cmCxxModuleMapper: Fix transitive requirements computation

Previously CMake may generate incomplete transitive requirements in
CMakeFiles/<target>.dir/CXXModules.json and therefore in module mapper
for compiler, when source files were listed in CMakeList.txt in a
certain order.

This commit fixes the problem by correctly tracking unfinished
transitive requirements computation of module units.

There have been a simple circular test case whose circular dependency
was reported by build system. Now with this correct implementation it's
reported by CMake generating module mappers.

Add two test cases for transitive requirements computation, one with
adding source files in hardcoded order, and the other in randomized
order.

Fixes: #25465

35 files changed, 384 insertions, 1 deletions

diff --git a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
index 0c0f1b4..fcfa60a 100644
--- a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
+++ b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
@@ -174,6 +174,8 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(object-library)
   run_cxx_module_test(generated)
   run_cxx_module_test(deep-chain)
+  run_cxx_module_test(non-trivial-collation-order)
+  run_cxx_module_test(non-trivial-collation-order-randomized)
   run_cxx_module_test(duplicate)
   set(RunCMake_CXXModules_NO_TEST 1)
   run_cxx_module_test(circular)
diff --git a/Tests/RunCMake/CXXModules/examples/circular-build-stdout.txt b/Tests/RunCMake/CXXModules/examples/circular-build-stdout.txt
index ac80356..f6f5e5f 100644
--- a/Tests/RunCMake/CXXModules/examples/circular-build-stdout.txt
+++ b/Tests/RunCMake/CXXModules/examples/circular-build-stdout.txt
@@ -1 +1 @@
-((Ninja generators)?(build stopped: dependency cycle:)|(Visual Studio generators)?(error : Cannot build the following source files because there is a cyclic dependency between them))
+((Ninja generators)?(CMake Error: Circular dependency detected in the C\+\+ module import graph. See modules named: "a", "b")|(Visual Studio generators)?(error : Cannot build the following source files because there is a cyclic dependency between them))
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order-randomized/CMakeLists.txt b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order-randomized/CMakeLists.txt
new file mode 100644
index 0000000..4e1e7af
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order-randomized/CMakeLists.txt
@@ -0,0 +1,9 @@
+cmake_minimum_required(VERSION 3.24...3.28)
+project(cxx_modules_non_trivial_collation_order_randomized CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+set(non_trivial_collation_order_randomized TRUE)
+
+add_subdirectory("${CMAKE_CURRENT_SOURCE_DIR}/../non-trivial-collation-order/"
+                 "${CMAKE_CURRENT_BINARY_DIR}/non-trivial-collation-order")
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/CMakeLists.txt b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/CMakeLists.txt
new file mode 100644
index 0000000..218b45f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/CMakeLists.txt
@@ -0,0 +1,132 @@
+cmake_minimum_required(VERSION 3.24...3.28)
+project(cxx_modules_non_trivial_collation_order CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+  include(CheckCompilerFlag)
+  check_compiler_flag(CXX "-Wread-modules-implicitly" have_implicit_module_warning)
+  if (have_implicit_module_warning)
+    add_compile_options(-Werror=read-modules-implicitly)
+  endif ()
+endif ()
+
+# Returns an random integer from range [0,length)
+function(get_random_index result length)
+  # math(EXPR) parses and evaluates as 64-bit signed integer.
+  string(RANDOM LENGTH 18 ALPHABET "0123456789" value)
+  # Modulo is not uniformly distributed, but we doesn't need that here.
+  math(EXPR ${result} "${value} % ${length}")
+  return(PROPAGATE ${result})
+endfunction()
+
+# Shuffle list_var randomly
+function(shuffle_list list_var)
+  set(result)
+  set(length 0)
+  foreach(item IN LISTS "${list_var}")
+    math(EXPR length "${length} + 1")
+    get_random_index(index ${length})
+    list(INSERT result ${index} "${item}")
+  endforeach()
+  set("${list_var}" "${result}" PARENT_SCOPE)
+endfunction()
+
+# The import chain is:
+# impl-non-partition  --(implicitly)-->  partition_level(primary interface unit)
+# --> :intf7 --> :intf6 --> :intf5 --> :intf4 --> :intf3 --> :intf2 --> :intf1
+# --> :impl7 --> :impl6 --> :impl5 --> :impl4 --> :impl3 --> :impl2 --> :impl1
+set(partition_level_srcs
+  partition_level/partition_level.cxx
+  partition_level/intf1.cxx
+  partition_level/intf3.cxx
+  partition_level/intf4.cxx
+  partition_level/intf2.cxx   # intentional order
+  partition_level/intf5.cxx
+  partition_level/intf6.cxx
+  partition_level/intf7.cxx
+  partition_level/impl1.cxx
+  partition_level/impl3.cxx
+  partition_level/impl4.cxx
+  partition_level/impl2.cxx   # intentional order
+  partition_level/impl5.cxx
+  partition_level/impl6.cxx
+  partition_level/impl7.cxx
+)
+
+# The import chain is:
+# mod7 --> mod6 --> mod5 --> mod4 --> mod3 --> mod2 --> mod1 --> partition_level
+set(module_level_srcs
+  ${partition_level_srcs}
+  module_level/mod1.cxx
+  module_level/mod3.cxx
+  module_level/mod4.cxx
+  module_level/mod2.cxx   # intentional order
+  module_level/mod5.cxx
+  module_level/mod6.cxx
+  module_level/mod7.cxx
+)
+
+# The import chain is:
+# target7 --> target6 --> target5 --> target4 -->
+# target3 --> target2 --> target1 --> target_module_level
+set(targets
+  target_module_level
+  target1
+  target3
+  target4
+  target2 # intentional order
+  target5
+  target6
+  target7
+)
+
+if(non_trivial_collation_order_randomized)
+  shuffle_list(module_level_srcs)
+  shuffle_list(targets)
+endif()
+
+message(STATUS "module_level_srcs: ${module_level_srcs}")
+message(STATUS "targets: ${targets}")
+
+foreach(tgt IN LISTS targets)
+  if (tgt STREQUAL "target_module_level")
+
+    add_library(target_module_level STATIC)
