @@ bool cmExportBuildFileGenerator::GenerateMainFile(std::ostream& os)
 
   // Create all the imported targets.
   for (cmGeneratorTarget* gte : this->Exports) {
-    this->GenerateImportTargetCode(os, gte);
+    this->GenerateImportTargetCode(os, gte, this->GetExportTargetType(gte));
 
     gte->Target->AppendBuildInterfaceIncludes();
 
@@ -128,12 +128,13 @@ void cmExportBuildFileGenerator::GenerateImportTargetsConfig(
     // Collect import properties for this target.
     ImportPropertyMap properties;
 
-    if (target->GetType() != cmStateEnums::INTERFACE_LIBRARY) {
+    if (this->GetExportTargetType(target) != cmStateEnums::INTERFACE_LIBRARY) {
       this->SetImportLocationProperty(config, suffix, target, properties);
     }
     if (!properties.empty()) {
       // Get the rest of the target details.
-      if (target->GetType() != cmStateEnums::INTERFACE_LIBRARY) {
+      if (this->GetExportTargetType(target) !=
+          cmStateEnums::INTERFACE_LIBRARY) {
         this->SetImportDetailProperties(config, suffix, target, properties,
                                         missingTargets);
         this->SetImportLinkInterface(config, suffix,
@@ -153,6 +154,19 @@ void cmExportBuildFileGenerator::GenerateImportTargetsConfig(
   }
 }
 
+cmStateEnums::TargetType cmExportBuildFileGenerator::GetExportTargetType(
+  cmGeneratorTarget const* target) const
+{
+  cmStateEnums::TargetType targetType = target->GetType();
+  // An object library exports as an interface library if we cannot
+  // tell clients where to find the objects.
+  if (targetType == cmStateEnums::OBJECT_LIBRARY &&
+      !this->LG->GetGlobalGenerator()->HasKnownObjectFileLocation(nullptr)) {
+    targetType = cmStateEnums::INTERFACE_LIBRARY;
+  }
+  return targetType;
+}
+
 void cmExportBuildFileGenerator::SetExportSet(cmExportSet* exportSet)
 {
   this->ExportSet = exportSet;

Source/cmExportBuildFileGenerator.h

@@ -6,6 +6,7 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include "cmExportFileGenerator.h"
+#include "cmStateTypes.h"
 
 #include <iosfwd>
 #include <string>
@@ -53,6 +54,8 @@ protected:
   void GenerateImportTargetsConfig(
     std::ostream& os, const std::string& config, std::string const& suffix,
     std::vector<std::string>& missingTargets) override;
+  cmStateEnums::TargetType GetExportTargetType(
+    cmGeneratorTarget const* target) const;
   void HandleMissingTarget(std::string& link_libs,
                            std::vector<std::string>& missingTargets,
                            cmGeneratorTarget* depender,

Source/cmExportCommand.cxx

@@ -146,17 +146,6 @@ bool cmExportCommand::InitialPass(std::vector<std::string> const& args,
       }
 
       if (cmTarget* target = gg->FindTarget(currentTarget)) {
-        if (target->GetType() == cmStateEnums::OBJECT_LIBRARY) {
-          std::string reason;
-          if (!this->Makefile->GetGlobalGenerator()
-                 ->HasKnownObjectFileLocation(&reason)) {
-            std::ostringstream e;
-            e << "given OBJECT library \"" << currentTarget
-              << "\" which may not be exported" << reason << ".";
-            this->SetError(e.str());
-            return false;
-          }
-        }
         if (target->GetType() == cmStateEnums::UTILITY) {
           this->SetError("given custom target \"" + currentTarget +
                          "\" which may not be exported.");

Source/cmExportFileGenerator.cxx

@@ -901,8 +901,10 @@ void cmExportFileGenerator::GenerateExpectedTargetsCode(
         "\n\n";
   /* clang-format on */
 }
+
 void cmExportFileGenerator::GenerateImportTargetCode(
-  std::ostream& os, const cmGeneratorTarget* target)
+  std::ostream& os, cmGeneratorTarget const* target,
+  cmStateEnums::TargetType targetType)
 {
   // Construct the imported target name.
   std::string targetName = this->Namespace;
@@ -911,7 +913,7 @@ void cmExportFileGenerator::GenerateImportTargetCode(
 
