Don't put an sret attribute on the this argument.

Author: martinboehme

lib/IRGen/GenCall.cpp

@@ -1435,8 +1435,13 @@ void SignatureExpansion::expandExternalSignatureTypes() {
   if (formalIndirectResult) {
     // If the result is a formal indirect result in SIL, that means that the
     // Clang function has an explicit output parameter (e.g. it's a C++
-    // constructor), which it might capture -- so don't specify "nocapture".
-    addIndirectResultAttributes(IGM, Attrs, 0, claimSRet(), /* nocapture = */ false);
+    // constructor). This means:
+    // - Don't mark it `sret`, as this should only be used for C++ return
+    //   values.
+    // - The Clang function might capture the pointer, so don't specify
+    //   `nocapture`.
+    addIndirectResultAttributes(IGM, Attrs, 0, /* allowSRet = */ false,
+                                /* noCapture = */ false);
   }
 
   if (returnInfo.isIndirect() || returnInfo.isIgnore()) {

test/Interop/Cxx/class/constructors-ir.swift

@@ -11,20 +11,21 @@ typealias Void = ()
 public func createHasVirtualBase() -> HasVirtualBase {
   // - The `this` parameter should carry a `noalias` attribute, as it is
   //   guaranteed that nothing will alias the object before it has been fully
-  //   constructed. It should also carry an `sret` attribute to indicate that
-  //   this is an out parameter for a structure being returned by the function.
-  //   Note that this doesn't apply on ABIs (Itanium ARM, Microsoft x64)
-  //   where we insert an (inlined) thunk; in this case, we're getting the
-  //   attributes of the constructor that was generated by Clang, which doesn't
-  //   insert these attributes.
+  //   constructed. Note that this doesn't apply on ABIs (Itanium ARM,
+  //   Microsoft x64) where we insert an (inlined) thunk; in this case, we're
+  //   getting the attributes of the constructor that was generated by Clang,
+  //   which doesn't insert these attributes.
+  //
+  // - The `this` parameter should _not_ carry an `sret` attribute because the
+  //   constructor doesn't return the constructed object as a return value.
   //
   // - The `this` parameter should _not_ carry a `nocapture` attribute (unlike
   //   Swift constructors that return their result indirectly) because the C++
   //   constructor has explicit access to `this` and may capture it.
   //
   // ITANIUM_X64: define swiftcc void @"$ss20createHasVirtualBaseSo0bcD0VyF"(%TSo14HasVirtualBaseV* noalias nocapture sret %0)
   // ITANIUM_X64-NOT: define
-  // ITANIUM_X64: call void @_ZN14HasVirtualBaseC1E7ArgType(%struct.HasVirtualBase* noalias sret %{{[0-9]+}}, i32 %{{[0-9]+}})
+  // ITANIUM_X64: call void @_ZN14HasVirtualBaseC1E7ArgType(%struct.HasVirtualBase* noalias %{{[0-9]+}}, i32 %{{[0-9]+}})
   //
   // ITANIUM_ARM: define protected swiftcc void @"$ss20createHasVirtualBaseSo0bcD0VyF"(%TSo14HasVirtualBaseV* noalias nocapture sret %0)
   // To verify that the thunk is inlined, make sure there's no intervening