+    target_sources(target_module_level
+      PUBLIC
+        FILE_SET CXX_MODULES
+          BASE_DIRS "${CMAKE_CURRENT_SOURCE_DIR}"
+          FILES ${module_level_srcs}
+      PRIVATE
+        partition_level/impl-non-partition.cxx
+    )
+    target_compile_features(target_module_level PUBLIC cxx_std_20)
+
+  else()
+
+    add_library(${tgt} STATIC)
+    target_sources(${tgt}
+      PUBLIC
+        FILE_SET CXX_MODULES
+          BASE_DIRS "${CMAKE_CURRENT_SOURCE_DIR}"
+          FILES target_level/${tgt}.cxx
+    )
+    target_compile_features(${tgt} PUBLIC cxx_std_20)
+    if (tgt MATCHES "^target([2-9])$")
+      math(EXPR dep_number "${CMAKE_MATCH_1} - 1")
+      target_link_libraries(${tgt} PRIVATE "target${dep_number}")
+    else()
+      target_link_libraries(${tgt} PRIVATE target_module_level)
+    endif()
+
+  endif()
+endforeach()
+
+add_executable(exe)
+target_link_libraries(exe PRIVATE target7)
+target_sources(exe
+  PRIVATE
+    main.cxx)
+
+add_test(NAME exe COMMAND exe)
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/main.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/main.cxx
new file mode 100644
index 0000000..6496776
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/main.cxx
@@ -0,0 +1,6 @@
+import target7;
+
+int main()
+{
+  return target7();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod1.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod1.cxx
new file mode 100644
index 0000000..1999688
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod1.cxx
@@ -0,0 +1,7 @@
+export module mod1;
+import partition_level;
+
+export int mod1()
+{
+  return partition_level::get();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod2.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod2.cxx
new file mode 100644
index 0000000..454899f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod2.cxx
@@ -0,0 +1,7 @@
+export module mod2;
+import mod1;
+
+export int mod2()
+{
+  return mod1();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod3.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod3.cxx
new file mode 100644
index 0000000..85dea1e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod3.cxx
@@ -0,0 +1,7 @@
+export module mod3;
+import mod2;
+
+export int mod3()
+{
+  return mod2();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod4.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod4.cxx
new file mode 100644
index 0000000..e6143d7
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod4.cxx
@@ -0,0 +1,7 @@
+export module mod4;
+import mod3;
+
+export int mod4()
+{
+  return mod3();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod5.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod5.cxx
new file mode 100644
index 0000000..94355a9
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod5.cxx
@@ -0,0 +1,7 @@
+export module mod5;
+import mod4;
+
+export int mod5()
+{
+  return mod4();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod6.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod6.cxx
new file mode 100644
index 0000000..998ed8b
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod6.cxx
@@ -0,0 +1,7 @@
+export module mod6;
+import mod5;
+
+export int mod6()
+{
+  return mod5();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod7.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod7.cxx
new file mode 100644
index 0000000..15c52b0
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/module_level/mod7.cxx
@@ -0,0 +1,7 @@
+export module mod7;
+import mod6;
+
+export int mod7()
+{
+  return mod6();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl-non-partition.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl-non-partition.cxx
new file mode 100644
index 0000000..0cf184f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl-non-partition.cxx
@@ -0,0 +1,10 @@
+module partition_level;
+
+namespace partition_level {
+int non_partition()
+{
+  return primary() + intf1() + intf2() + intf3() + intf4() + intf5() +
+    intf6() + intf7() + impl1() + impl2() + impl3() + impl4() + impl5() +
+    impl6() + impl7();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl1.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl1.cxx
new file mode 100644
index 0000000..86c27c6
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl1.cxx
@@ -0,0 +1,7 @@
+module partition_level:impl1;
+namespace partition_level {
+int impl1()
+{
+  return 0;
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl2.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl2.cxx
new file mode 100644
index 0000000..49d10e3
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl2.cxx
@@ -0,0 +1,8 @@
+module partition_level:impl2;
+import :impl1;
+namespace partition_level {
+int impl2()
+{
+  return impl1();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl3.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl3.cxx
new file mode 100644
index 0000000..cbd183b
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl3.cxx
@@ -0,0 +1,8 @@
+module partition_level:impl3;
+import :impl2;
+namespace partition_level {
+int impl3()
+{
+  return impl2();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl4.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl4.cxx
new file mode 100644
index 0000000..7ce9390
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl4.cxx
@@ -0,0 +1,8 @@
+module partition_level:impl4;
+import :impl3;
+namespace partition_level {
+int impl4()
+{
+  return impl3();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl5.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl5.cxx
new file mode 100644
index 0000000..901eda9
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl5.cxx
@@ -0,0 +1,8 @@
+module partition_level:impl5;
+import :impl4;
+namespace partition_level {
+int impl5()
+{
+  return impl4();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl6.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl6.cxx
new file mode 100644
index 0000000..38130c7