   // Create the imported target.
   os << "# Create imported target " << targetName << "\n";
-  switch (target->GetType()) {
+  switch (targetType) {
     case cmStateEnums::EXECUTABLE:
       os << "add_executable(" << targetName << " IMPORTED)\n";
       break;

Source/cmExportFileGenerator.h

@@ -6,6 +6,7 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include "cmGeneratorExpression.h"
+#include "cmStateTypes.h"
 #include "cmVersion.h"
 #include "cmVersionConfig.h"
 
@@ -76,7 +77,8 @@ protected:
   virtual void GenerateImportFooterCode(std::ostream& os);
   void GenerateImportVersionCode(std::ostream& os);
   virtual void GenerateImportTargetCode(std::ostream& os,
-                                        cmGeneratorTarget const* target);
+                                        cmGeneratorTarget const* target,
+                                        cmStateEnums::TargetType targetType);
   virtual void GenerateImportPropertyCode(std::ostream& os,
                                           const std::string& config,
                                           cmGeneratorTarget const* target,

Source/cmExportInstallAndroidMKGenerator.cxx

@@ -55,7 +55,8 @@ void cmExportInstallAndroidMKGenerator::GenerateImportFooterCode(std::ostream&)
 }
 
 void cmExportInstallAndroidMKGenerator::GenerateImportTargetCode(
-  std::ostream& os, const cmGeneratorTarget* target)
+  std::ostream& os, cmGeneratorTarget const* target,
+  cmStateEnums::TargetType /*targetType*/)
 {
   std::string targetName = this->Namespace;
   targetName += target->GetExportName();

Source/cmExportInstallAndroidMKGenerator.h

@@ -12,6 +12,7 @@
 
 #include "cmExportFileGenerator.h"
 #include "cmExportInstallFileGenerator.h"
+#include "cmStateTypes.h"
 
 class cmGeneratorTarget;
 class cmInstallExportGenerator;
@@ -41,8 +42,9 @@ protected:
   void GenerateImportHeaderCode(std::ostream& os,
                                 const std::string& config = "") override;
   void GenerateImportFooterCode(std::ostream& os) override;
-  void GenerateImportTargetCode(std::ostream& os,
-                                const cmGeneratorTarget* target) override;
+  void GenerateImportTargetCode(
+    std::ostream& os, cmGeneratorTarget const* target,
+    cmStateEnums::TargetType /*targetType*/) override;
   void GenerateExpectedTargetsCode(
     std::ostream& os, const std::string& expectedTargets) override;
   void GenerateImportPropertyCode(

Source/cmExportInstallFileGenerator.cxx

@@ -75,11 +75,12 @@ bool cmExportInstallFileGenerator::GenerateMainFile(std::ostream& os)
   // Create all the imported targets.
   for (cmTargetExport* te : allTargets) {
     cmGeneratorTarget* gt = te->Target;
+    cmStateEnums::TargetType targetType = this->GetExportTargetType(te);
 
     requiresConfigFiles =
-      requiresConfigFiles || gt->GetType() != cmStateEnums::INTERFACE_LIBRARY;
+      requiresConfigFiles || targetType != cmStateEnums::INTERFACE_LIBRARY;
 
-    this->GenerateImportTargetCode(os, gt);
+    this->GenerateImportTargetCode(os, gt, targetType);
 
     ImportPropertyMap properties;
 
@@ -114,7 +115,7 @@ bool cmExportInstallFileGenerator::GenerateMainFile(std::ostream& os)
         require2_8_12 = true;
       }
     }
-    if (gt->GetType() == cmStateEnums::INTERFACE_LIBRARY) {
+    if (targetType == cmStateEnums::INTERFACE_LIBRARY) {
       require3_0_0 = true;
     }
     if (gt->GetProperty("INTERFACE_SOURCES")) {
@@ -308,7 +309,7 @@ void cmExportInstallFileGenerator::GenerateImportTargetsConfig(
   // Add each target in the set to the export.
   for (cmTargetExport* te : *this->IEGen->GetExportSet()->GetTargetExports()) {
     // Collect import properties for this target.
-    if (te->Target->GetType() == cmStateEnums::INTERFACE_LIBRARY) {
+    if (this->GetExportTargetType(te) == cmStateEnums::INTERFACE_LIBRARY) {
       continue;
     }
 
