both_libraries: Make sure to select the right linker for static lib

Regression test: libccpp has both C and C++ sources. The executable only
has C sources. It should still link using the C++ compiler. When using
both_libraries the static has no sources and thus no compilers,
resulting in the executable linking using the C compiler.
https://github.com/Netflix/vmaf/issues/1107

mesonbuild/interpreter/interpreter.py

@@ -101,7 +101,6 @@ import typing as T
 import textwrap
 import importlib
 import copy
-import itertools
 
 if T.TYPE_CHECKING:
     import argparse
@@ -3093,8 +3092,8 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
             static_lib.sources = []
             static_lib.generated = []
             # Compilers with no corresponding sources confuses the backend.
-            # Keep only the first compiler because it is the linker.
-            static_lib.compilers = dict(itertools.islice(static_lib.compilers.items(), 1))
+            # Keep only compilers used for linking
+            static_lib.compilers = {k: v for k, v in static_lib.compilers.items() if k in compilers.clink_langs}
 
         return build.BothLibraries(shared_lib, static_lib)
 

test cases/common/178 bothlibraries/foo.cpp

@@ -0,0 +1,11 @@
+#include <memory>
+#include "mylib.h"
+
+extern "C" {
+    DO_EXPORT int foo(void);
+}
+
+int foo(void) {
+    auto bptr = std::make_shared<int>(0);
+    return *bptr;
+}

test cases/common/178 bothlibraries/main2.c

@@ -0,0 +1,9 @@
+#include "mylib.h"
+
+DO_IMPORT int func(void);
+DO_IMPORT int foo(void);
+DO_IMPORT int retval;
+
+int main(void) {
+    return func() + foo() == retval ? 0 : 1;
+}

test cases/common/178 bothlibraries/meson.build

@@ -1,4 +1,4 @@
-project('both libraries linking test', 'c')
+project('both libraries linking test', 'c', 'cpp')
 
 both_libs = both_libraries('mylib', 'libfile.c')
 dep = declare_dependency(link_with: both_libs)
@@ -48,3 +48,15 @@ assert(both_libs3.get_static_lib().name() == 'mylib')
 test('runtest-shared-2', exe_shared2)
 test('runtest-static-2', exe_static2)
 test('runtest-both-2', exe_both2)
+
+# Regression test: libccpp has both C and C++ sources. The executable only has
+# C sources. It should still link using the C++ compiler. When using
+# both_libraries the static has no sources and thus no compilers, resulting in
+# the executable linking using the C compiler.
+# https://github.com/Netflix/vmaf/issues/1107
+libccpp = both_libraries('ccpp', 'foo.cpp', 'libfile.c')
+exe = executable('prog-ccpp', 'main2.c',
+  link_with: libccpp.get_static_lib(),
+  c_args : ['-DSTATIC_COMPILATION'],
+)
+test('runtest-ccpp', exe)