INTERFACE_SOURCES: Fix per-config link libs on multi-config generators

In multi-config generators we memoize the computed set of source files
for a target to avoid repeating the computation when the set does not
depend on the configuration.  We already track whether generator
expressions in `SOURCES` or `INTERFACE_SOURCES` reference the
configuration (`$<CONFIG:...>`).  However, we previously forgot to track
whether the set of libraries whose `INTERFACE_SOURCES` are considered
depends on the configuration.  This caused multi-config generators to
use the first configuration's set of sources for all configurations
in cases such as

    target_link_libraries(tgt PRIVATE $<$<CONFIG:Debug>:iface_debug>)

where the `iface_debug` target has `INTERFACE_SOURCES`.

Fix this by also tracking config-dependence of the list of libraries for
evaluation of the list of source files.

Fixes: #20683

Source/cmGeneratorTarget.cxx

@@ -241,6 +241,7 @@ EvaluatedTargetPropertyEntry EvaluateTargetPropertyEntry(
 struct EvaluatedTargetPropertyEntries
 {
   std::vector<EvaluatedTargetPropertyEntry> Entries;
+  bool HadContextSensitiveCondition = false;
 };
 
 EvaluatedTargetPropertyEntries EvaluateTargetPropertyEntries(
@@ -1251,6 +1252,9 @@ std::string cmGeneratorTarget::EvaluateInterfaceProperty(
 
   if (cmLinkInterfaceLibraries const* iface = this->GetLinkInterfaceLibraries(
         context->Config, headTarget, usage_requirements_only)) {
+    context->HadContextSensitiveCondition =
+      context->HadContextSensitiveCondition ||
+      iface->HadContextSensitiveCondition;
     for (cmLinkItem const& lib : iface->Libraries) {
       // Broken code can have a target in its own link interface.
       // Don't follow such link interface entries so as not to create a
@@ -1378,6 +1382,7 @@ void AddInterfaceEntries(cmGeneratorTarget const* headTarget,
 {
   if (cmLinkImplementationLibraries const* impl =
         headTarget->GetLinkImplementationLibraries(config)) {
+    entries.HadContextSensitiveCondition = impl->HadContextSensitiveCondition;
     for (cmLinkImplItem const& lib : impl->Libraries) {
       if (lib.Target) {
         EvaluatedTargetPropertyEntry ee(lib, lib.Backtrace);
@@ -1404,6 +1409,7 @@ void AddObjectEntries(cmGeneratorTarget const* headTarget,
 {
   if (cmLinkImplementationLibraries const* impl =
         headTarget->GetLinkImplementationLibraries(config)) {
+    entries.HadContextSensitiveCondition = impl->HadContextSensitiveCondition;
     for (cmLinkImplItem const& lib : impl->Libraries) {
       if (lib.Target &&
           lib.Target->GetType() == cmStateEnums::OBJECT_LIBRARY) {
@@ -1436,7 +1442,7 @@ bool processSources(cmGeneratorTarget const* tgt,
 {
   cmMakefile* mf = tgt->Target->GetMakefile();
 
-  bool contextDependent = false;
+  bool contextDependent = entries.HadContextSensitiveCondition;
 
   for (EvaluatedTargetPropertyEntry& entry : entries.Entries) {
     if (entry.ContextDependent) {

Tests/ConfigSources/CMakeLists.txt

@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 3.0)
 project(ConfigSources CXX)
 
+# Per-config sources via INTERFACE_SOURCES.
 add_library(iface INTERFACE)
 target_sources(iface INTERFACE
   "${CMAKE_CURRENT_SOURCE_DIR}/iface_src.cpp"
@@ -12,10 +13,45 @@ target_compile_definitions(iface INTERFACE
   "$<$<CONFIG:Debug>:CFG_DEBUG>"
   "$<$<NOT:$<CONFIG:Debug>>:CFG_OTHER>"
   )
-
 add_executable(ConfigSources
   $<$<CONFIG:Debug>:main_debug.cpp>
   $<$<NOT:$<CONFIG:Debug>>:main_other.cpp>
   $<$<CONFIG:NotAConfig>:does_not_exist.cpp>
   )
 target_link_libraries(ConfigSources iface)
+
+# Per-config sources via LINK_LIBRARIES.
+add_library(iface_debug INTERFACE)
+target_sources(iface_debug INTERFACE
+  "${CMAKE_CURRENT_SOURCE_DIR}/iface_src.cpp"
+  "${CMAKE_CURRENT_SOURCE_DIR}/iface_debug_src.cpp"
+  )
+add_library(iface_other INTERFACE)
+target_sources(iface_other INTERFACE
+  "${CMAKE_CURRENT_SOURCE_DIR}/iface_src.cpp"
+  "${CMAKE_CURRENT_SOURCE_DIR}/iface_other_src.cpp"
+  )
+add_executable(ConfigSourcesLink main.cpp)
+target_compile_definitions(ConfigSourcesLink PRIVATE
+  "$<$<CONFIG:Debug>:CFG_DEBUG>"
+  "$<$<NOT:$<CONFIG:Debug>>:CFG_OTHER>"
+  )
+target_link_libraries(ConfigSourcesLink PRIVATE
+  "$<$<CONFIG:Debug>:iface_debug>"
+  "$<$<NOT:$<CONFIG:Debug>>:iface_other>"
+  "$<$<CONFIG:NotAConfig>:iface_does_not_exist>"
+  )
+
+# Per-config sources via INTERFACE_LINK_LIBRARIES.
+add_library(ConfigSourcesIface INTERFACE)
+target_link_libraries(ConfigSourcesIface INTERFACE
+  "$<$<CONFIG:Debug>:iface_debug>"
+  "$<$<NOT:$<CONFIG:Debug>>:iface_other>"
+  "$<$<CONFIG:NotAConfig>:iface_does_not_exist>"
+  )
+add_executable(ConfigSourcesLinkIface main.cpp)
+target_compile_definitions(ConfigSourcesLinkIface PRIVATE
+  "$<$<CONFIG:Debug>:CFG_DEBUG>"
+  "$<$<NOT:$<CONFIG:Debug>>:CFG_OTHER>"
+  )
+target_link_libraries(ConfigSourcesLinkIface ConfigSourcesIface)

Tests/ConfigSources/main.cpp

@@ -0,0 +1,9 @@
+#if !defined(CFG_DEBUG) && !defined(CFG_OTHER)
+#  error "Neither CFG_DEBUG or CFG_OTHER is defined."
+#endif
+#ifdef CFG_DEBUG
+#  include "main_debug.cpp"
+#endif
+#ifdef CFG_OTHER
+#  include "main_other.cpp"
+#endif