[clang] d052a57 - [c++2a] Allow comparison functions to be explicitly defaulted.

Richard Smith via cfe-commits cfe-commits at lists.llvm.org
Tue Mar 10 16:35:03 PDT 2020


Should be fixed in llvmorg-11-init-5426-g4cba668ac13.

On Sat, 7 Mar 2020 at 08:05, Hubert Tong via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> Following this commit, the error recovery for invalid cases that
> explicitly define (out-of-line) a member function template as deleted and
> attempts to instantiate said function appears broken.
>
> <stdin>:4:35: error: deleted definition must be first declaration
> template <typename> void A::f() = delete;
>                                   ^
> <stdin>:2:35: note: previous declaration is here
>   template <typename> static void f();
>                                   ^
> clang:
> /src_d052a578de58cbbb638cbe2dba05242d1ff443b9/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp:4288:
> void clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation,
> clang::FunctionDecl *, bool, bool, bool): Assertion `(Pattern ||
> PatternDecl->isDefaulted() || PatternDecl->hasSkippedBody()) && "unexpected
> kind of function template definition"' failed.
> Stack dump:
> 0.      Program arguments:
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/clang -cc1 -std=c++11
> -xc++ -
> 1.      <stdin>:5:26: current parser token ';'
>  #0 0x00003fff7fe6a024 PrintStackTraceSignalHandler(void*)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libLLVMSupport.so.10svn+0x1ea024)
>  #1 0x00003fff7fe670c8 llvm::sys::RunSignalHandlers()
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libLLVMSupport.so.10svn+0x1e70c8)
>  #2 0x00003fff7fe6a49c SignalHandler(int)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libLLVMSupport.so.10svn+0x1ea49c)
>  #3 0x00003fff82030478  0x478 abort
>  #4 0x00003fff82030478
>  #5 0x00003fff82030478 __assert_fail_base (+0x478)
>  #6 0x00003fff7e0a1f94 __assert_fail (/lib64/libc.so.6+0x41f94)
>  #7 0x00003fff7e0955d4
> clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation,
> clang::FunctionDecl*, bool, bool, bool) (/lib64/libc.so.6+0x355d4)
>  #8 0x00003fff7e0956c4
> clang::Sema::ActOnExplicitInstantiation(clang::Scope*,
> clang::SourceLocation, clang::SourceLocation, clang::Declarator&)
> (/lib64/libc.so.6+0x356c4)
>  #9 0x00003fff7c28d604
> clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&,
> clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangSema.so.10svn+0x8ad604)
> #10 0x00003fff7c15c2b0
> clang::Parser::ParseDeclarationAfterDeclarator(clang::Declarator&,
> clang::Parser::ParsedTemplateInfo const&)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangSema.so.10svn+0x77c2b0)
> #11 0x00003fff7c4cc8f8
> clang::Parser::ParseSingleDeclarationAfterTemplate(clang::DeclaratorContext,
> clang::Parser::ParsedTemplateInfo const&, clang::ParsingDeclRAIIObject&,
> clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0x4c8f8)
> #12 0x00003fff7c4cdf48
> clang::Parser::ParseExplicitInstantiation(clang::DeclaratorContext,
> clang::SourceLocation, clang::SourceLocation, clang::SourceLocation&,
> clang::ParsedAttributes&, clang::AccessSpecifier)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0x4df48)
> #13 0x00003fff7c57c1f0
> clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext,
> clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0xfc1f0)
> #14 0x00003fff7c57a6c0
> clang::Parser::ParseDeclaration(clang::DeclaratorContext,
> clang::SourceLocation&, clang::Parser::ParsedAttributesWithRange&,
> clang::SourceLocation*)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0xfa6c0)
> #15 0x00003fff7c57a4f8
> clang::Parser::ParseExternalDeclaration(clang::Parser::ParsedAttributesWithRange&,
> clang::ParsingDeclSpec*)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0xfa4f8)
> #16 0x00003fff7c4c5db0
> clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&,
> bool)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0x45db0)
> #17 0x00003fff7c58fffc clang::ParseAST(clang::Sema&, bool, bool)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0x10fffc)
> #18 0x00003fff7c58def4 clang::ASTFrontendAction::ExecuteAction()
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0x10def4)
> #19 0x00003fff7c4b01e0 clang::FrontendAction::Execute()
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn+0x301e0)
> #20 0x00003fff7e93d57c
> clang::CompilerInstance::ExecuteAction(clang::FrontendAction&)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontend.so.10svn+0x10d57c)
