[cxx-interop][windows] additional fix for indirect return in methods for MSVC ABI

Fixes #67685

Author: hyp

lib/IRGen/GenCall.cpp

@@ -1424,22 +1424,7 @@ void SignatureExpansion::expandExternalSignatureTypes() {
                                   attrKindForExtending(signExt));
   }
 
-  // If we return indirectly, that is the first parameter type.
-  if (returnInfo.isIndirect()) {
-    addIndirectResult();
-  }
-
-  size_t firstParamToLowerNormally = 0;
-
-  // Use a special IR type for passing block pointers.
-  if (FnType->getRepresentation() == SILFunctionTypeRepresentation::Block) {
-    assert(FI.arg_begin()[0].info.isDirect() &&
-           "block pointer not passed directly?");
-    ParamIRTypes.push_back(IGM.ObjCBlockPtrTy);
-    firstParamToLowerNormally = 1;
-  }
-
-  for (auto i : indices(paramTys).slice(firstParamToLowerNormally)) {
+  auto emitArg = [&](size_t i) {
     auto &AI = FI.arg_begin()[i].info;
 
     // Add a padding argument if required.
@@ -1528,8 +1513,35 @@ void SignatureExpansion::expandExternalSignatureTypes() {
     case clang::CodeGen::ABIArgInfo::InAlloca:
       llvm_unreachable("Need to handle InAlloca during signature expansion");
     }
+  };
+
+  size_t firstParamToLowerNormally = 0;
+
+  // If we return indirectly, that is the first parameter type.
+  if (returnInfo.isIndirect()) {
+    if (IGM.Triple.isWindowsMSVCEnvironment() &&
+        FnType->getRepresentation() ==
+            SILFunctionTypeRepresentation::CXXMethod) {
+      // Windows ABI places `this` before the
+      // returned indirect values.
+      emitArg(0);
+      firstParamToLowerNormally = 1;
+      addIndirectResult();
+    } else
+      addIndirectResult();
   }
 
+  // Use a special IR type for passing block pointers.
+  if (FnType->getRepresentation() == SILFunctionTypeRepresentation::Block) {
+    assert(FI.arg_begin()[0].info.isDirect() &&
+           "block pointer not passed directly?");
+    ParamIRTypes.push_back(IGM.ObjCBlockPtrTy);
+    firstParamToLowerNormally = 1;
+  }
+
+  for (auto i : indices(paramTys).slice(firstParamToLowerNormally))
+    emitArg(i);
+
   if (returnInfo.isIndirect() || returnInfo.isIgnore()) {
     ResultIRType = IGM.VoidTy;
   } else {

test/Interop/Cxx/stdlib/Inputs/module.modulemap

@@ -27,3 +27,9 @@ module StdPair {
   header "std-pair.h"
   requires cplusplus
 }
+
+module MsvcUseVecIt {
+  header "msvc-std-vector-it.h"
+  requires cplusplus
+  export *
+}

test/Interop/Cxx/stdlib/Inputs/msvc-std-vector-it.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <vector>
+#include <string>
+
+struct FetchProvidersResult {
+  std::vector<std::string> providers;
+};
+
+inline FetchProvidersResult *f() noexcept {
+    return new FetchProvidersResult();
+}

test/Interop/Cxx/stdlib/msvc-abi-use-vector-iterator.swift

@@ -0,0 +1,15 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -std=c++17)
+
+// REQUIRES: OS=windows-msvc
+// REQUIRES: executable_test
+
+import MsvcUseVecIt
+
+func test() -> Bool {
+    let result = f()
+    let begin = result.pointee.providers.__beginUnsafe()
+    let end = result.pointee.providers.__endUnsafe()
+    return begin != end
+}
+
+let _ = test()