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl6.cxx
@@ -0,0 +1,8 @@
+module partition_level:impl6;
+import :impl5;
+namespace partition_level {
+int impl6()
+{
+  return impl5();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl7.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl7.cxx
new file mode 100644
index 0000000..c3b1322
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/impl7.cxx
@@ -0,0 +1,8 @@
+module partition_level:impl7;
+import :impl6;
+namespace partition_level {
+int impl7()
+{
+  return impl6();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf1.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf1.cxx
new file mode 100644
index 0000000..5d8500d
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf1.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf1;
+import :impl7;
+namespace partition_level {
+int intf1()
+{
+  return impl7();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf2.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf2.cxx
new file mode 100644
index 0000000..76c6eed
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf2.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf2;
+export import :intf1;
+namespace partition_level {
+int intf2()
+{
+  return intf1();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf3.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf3.cxx
new file mode 100644
index 0000000..3b16053
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf3.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf3;
+export import :intf2;
+namespace partition_level {
+int intf3()
+{
+  return intf2();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf4.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf4.cxx
new file mode 100644
index 0000000..2826717
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf4.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf4;
+export import :intf3;
+namespace partition_level {
+int intf4()
+{
+  return intf3();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf5.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf5.cxx
new file mode 100644
index 0000000..ede117e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf5.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf5;
+export import :intf4;
+namespace partition_level {
+int intf5()
+{
+  return intf4();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf6.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf6.cxx
new file mode 100644
index 0000000..c89a554
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf6.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf6;
+export import :intf5;
+namespace partition_level {
+int intf6()
+{
+  return intf5();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf7.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf7.cxx
new file mode 100644
index 0000000..ee0b250
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/intf7.cxx
@@ -0,0 +1,8 @@
+export module partition_level:intf7;
+export import :intf6;
+namespace partition_level {
+int intf7()
+{
+  return intf6();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/partition_level.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/partition_level.cxx
new file mode 100644
index 0000000..7f3ddb2
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/partition_level/partition_level.cxx
@@ -0,0 +1,15 @@
+export module partition_level;
+export import :intf7;
+namespace partition_level {
+int primary()
+{
+  return intf7();
+}
+
+int non_partition();
+
+export int get()
+{
+  return non_partition();
+}
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target1.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target1.cxx
new file mode 100644
index 0000000..5487e21
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target1.cxx
@@ -0,0 +1,7 @@
+export module target1;
+import mod7;
+
+export int target1()
+{
+  return mod7();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target2.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target2.cxx
new file mode 100644
index 0000000..52c87dc
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target2.cxx
@@ -0,0 +1,7 @@
+export module target2;
+import target1;
+
+export int target2()
+{
+  return target1();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target3.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target3.cxx
new file mode 100644
index 0000000..4a1a3c0
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target3.cxx
@@ -0,0 +1,7 @@
+export module target3;
+import target2;
+
+export int target3()
+{
+  return target2();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target4.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target4.cxx
new file mode 100644
index 0000000..da1bf9d
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target4.cxx
@@ -0,0 +1,7 @@
+export module target4;
+import target3;
+
+export int target4()
+{
+  return target3();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target5.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target5.cxx
new file mode 100644
index 0000000..f18df7a
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target5.cxx
@@ -0,0 +1,7 @@
+export module target5;
+import target4;
+
+export int target5()
+{
+  return target4();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target6.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target6.cxx
new file mode 100644
index 0000000..a598df8
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target6.cxx
@@ -0,0 +1,7 @@
+export module target6;
+import target5;
+
+export int target6()
+{
+  return target5();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target7.cxx b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target7.cxx
new file mode 100644
index 0000000..641cfbc
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/non-trivial-collation-order/target_level/target7.cxx
@@ -0,0 +1,7 @@
+export module target7;
+import target6;
+
+export int target7()
+{
+  return target6();
+}