fix(c++/modules): avoid leaking private dep flags to rebuilt BMIs

Only inherit interface-visible defines/undefines/includedirs when
collecting module fileconfigs from dependency targets.

This fixes consumer-side BMI rebuilds under
build.c++.modules.reuse.strict picking up private producer flags.

Add a regression test for strict module reuse with a shared dependency.

6 files changed, 68 insertions, 0 deletions

diff --git a/tests/projects/c++/modules/reuse_strict_private_defines/src/include/public_config.h b/tests/projects/c++/modules/reuse_strict_private_defines/src/include/public_config.h
new file mode 100644
index 000000000..3ceb5dcd5
--- /dev/null
+++ b/tests/projects/c++/modules/reuse_strict_private_defines/src/include/public_config.h
@@ -0,0 +1,3 @@
+#pragma once
+
+#define PUBLIC_CONFIG_VALUE 0
diff --git a/tests/projects/c++/modules/reuse_strict_private_defines/src/main.cpp b/tests/projects/c++/modules/reuse_strict_private_defines/src/main.cpp
new file mode 100644
index 000000000..5b477f2d6
--- /dev/null
+++ b/tests/projects/c++/modules/reuse_strict_private_defines/src/main.cpp
@@ -0,0 +1,5 @@
+import mod;
+
+int main() {
+    return foo();
+}
diff --git a/tests/projects/c++/modules/reuse_strict_private_defines/src/mod.cpp b/tests/projects/c++/modules/reuse_strict_private_defines/src/mod.cpp
new file mode 100644
index 000000000..478993dfb
--- /dev/null
+++ b/tests/projects/c++/modules/reuse_strict_private_defines/src/mod.cpp
@@ -0,0 +1,7 @@
+#include <public_config.h>
+
+module mod;
+
+int foo() {
+    return PUBLIC_CONFIG_VALUE;
+}
diff --git a/tests/projects/c++/modules/reuse_strict_private_defines/src/mod.mpp b/tests/projects/c++/modules/reuse_strict_private_defines/src/mod.mpp
new file mode 100644
index 000000000..b55b834a7
--- /dev/null
+++ b/tests/projects/c++/modules/reuse_strict_private_defines/src/mod.mpp
@@ -0,0 +1,3 @@
+export module mod;
+
+export int foo();
diff --git a/tests/projects/c++/modules/reuse_strict_private_defines/test.lua b/tests/projects/c++/modules/reuse_strict_private_defines/test.lua
new file mode 100644
index 000000000..bd9b241cd
--- /dev/null
+++ b/tests/projects/c++/modules/reuse_strict_private_defines/test.lua
@@ -0,0 +1,34 @@
+inherit(".test_base")
+
+local PRIVATE_DEFINE = "PRIVATE_DEP_DEFINE_DO_NOT_PROPAGATE"
+local PUBLIC_SYSINCLUDEDIR = path.translate(path.absolute("src/include"))
+
+function _build()
+    local outdata = os.iorun("xmake -r -vD")
+    local leaked = false
+    local missing_sysincludedir = true
+    for line in outdata:gmatch("[^\r\n]+") do
+        if line:find("Consumer", 1, true) then
+            if line:find(PRIVATE_DEFINE, 1, true) then
+                leaked = true
+            end
+            if line:find(PUBLIC_SYSINCLUDEDIR, 1, true) then
+                missing_sysincludedir = false
+            end
+        end
+    end
+    if leaked then
+        raise("Private dependency defines leaked into Consumer module rebuilds under reuse.strict\n%s", outdata)
+    end
+    if missing_sysincludedir then
+        raise("Missing public sysincludedir in Consumer module rebuilds under reuse.strict\n%s", outdata)
+    end
+    os.run("xmake -vD")
+end
+
+function main(_)
+    local clang_options = {compiler = "clang", version = CLANG_MIN_VER, build = _build}
+    local gcc_options = {compiler = "gcc", version = GCC_MIN_VER, build = _build}
+    local msvc_options = {version = MSVC_MIN_VER, build = _build}
+    run_tests(clang_options, gcc_options, msvc_options)
+end
diff --git a/tests/projects/c++/modules/reuse_strict_private_defines/xmake.lua b/tests/projects/c++/modules/reuse_strict_private_defines/xmake.lua
new file mode 100644
index 000000000..aba5cc895
--- /dev/null
+++ b/tests/projects/c++/modules/reuse_strict_private_defines/xmake.lua
@@ -0,0 +1,16 @@
+add_rules("mode.debug", "mode.release")
+
+set_languages("cxx20")
+
+target("Producer")
+    set_kind("shared")
+    add_defines("PRIVATE_DEP_DEFINE_DO_NOT_PROPAGATE")
+    add_sysincludedirs("src/include", {public = true})
+    add_files("src/mod.mpp", {public = true})
+    add_files("src/mod.cpp")
+
+target("Consumer")
+    set_kind("binary")
+    set_policy("build.c++.modules.reuse.strict", true)
+    add_deps("Producer")
+    add_files("src/main.cpp")