From b722eea9253acd9e33921779175fc615d064577a Mon Sep 17 00:00:00 2001
From: Marc Chevrier <marc.chevrier@gmail.com>
Date: Mon, 22 Feb 2021 11:45:11 +0100
Subject: Genex: LINK_LANGUAGE respects LINKER_LANGUAGE property

If target property LINKER_LANGUAGE is set, LINK_LANGUAGE generator
expression evaluation must be always successful.

This fix can be helpful to elaborate a solution for issue #21818.
---
 Source/cmGeneratorTarget.cxx                            |  6 +++++-
 .../RunCMakeTest.cmake                                  |  1 +
 .../target_link_libraries-LINK_LANGUAGE/func.cxx        | 17 ++++++++++++++---
 .../target_link_libraries-LINK_LANGUAGE/genex.cmake     |  9 +++++++++
 .../RunCMake/target_link_libraries-LINK_LANGUAGE/main.c | 12 ++++++++++--
 5 files changed, 39 insertions(+), 6 deletions(-)

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 9235faa..d7e9952 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -2648,8 +2648,12 @@ void cmGeneratorTarget::ComputeLinkClosure(const std::string& config,
     LinkClosure linkClosure;
     linkClosure.LinkerLanguage = this->LinkerLanguage;
 
+    bool hasHardCodedLinkerLanguage = this->Target->GetProperty("HAS_CXX") ||
+      !this->Target->GetSafeProperty("LINKER_LANGUAGE").empty();
+
     // Get languages built in this target.
-    secondPass = this->ComputeLinkClosure(config, linkClosure, false);
+    secondPass = this->ComputeLinkClosure(config, linkClosure, false) &&
+      !hasHardCodedLinkerLanguage;
     this->LinkerLanguage = linkClosure.LinkerLanguage;
     if (!secondPass) {
       lc = std::move(linkClosure);
diff --git a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/RunCMakeTest.cmake b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/RunCMakeTest.cmake
index 0f0e5d8..f43a7c6 100644
--- a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/RunCMakeTest.cmake
+++ b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/RunCMakeTest.cmake
@@ -34,6 +34,7 @@ if (RunCMake_GENERATOR MATCHES "Makefiles|Ninja|Visual Studio|Xcode|Watcom WMake
   run_cmake_target(genex CXX_interface2 LinkLibraries_CXX_interface2 --config Release)
   run_cmake_target(genex C_static LinkLibraries_C_static --config Release)
   run_cmake_target(genex CXX_static LinkLibraries_CXX_static --config Release)
+  run_cmake_target(genex C_static_CXX LinkLibraries_C_static_CXX --config Release)
 
   unset(RunCMake_TEST_OPTIONS)
   unset(RunCMake_TEST_OUTPUT_MERGE)
diff --git a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/func.cxx b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/func.cxx
index a12caca..ee6811c 100644
--- a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/func.cxx
+++ b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/func.cxx
@@ -1,7 +1,18 @@
 
-#if defined(_WIN32)
-__declspec(dllexport)
+#if !defined(BUILD_STATIC) && defined(_WIN32)
+#  define EXPORT_SYMBOL __declspec(dllexport)
+#else
+#  define EXPORT_SYMBOL
 #endif
-  void func_cxx()
+
+EXPORT_SYMBOL
+void func_cxx()
 {
 }
+
+extern "C" {
+EXPORT_SYMBOL
+void func_c_cxx()
+{
+}
+}
diff --git a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/genex.cmake b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/genex.cmake
index f3fe955..22d3df7 100644
--- a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/genex.cmake
+++ b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/genex.cmake
@@ -7,6 +7,8 @@ enable_language(CXX)
 add_library(shared_C SHARED func.c)
 add_library(shared_CXX SHARED func.cxx)
 
+add_library(static_cxx STATIC func.cxx)
+target_compile_definitions(static_cxx PRIVATE BUILD_STATIC)
 
 add_library(static1_C STATIC empty.c)
 target_link_libraries (static1_C INTERFACE $<$<LINK_LANGUAGE:C>:shared_C>)
@@ -70,3 +72,10 @@ add_executable(LinkLibraries_C_static main.c)
 target_link_libraries (LinkLibraries_C_static PRIVATE static3)
 add_executable(LinkLibraries_CXX_static main.cxx)
 target_link_libraries (LinkLibraries_CXX_static PRIVATE static3)
+
+# $<LINK_LANGUAGE:> change, by default, link language from C to CXX
+# but because LINKER_LANGUAGE property is set, keep C as link language
+add_executable(LinkLibraries_C_static_CXX main.c)
+target_compile_definitions (LinkLibraries_C_static_CXX PRIVATE C_USE_CXX)
+target_link_libraries (LinkLibraries_C_static_CXX PRIVATE $<$<LINK_LANGUAGE:C>:static_cxx>)
+set_property(TARGET LinkLibraries_C_static_CXX PROPERTY LINKER_LANGUAGE C)
diff --git a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/main.c b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/main.c
index a908dea..689dbd7 100644
--- a/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/main.c
+++ b/Tests/RunCMake/target_link_libraries-LINK_LANGUAGE/main.c
@@ -1,12 +1,20 @@
 
-#if defined(_WIN32)
+#if defined(C_USE_CXX)
+void func_c_cxx();
+#else
+#  if defined(_WIN32)
 __declspec(dllimport)
-#endif
+#  endif
   void func_c();
+#endif
 
 int main()
 {
+#if defined(C_USE_CXX)
+  func_c_cxx();
+#else
   func_c();
+#endif
 
   return 0;
 }
-- 
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