Revert "[Clang] [AST] Fix placeholder return type name mangling for MSVC 1920+ / VS2019+ (#102848)"

It cause builds to start failing with

  Invalid type expected
  UNREACHABLE executed at clang/lib/AST/MicrosoftMangle.cpp:2551!

see comments on the PR.

> Partial fix for #92204.
> This PR just fixes VS2019+ since that is the suite of compilers that I
> require link compatibility with at the moment.
> I still intend to fix VS2017 and to update llvm-undname in future PRs.
> Once those are also finished and merged I'll close out
> #92204.
> I am hoping to get the llvm-undname PR up in a couple of weeks to be
> able to demangle the VS2019+ name mangling.
>
> MSVC 1920+ mangles placeholder return types for non-templated functions
> with "@".
> For example `auto foo() { return 0; }` is mangled as `?foo@@ya@XZ`.
>
> MSVC 1920+ mangles placeholder return types for templated functions as
> the qualifiers of the AutoType followed by "_P" for `auto` and "_T" for
> `decltype(auto)`.
> For example `template<class T> auto foo() { return 0; }` is mangled as
> `??$foo@H@@ya?A_PXZ` when `foo` is instantiated as follows `foo<int>()`.
>
> Lambdas with placeholder return types are still mangled with clang's
> custom mangling since MSVC lambda mangling hasn't been deciphered yet.
> Similarly any pointers in the return type with an address space are
> mangled with clang's custom mangling since that is a clang extension.
>
> We cannot augment `mangleType` to support this mangling scheme as the
> mangling schemes for variables and functions differ.
> auto variables are encoded with the fully deduced type where auto return
> types are not.
> The following two functions with a static variable are mangled the same
> ```
> template<class T>
> int test()
> {
>     static int i = 0; // "?i@?1???$test@H@@yahxz@4HA"
>     return i;
> }
>
> template<class T>
> int test()
> {
>     static auto i = 0; // "?i@?1???$test@H@@yahxz@4HA"
>     return i;
> }
> ```
> Inside `mangleType` once we get to mangling the `AutoType` we have no
> context if we are from a variable encoding or some other encoding.
> Therefore it was easier to handle any special casing for `AutoType`
> return types with a separate function instead of using the `mangleType`
> infrastructure.

This reverts commit e0d173d
and the wollow-up fa343be.

Author: zmodem

clang/docs/ReleaseNotes.rst

@@ -77,8 +77,6 @@ C++ Specific Potentially Breaking Changes
 ABI Changes in This Version
 ---------------------------
 
-- Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.
-
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 

clang/lib/AST/MicrosoftMangle.cpp

@@ -408,8 +408,6 @@ class MicrosoftCXXNameMangler {
   void mangleSourceName(StringRef Name);
   void mangleNestedName(GlobalDecl GD);
 
-  void mangleAutoReturnType(QualType T, QualifierMangleMode QMM);
-
 private:
   bool isStructorDecl(const NamedDecl *ND) const {
     return ND == Structor || getStructor(ND) == Structor;
@@ -479,11 +477,6 @@ class MicrosoftCXXNameMangler {
                           SourceRange Range);
   void mangleObjCKindOfType(const ObjCObjectType *T, Qualifiers Quals,
                             SourceRange Range);
-
-  void mangleAutoReturnType(const MemberPointerType *T, Qualifiers Quals);
-  void mangleAutoReturnType(const PointerType *T, Qualifiers Quals);
-  void mangleAutoReturnType(const LValueReferenceType *T, Qualifiers Quals);
-  void mangleAutoReturnType(const RValueReferenceType *T, Qualifiers Quals);
 };
 }
 
@@ -2501,57 +2494,6 @@ void MicrosoftCXXNameMangler::mangleAddressSpaceType(QualType T,
   mangleArtificialTagType(TagTypeKind::Struct, ASMangling, {"__clang"});
 }
 
