[cxx-interop] Emit IR for virtual methods

lib/IRGen/GenClangDecl.cpp

@@ -201,6 +201,17 @@ void IRGenModule::emitClangDecl(const clang::Decl *decl) {
       }
     }
 
+    // If something from a C++ class is used, emit all virtual methods of this
+    // class because they might be emitted in the vtable even if not used
+    // directly from Swift.
+    if (auto *record = dyn_cast<clang::CXXRecordDecl>(next->getDeclContext())) {
+      for (auto *method : record->methods()) {
+        if (method->isVirtual()) {
+          callback(method);
+        }
+      }
+    }
+
     if (auto var = dyn_cast<clang::VarDecl>(next))
       if (!var->isFileVarDecl())
         continue;

test/Interop/Cxx/class/inheritance/Inputs/module.modulemap

@@ -13,3 +13,8 @@ module SubTypes {
 module TypeAliases {
   header "type-aliases.h"
 }
+
+module VirtualMethods {
+  header "virtual-methods.h"
+  requires cplusplus
+}

test/Interop/Cxx/class/inheritance/Inputs/virtual-methods.h

@@ -0,0 +1,29 @@
+extern "C" void puts(const char *);
+
+inline void testFunctionCollected() {
+  puts("test\n");
+}
+
+struct Base {
+  virtual void foo() = 0;
+};
+
+template <class T>
+struct Derived : Base {
+  inline void foo() override {
+    testFunctionCollected();
+  }
+
+  void callMe() {
+  }
+};
+
+using DerivedInt = Derived<int>;
+
+template <class T>
+struct Unused : Base {
+  inline void foo() override {
+  }
+};
+
+using UnusedInt = Unused<int>;

test/Interop/Cxx/class/inheritance/virtual-methods-irgen.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-emit-ir -I %S/Inputs -enable-experimental-cxx-interop %s -validate-tbd-against-ir=none | %FileCheck %s
+
+// FIXME: enable on Windows
+// XFAIL: OS=windows-msvc
+
+import VirtualMethods
+
+var x = DerivedInt()
+x.callMe()
+
+// CHECK: define {{.*}}void @{{_ZN7DerivedIiE3fooEv|"\?foo@\?$Derived@H@@UEAAXXZ"}}
+// CHECK:   call void @{{_Z21testFunctionCollectedv|"\?testFunctionCollected@@YAXXZ"}}
+
+// CHECK: define {{.*}}void @{{_Z21testFunctionCollectedv|"\?testFunctionCollected@@YAXXZ"}}
+
+// CHECK-NOT: _ZN6UnusedIiE3fooEv
+// CHECK-NOT: "\?foo@\?$Unused@H@@UEAAXXZ"