[clang] fix oops: enable implicit moves in MSVC compatibility mode

[mizvekov]

Summary

When disabling simpler implicit moves in MSVC compatibility mode as
a workaround in D105518, we forgot to make the opposite change and
enable regular (P1825) implicit moves in the same mode.

As a result, we were not doing any implicit moves at all. OOPS!

This fixes it and adds test for this.

This is a fix to a temporary workaround, there is ongoing
work to replace this, applying the workaround only to
system headers and the ::stl namespace.

Signed-off-by: Matheus Izvekov <[e-mail]>

[aaron.ballman]

Oops! Good catch, LGTM!

clang/lib/Sema/SemaStmt.cpp

 ///
 /// This routine implements C++20 [class.copy.elision]p3, which attempts to
 /// treat returned lvalues as rvalues in certain cases (to prefer move
 /// construction), then falls back to treating them as lvalues if that failed.
 ExprResult
 Sema::PerformMoveOrCopyInitialization(const InitializedEntity &Entity,
                                       const NamedReturnInfo &NRInfo,
                                       Expr *Value) {
-  if (!getLangOpts().CPlusPlus2b && NRInfo.isMoveEligible()) {
+  // FIXME: We force P1825 implicit moves here in msvc compatibility mode
+  // because we are disabling simpler implicit moves as a temporary
+  // work around, as the MSVC STL has issues with this change.
+  // We will come back later with a more targeted approach.
+  if ((!getLangOpts().CPlusPlus2b || getLangOpts().MSVCCompat) &&
+      NRInfo.isMoveEligible()) {
     ImplicitCastExpr AsRvalue(ImplicitCastExpr::OnStack, Value->getType(),
                               CK_NoOp, Value, VK_XValue, FPOptionsOverride());
     Expr *InitExpr = &AsRvalue;
     auto Kind = InitializationKind::CreateCopy(Value->getBeginLoc(),
                                                Value->getBeginLoc());
     InitializationSequence Seq(*this, Entity, Kind, InitExpr);
     auto Res = Seq.getFailedOverloadResult();
     if ((Res == OR_Success || Res == OR_Deleted) &&

clang/test/SemaCXX/cxx2b-p2266-disable-with-msvc-compat.cpp

MoveOnly &&test4() {
   return x; // old-error {{cannot bind to lvalue of type}}
 }
 
 void test5() try {
   CopyOnly x;
   throw x; // new-error {{no matching constructor for initialization}}
 } catch (...) {
 }
+
+MoveOnly test6(MoveOnly x) { return x; }