-void MicrosoftCXXNameMangler::mangleAutoReturnType(QualType T,
-                                                   QualifierMangleMode QMM) {
-  assert(getASTContext().getLangOpts().isCompatibleWithMSVC(
-             LangOptions::MSVC2019) &&
-         "Cannot mangle MSVC 2017 auto return types!");
-
-  if (isa<AutoType>(T)) {
-    const auto *AT = T->getContainedAutoType();
-    Qualifiers Quals = T.getLocalQualifiers();
-
-    if (QMM == QMM_Result)
-      Out << '?';
-    if (QMM != QMM_Drop)
-      mangleQualifiers(Quals, false);
-    Out << (AT->isDecltypeAuto() ? "_T" : "_P");
-    return;
-  }
-
-  T = T.getDesugaredType(getASTContext());
-  Qualifiers Quals = T.getLocalQualifiers();
-
-  switch (QMM) {
-  case QMM_Drop:
-  case QMM_Result:
-    break;
-  case QMM_Mangle:
-    mangleQualifiers(Quals, false);
-    break;
-  default:
-    llvm_unreachable("QMM_Escape unexpected");
-  }
-
-  const Type *ty = T.getTypePtr();
-  switch (ty->getTypeClass()) {
-  case Type::MemberPointer:
-    mangleAutoReturnType(cast<MemberPointerType>(ty), Quals);
-    break;
-  case Type::Pointer:
-    mangleAutoReturnType(cast<PointerType>(ty), Quals);
-    break;
-  case Type::LValueReference:
-    mangleAutoReturnType(cast<LValueReferenceType>(ty), Quals);
-    break;
-  case Type::RValueReference:
-    mangleAutoReturnType(cast<RValueReferenceType>(ty), Quals);
-    break;
-  default:
-    llvm_unreachable("Invalid type expected");
-  }
-}
-
 void MicrosoftCXXNameMangler::mangleType(QualType T, SourceRange Range,
                                          QualifierMangleMode QMM) {
   // Don't use the canonical types.  MSVC includes things like 'const' on
@@ -2965,52 +2907,17 @@ void MicrosoftCXXNameMangler::mangleFunctionType(const FunctionType *T,
       // can differ by their calling convention and are typically deduced.  So
       // we make sure that this type gets mangled properly.
       mangleType(ResultType, Range, QMM_Result);
-    } else if (IsInLambda) {
-      if (const auto *AT = ResultType->getContainedAutoType()) {
-        assert(AT->getKeyword() == AutoTypeKeyword::Auto &&
-               "should only need to mangle auto!");
-        (void)AT;
-        Out << '?';
-        mangleQualifiers(ResultType.getLocalQualifiers(), /*IsMember=*/false);
-        Out << '?';
-        mangleSourceName("<auto>");
-        Out << '@';
-      } else {
-        Out << '@';
-      }
-    } else if (const auto *AT = ResultType->getContainedAutoType()) {
+    } else if (const auto *AT = dyn_cast_or_null<AutoType>(
+                   ResultType->getContainedAutoType())) {
+      Out << '?';
+      mangleQualifiers(ResultType.getLocalQualifiers(), /*IsMember=*/false);
+      Out << '?';
       assert(AT->getKeyword() != AutoTypeKeyword::GNUAutoType &&
              "shouldn't need to mangle __auto_type!");
-
-      // If we have any pointer types with the clang address space extension
-      // then defer to the custom clang mangling to keep backwards
-      // compatibility. See `mangleType(const PointerType *T, Qualifiers Quals,
-      // SourceRange Range)` for details.
-      auto UseClangMangling = [](QualType ResultType) {
-        QualType T = ResultType;
-        while (isa<PointerType>(T.getTypePtr())) {
-          T = T->getPointeeType();
-          if (T.getQualifiers().hasAddressSpace())
-            return true;
-        }
-        return false;
-      };
-
-      if (getASTContext().getLangOpts().isCompatibleWithMSVC(
-              LangOptions::MSVC2019) &&
-          !UseClangMangling(ResultType)) {
-        if (D && !D->getPrimaryTemplate()) {
-          Out << '@';
-        } else {
-          mangleAutoReturnType(ResultType, QMM_Result);
-        }
-      } else {
-        Out << '?';
-        mangleQualifiers(ResultType.getLocalQualifiers(), /*IsMember=*/false);
-        Out << '?';
-        mangleSourceName(AT->isDecltypeAuto() ? "<decltype-auto>" : "<auto>");
-        Out << '@';
-      }
+      mangleSourceName(AT->isDecltypeAuto() ? "<decltype-auto>" : "<auto>");
+      Out << '@';
+    } else if (IsInLambda) {
+      Out << '@';
     } else {
       if (ResultType->isVoidType())
         ResultType = ResultType.getUnqualifiedType();
@@ -4313,57 +4220,6 @@ void MicrosoftMangleContextImpl::mangleStringLiteral(const StringLiteral *SL,
   Mangler.getStream() << '@';
 }
 
-void MicrosoftCXXNameMangler::mangleAutoReturnType(const MemberPointerType *T,
-                                                   Qualifiers Quals) {
-  QualType PointeeType = T->getPointeeType();
-  manglePointerCVQualifiers(Quals);
-  manglePointerExtQualifiers(Quals, PointeeType);
-  if (const FunctionProtoType *FPT = PointeeType->getAs<FunctionProtoType>()) {
-    Out << '8';
-    mangleName(T->getClass()->castAs<RecordType>()->getDecl());
-    mangleFunctionType(FPT, nullptr, true);
-  } else {
-    mangleQualifiers(PointeeType.getQualifiers(), true);
-    mangleName(T->getClass()->castAs<RecordType>()->getDecl());
-    mangleAutoReturnType(PointeeType, QMM_Drop);
-  }
-}
-
-void MicrosoftCXXNameMangler::mangleAutoReturnType(const PointerType *T,
-                                                   Qualifiers Quals) {
-  QualType PointeeType = T->getPointeeType();
-  assert(!PointeeType.getQualifiers().hasAddressSpace() &&
-         "Unexpected address space mangling required");
-
-  manglePointerCVQualifiers(Quals);
-  manglePointerExtQualifiers(Quals, PointeeType);
-
-  if (const FunctionProtoType *FPT = PointeeType->getAs<FunctionProtoType>()) {
-    Out << '6';
-    mangleFunctionType(FPT);
-  } else {
-    mangleAutoReturnType(PointeeType, QMM_Mangle);
-  }
-}
-
-void MicrosoftCXXNameMangler::mangleAutoReturnType(const LValueReferenceType *T,
-                                                   Qualifiers Quals) {
-  QualType PointeeType = T->getPointeeType();
-  assert(!Quals.hasConst() && !Quals.hasVolatile() && "unexpected qualifier!");
-  Out << 'A';
-  manglePointerExtQualifiers(Quals, PointeeType);
-  mangleAutoReturnType(PointeeType, QMM_Mangle);
-}
-
-void MicrosoftCXXNameMangler::mangleAutoReturnType(const RValueReferenceType *T,
-                                                   Qualifiers Quals) {
-  QualType PointeeType = T->getPointeeType();
-  assert(!Quals.hasConst() && !Quals.hasVolatile() && "unexpected qualifier!");
-  Out << "$$Q";
-  manglePointerExtQualifiers(Quals, PointeeType);
-  mangleAutoReturnType(PointeeType, QMM_Mangle);
-}
-
 MicrosoftMangleContext *MicrosoftMangleContext::create(ASTContext &Context,
                                                        DiagnosticsEngine &Diags,
                                                        bool IsAux) {