Parsing CPPASTLinkageSpecification (#1579)

It seems we previously ignored it, which meant that all code that was behind `extern "C"` was ignored.

Fixes #1578

cpg-language-cxx/src/main/kotlin/de/fraunhofer/aisec/cpg/frontends/cxx/DeclarationHandler.kt

@@ -73,6 +73,7 @@ class DeclarationHandler(lang: CXXLanguageFrontend) :
             is CPPASTUsingDeclaration -> handleUsingDeclaration(node)
             is CPPASTAliasDeclaration -> handleAliasDeclaration(node)
             is CPPASTNamespaceAlias -> handleNamespaceAlias(node)
+            is CPPASTLinkageSpecification -> handleLinkageSpecification(node)
             else -> {
                 return handleNotSupported(node, node.javaClass.name)
             }
@@ -668,6 +669,23 @@ class DeclarationHandler(lang: CXXLanguageFrontend) :
         return enum
     }
 
+    /**
+     * Translates a C++ (linkage
+     * specification)[https://en.cppreference.com/w/cpp/language/language_linkage]. Actually, we do
+     * not care about the linkage specification per-se, but we need to parse the declaration(s) it
+     * contains.
+     */
+    private fun handleLinkageSpecification(ctx: CPPASTLinkageSpecification): Declaration {
+        var sequence = DeclarationSequence()
+
+        // Just forward its declaration(s) to our handler
+        for (decl in ctx.declarations) {
+            handle(decl)?.let { sequence += it }
+        }
+
+        return simplifySequence(sequence)
+    }
+
     /**
      * @param sequence
      * @return First Element of DeclarationSequence if the Sequence consist of only one element,
@@ -707,9 +725,6 @@ class DeclarationHandler(lang: CXXLanguageFrontend) :
         val problematicIncludes = HashMap<String?, HashSet<ProblemDeclaration>>()
 
         for (declaration in translationUnit.declarations) {
-            if (declaration is CPPASTLinkageSpecification) {
-                continue // do not care about these for now
-            }
             val decl = handle(declaration) ?: continue
             (decl as? ProblemDeclaration)?.location?.let {
                 val problems =

cpg-language-cxx/src/test/kotlin/de/fraunhofer/aisec/cpg/frontends/cxx/CXXLanguageFrontendTest.kt

@@ -1721,4 +1721,17 @@ internal class CXXLanguageFrontendTest : BaseTest() {
         assertTrue(printf.prevEOG.isNotEmpty())
         assertTrue(printf.invokes.isNotEmpty())
     }
+
+    @Test
+    fun testExternC() {
+        val file = File("src/test/resources/cxx/extern_c.cpp")
+        val result =
+            analyze(listOf(file), file.parentFile.toPath(), true) {
+                it.registerLanguage<CPPLanguage>()
+            }
+        assertNotNull(result)
+
+        val test = result.functions["test"]
+        assertNotNull(test)
+    }
 }

cpg-language-cxx/src/test/resources/cxx/extern_c.cpp

@@ -0,0 +1,3 @@
+extern "C" {
+void test();
+}