@@ -426,6 +427,19 @@ void cmExportInstallFileGenerator::SetImportLocationProperty(
   }
 }
 
+cmStateEnums::TargetType cmExportInstallFileGenerator::GetExportTargetType(
+  cmTargetExport const* targetExport) const
+{
+  cmStateEnums::TargetType targetType = targetExport->Target->GetType();
+  // An OBJECT library installed with no OBJECTS DESTINATION
+  // is transformed to an INTERFACE library.
+  if (targetType == cmStateEnums::OBJECT_LIBRARY &&
+      targetExport->ObjectsGenerator == nullptr) {
+    targetType = cmStateEnums::INTERFACE_LIBRARY;
+  }
+  return targetType;
+}
+
 void cmExportInstallFileGenerator::HandleMissingTarget(
   std::string& link_libs, std::vector<std::string>& missingTargets,
   cmGeneratorTarget* depender, cmGeneratorTarget* dependee)

Source/cmExportInstallFileGenerator.h

@@ -6,6 +6,7 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include "cmExportFileGenerator.h"
+#include "cmStateTypes.h"
 
 #include <iosfwd>
 #include <map>
@@ -17,6 +18,7 @@ class cmGeneratorTarget;
 class cmGlobalGenerator;
 class cmInstallExportGenerator;
 class cmInstallTargetGenerator;
+class cmTargetExport;
 
 /** \class cmExportInstallFileGenerator
  * \brief Generate a file exporting targets from an install tree.
@@ -57,6 +59,8 @@ protected:
   void GenerateImportTargetsConfig(
     std::ostream& os, const std::string& config, std::string const& suffix,
     std::vector<std::string>& missingTargets) override;
+  cmStateEnums::TargetType GetExportTargetType(
+    cmTargetExport const* targetExport) const;
   void HandleMissingTarget(std::string& link_libs,
                            std::vector<std::string>& missingTargets,
                            cmGeneratorTarget* depender,

Source/cmExportTryCompileFileGenerator.cxx

@@ -32,7 +32,7 @@ bool cmExportTryCompileFileGenerator::GenerateMainFile(std::ostream& os)
     this->Exports.pop_back();
     if (emitted.insert(te).second) {
       emittedDeps.insert(te);
-      this->GenerateImportTargetCode(os, te);
+      this->GenerateImportTargetCode(os, te, te->GetType());
 
       ImportPropertyMap properties;
 

Source/cmInstallCommand.cxx

@@ -360,17 +360,6 @@ bool cmInstallCommand::HandleTargetsMode(std::vector<std::string> const& args)
         this->SetError(e.str());
         return false;
       }
-      if (target->GetType() == cmStateEnums::OBJECT_LIBRARY) {
-        std::string reason;
-        if (!this->Makefile->GetGlobalGenerator()->HasKnownObjectFileLocation(
-              &reason)) {
-          std::ostringstream e;
-          e << "TARGETS given OBJECT library \"" << tgt
-            << "\" which may not be installed" << reason << ".";
-          this->SetError(e.str());
-          return false;
-        }
-      }
       // Store the target in the list to be installed.
       targets.push_back(target);
     } else {
@@ -534,15 +523,23 @@ bool cmInstallCommand::HandleTargetsMode(std::vector<std::string> const& args)
       case cmStateEnums::OBJECT_LIBRARY: {
         // Objects use OBJECT properties.
         if (!objectArgs.GetDestination().empty()) {
+          // Verify that we know where the objects are to install them.
+          std::string reason;
+          if (!this->Makefile->GetGlobalGenerator()
+                 ->HasKnownObjectFileLocation(&reason)) {
+            std::ostringstream e;
+            e << "TARGETS given OBJECT library \"" << target.GetName()
+              << "\" whose objects may not be installed" << reason << ".";
+            this->SetError(e.str());
+            return false;
+          }
+
           objectGenerator =
             CreateInstallTargetGenerator(target, objectArgs, false);
         } else {
-          std::ostringstream e;
-          e << "TARGETS given no OBJECTS DESTINATION for object library "
-               "target \""
-            << target.GetName() << "\".";
-          this->SetError(e.str());
-          return false;
+          // Installing an OBJECT library without a destination transforms
+          // it to an INTERFACE library.  It installs no files but can be
+          // exported.
         }
       } break;
       case cmStateEnums::EXECUTABLE: {