file(COPY_FILE): Report if failure occurred on input or output path

When we know whether a failure was associated with the input or the
output path, include this information in the error message.

Source/cmSystemTools.cxx

@@ -1140,7 +1140,7 @@ cmSystemTools::CopyResult cmSystemTools::CopySingleFile(
     return CopyResult::Success;
   }
 
-  cmsys::Status status;
+  cmsys::SystemTools::CopyStatus status;
   status = cmsys::SystemTools::CloneFileContent(oldname, newname);
   if (!status) {
     // if cloning did not succeed, fall back to blockwise copy
@@ -1149,14 +1149,24 @@ cmSystemTools::CopyResult cmSystemTools::CopySingleFile(
   if (!status) {
     if (err) {
       *err = status.GetString();
+      switch (status.Path) {
+        case cmsys::SystemTools::CopyStatus::SourcePath:
+          *err = cmStrCat(*err, " (input)");
+          break;
+        case cmsys::SystemTools::CopyStatus::DestPath:
+          *err = cmStrCat(*err, " (output)");
+          break;
+        default:
+          break;
+      }
     }
     return CopyResult::Failure;
   }
   if (perms) {
-    status = SystemTools::SetPermissions(newname, perm);
-    if (!status) {
+    perms = SystemTools::SetPermissions(newname, perm);
+    if (!perms) {
       if (err) {
-        *err = status.GetString();
+        *err = cmStrCat(perms.GetString(), " (output)");
       }
       return CopyResult::Failure;
     }

Tests/RunCMake/file/COPY_FILE-input-missing-result.txt

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

Tests/RunCMake/file/COPY_FILE-input-missing-stderr.txt

@@ -0,0 +1,14 @@
+^CMake Error at [^
+]*/Tests/RunCMake/file/COPY_FILE-input-missing.cmake:[0-9]+ \(file\):
+  file COPY_FILE failed to copy
+
+    [^
+]*/Tests/RunCMake/file/COPY_FILE-input-missing-build/input-missing
+
+  to
+
+    [^
+]*/Tests/RunCMake/file/COPY_FILE-input-missing-build/output
+
+  because: [^
+]+ \(input\)$

Tests/RunCMake/file/COPY_FILE-input-missing.cmake

@@ -0,0 +1,3 @@
+set(oldname "${CMAKE_CURRENT_BINARY_DIR}/input-missing")
+set(newname "${CMAKE_CURRENT_BINARY_DIR}/output")
+file(COPY_FILE "${oldname}" "${newname}")

Tests/RunCMake/file/COPY_FILE-output-missing-result.txt

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

Tests/RunCMake/file/COPY_FILE-output-missing-stderr.txt

@@ -0,0 +1,14 @@
+^CMake Error at [^
+]*/Tests/RunCMake/file/COPY_FILE-output-missing.cmake:[0-9]+ \(file\):
+  file COPY_FILE failed to copy
+
+    [^
+]*/Tests/RunCMake/file/COPY_FILE-output-missing-build/input
+
+  to
+
+    [^
+]*/Tests/RunCMake/file/COPY_FILE-output-missing-build/output-missing/output
+
+  because: [^
+]+ \(output\)$

Tests/RunCMake/file/COPY_FILE-output-missing.cmake

@@ -0,0 +1,4 @@
+set(oldname "${CMAKE_CURRENT_BINARY_DIR}/input")
+set(newname "${CMAKE_CURRENT_BINARY_DIR}/output-missing/output")
+file(WRITE "${oldname}" "")
+file(COPY_FILE "${oldname}" "${newname}")

Tests/RunCMake/file/RunCMakeTest.cmake

@@ -65,6 +65,8 @@ run_cmake_script(COPY_FILE-file-ONLY_IF_DIFFERENT-no-overwrite)
 run_cmake_script(COPY_FILE-link-to-file)
 run_cmake_script(COPY_FILE-arg-missing)
 run_cmake_script(COPY_FILE-arg-unknown)
+run_cmake_script(COPY_FILE-input-missing)
+run_cmake_script(COPY_FILE-output-missing)
 
 run_cmake_script(RENAME-file-replace)
 run_cmake_script(RENAME-file-to-file)