[cxx-interop] Avoid ambiguous C++ overloads for functions from Swift extensions

Reverse interop has logic that avoids emitting ambiguous overloads into the generated header. That logic did not apply for functions declared within Swift extensions. This meant that the generated C++ header would sometimes not compile.

rdar://158252800

Author: egorzhdan

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -253,7 +253,6 @@ class DeclAndTypePrinter::Implementation
   /// Prints the members of a class, extension, or protocol.
   template <bool AllowDelayed = false, typename R>
   void printMembers(R &&members) {
-    CxxEmissionScopeRAII cxxScopeRAII(owningPrinter);
     // Using statements for nested types.
     if (outputLang == OutputLanguageMode::Cxx) {
       for (const Decl *member : members) {
@@ -373,7 +372,12 @@ class DeclAndTypePrinter::Implementation
     if (outputLang == OutputLanguageMode::Cxx) {
       // FIXME: Non objc class.
       ClangClassTypePrinter(os).printClassTypeDecl(
-          CD, [&]() { printMembers(CD->getAllMembers()); }, owningPrinter);
+          CD,
+          [&]() {
+            CxxEmissionScopeRAII cxxScopeRAII(owningPrinter);
+            printMembers(CD->getAllMembers());
+          },
+          owningPrinter);
       recordEmittedDeclInCurrentCxxLexicalScope(CD);
       return;
     }
@@ -426,6 +430,7 @@ class DeclAndTypePrinter::Implementation
     printer.printValueTypeDecl(
         SD, /*bodyPrinter=*/
         [&]() {
+          CxxEmissionScopeRAII cxxScopeRAII(owningPrinter);
           printMembers(SD->getAllMembers());
           for (const auto *ed :
                owningPrinter.interopContext.getExtensionsForNominalType(SD)) {
@@ -922,6 +927,7 @@ class DeclAndTypePrinter::Implementation
           os << "  }\n"; // operator cases()'s closing bracket
           os << "\n";
 
+          CxxEmissionScopeRAII cxxScopeRAII(owningPrinter);
           printMembers(ED->getAllMembers());
 
           for (const auto *ext :

test/Interop/SwiftToCxx/initializers/init-in-extension-in-cxx.swift

@@ -0,0 +1,22 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -module-name Init -clang-header-expose-decls=all-public -typecheck -verify -emit-clang-header-path %t/inits.h
+// RUN: %FileCheck %s < %t/inits.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/inits.h -Wno-unused-function -DSWIFT_CXX_INTEROP_HIDE_STL_OVERLAY)
+
+public struct OverloadedInitInExtension {
+    let x: Int
+
+    public init(_ x: Int) {
+        self.x = x
+    }
+}
+extension OverloadedInitInExtension {
+    public init(viaExtension x: Int) {
+        self.x = x
+    }
+}
+
+// Make sure we don't emit ambiguous overloads:
+// CHECK: static SWIFT_INLINE_THUNK OverloadedInitInExtension init(swift::Int x)
+// CHECK-NOT: static SWIFT_INLINE_THUNK OverloadedInitInExtension init(swift::Int x)

test/Interop/SwiftToCxx/methods/method-in-extension-in-cxx.swift

@@ -0,0 +1,22 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -module-name Method -clang-header-expose-decls=all-public -typecheck -verify -emit-clang-header-path %t/methods.h
+// RUN: %FileCheck %s < %t/methods.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/methods.h -Wno-unused-function -DSWIFT_CXX_INTEROP_HIDE_STL_OVERLAY)
+
+public struct OverloadedMethodInExtension {
+    var x: Int
+
+    public mutating func add(_ x: Int) {
+        self.x += x
+    }
+}
+extension OverloadedMethodInExtension {
+    public mutating func add(viaExtension x: Int) {
+        self.x += x
+    }
+}
+
+// Make sure we don't emit ambiguous overloads:
+// CHECK: SWIFT_INLINE_THUNK void OverloadedMethodInExtension::add(swift::Int x)
+// CHECK-NOT: SWIFT_INLINE_THUNK void OverloadedMethodInExtension::add(swift::Int x)