From 0cd4babe0342c994d28916ecbbb7cd2b91023b9b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin@martin.st>
Date: Thu, 21 Mar 2024 17:29:15 +0200
Subject: [PATCH] Reapply [libcxx] [modules] Fix relative paths with absolute
 LIBCXX_INSTALL_MODULES_DIR (#86020)

This reapplies 272d1b44efdedb68c194970a610f0ca1b7b769c5 (from #85756),
which was reverted in
407937036fa7640f61f225474b1ea6623a40dbdd.

In the previous attempt, empty CMAKE_INSTALL_PREFIX was handled by
quoting them, in d209d1340b99d4fbd325dffb5e13b757ab8264ea. That made the
calls to cmake_path(ABSOLUTE_PATH) succeed, but the output paths of that
weren't actually absolute, which was required by file(RELATIVE_PATH).

Avoid this issue by constructing a non-empty base directory variable
to use for calculating the relative path.

(cherry picked from commit 50801f1095d33e712c3a51fdeef82569bd09007f)
---
 libcxx/modules/CMakeLists.txt | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/libcxx/modules/CMakeLists.txt b/libcxx/modules/CMakeLists.txt
index 0dea8cfca94ac..d47d19a475531 100644
--- a/libcxx/modules/CMakeLists.txt
+++ b/libcxx/modules/CMakeLists.txt
@@ -206,9 +206,20 @@ add_custom_target(generate-cxx-modules
 # Configure the modules manifest.
 # Use the relative path between the installation and the module in the json
 # file. This allows moving the entire installation to a different location.
+if("${CMAKE_INSTALL_PREFIX}" STREQUAL "")
+  set(BASE_DIRECTORY "/")
+else()
+  set(BASE_DIRECTORY ${CMAKE_INSTALL_PREFIX})
+endif()
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_LIBRARY_DIR
+  BASE_DIRECTORY ${BASE_DIRECTORY}
+  OUTPUT_VARIABLE ABS_LIBRARY_DIR)
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_MODULES_DIR
+  BASE_DIRECTORY ${BASE_DIRECTORY}
+  OUTPUT_VARIABLE ABS_MODULES_DIR)
 file(RELATIVE_PATH LIBCXX_MODULE_RELATIVE_PATH
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_LIBRARY_DIR}
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_MODULES_DIR})
+  ${ABS_LIBRARY_DIR}
+  ${ABS_MODULES_DIR})
 configure_file(
   "modules.json.in"
   "${LIBCXX_LIBRARY_DIR}/libc++.modules.json"