> #21 0x00003fff7e93cbf0
> clang::ExecuteCompilerInvocation(clang::CompilerInstance*)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontend.so.10svn+0x10cbf0)
> #22 0x00003fff7e8d5bd4 cc1_main(llvm::ArrayRef<char const*>, char const*,
> void*)
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontend.so.10svn+0xa5bd4)
> #23 0x00003fff7e8042f0 main
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontendTool.so.10svn+0x42f0)
> #24 0x0000000010012594 generic_start_main.isra.0
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/clang+0x10012594)
> #25 0x000000001000f37c __libc_start_main
> (/build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/clang+0x1000f37c)
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libLLVMSupport.so.10svn(+0x1ea024)[0x3fff7fe6a024]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libLLVMSupport.so.10svn(_ZN4llvm3sys17RunSignalHandlersEv+0xc8)[0x3fff7fe670c8]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libLLVMSupport.so.10svn(+0x1ea49c)[0x3fff7fe6a49c]
> [0x3fff82030478]
> /lib64/libc.so.6(abort+0x2b4)[0x3fff7e0a1f94]
> /lib64/libc.so.6(+0x355d4)[0x3fff7e0955d4]
> /lib64/libc.so.6(__assert_fail+0x64)[0x3fff7e0956c4]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangSema.so.10svn(_ZN5clang4Sema29InstantiateFunctionDefinitionENS_14SourceLocationEPNS_12FunctionDeclEbbb+0x1244)[0x3fff7c28d604]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangSema.so.10svn(_ZN5clang4Sema26ActOnExplicitInstantiationEPNS_5ScopeENS_14SourceLocationES3_RNS_10DeclaratorE+0x2290)[0x3fff7c15c2b0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser44ParseDeclarationAfterDeclaratorAndAttributesERNS_10DeclaratorERKNS0_18ParsedTemplateInfoEPNS0_12ForRangeInitE+0xf8)[0x3fff7c4cc8f8]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser31ParseDeclarationAfterDeclaratorERNS_10DeclaratorERKNS0_18ParsedTemplateInfoE+0x98)[0x3fff7c4cdf48]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser35ParseSingleDeclarationAfterTemplateENS_17DeclaratorContextERKNS0_18ParsedTemplateInfoERNS_21ParsingDeclRAIIObjectERNS_14SourceLocationERNS_16ParsedAttributesENS_15AccessSpecifierE+0x950)[0x3fff7c57c1f0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser26ParseExplicitInstantiationENS_17DeclaratorContextENS_14SourceLocationES2_RS2_RNS_16ParsedAttributesENS_15AccessSpecifierE+0x80)[0x3fff7c57a6c0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser36ParseDeclarationStartingWithTemplateENS_17DeclaratorContextERNS_14SourceLocationERNS_16ParsedAttributesENS_15AccessSpecifierE+0x158)[0x3fff7c57a4f8]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser16ParseDeclarationENS_17DeclaratorContextERNS_14SourceLocationERNS0_25ParsedAttributesWithRangeEPS2_+0x350)[0x3fff7c4c5db0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser24ParseExternalDeclarationERNS0_25ParsedAttributesWithRangeEPNS_15ParsingDeclSpecE+0x2bc)[0x3fff7c58fffc]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang6Parser17ParseTopLevelDeclERNS_9OpaquePtrINS_12DeclGroupRefEEEb+0x614)[0x3fff7c58def4]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/../lib/libclangParse.so.10svn(_ZN5clang8ParseASTERNS_4SemaEbb+0x2c0)[0x3fff7c4b01e0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontend.so.10svn(_ZN5clang17ASTFrontendAction13ExecuteActionEv+0xdc)[0x3fff7e93d57c]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontend.so.10svn(_ZN5clang14FrontendAction7ExecuteEv+0x150)[0x3fff7e93cbf0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontend.so.10svn(_ZN5clang16CompilerInstance13ExecuteActionERNS_14FrontendActionE+0x714)[0x3fff7e8d5bd4]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/../lib/libclangFrontendTool.so.10svn(_ZN5clang25ExecuteCompilerInvocationEPNS_16CompilerInstanceE+0x830)[0x3fff7e8042f0]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/clang(_Z8cc1_mainN4llvm8ArrayRefIPKcEES2_Pv+0x674)[0x10012594]
>
> /build_d052a578de58cbbb638cbe2dba05242d1ff443b9/bin/clang(main+0x321c)[0x1000f37c]
> /lib64/libc.so.6(+0x25100)[0x3fff7e085100]
> /lib64/libc.so.6(__libc_start_main+0xc4)[0x3fff7e0852f4]
>
>
> On Tue, Oct 22, 2019 at 9:18 PM Richard Smith via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>>
>> Author: Richard Smith
>> Date: 2019-10-22T18:16:17-07:00
>> New Revision: d052a578de58cbbb638cbe2dba05242d1ff443b9
>>
>> URL:
>> https://github.com/llvm/llvm-project/commit/d052a578de58cbbb638cbe2dba05242d1ff443b9
>> DIFF:
>> https://github.com/llvm/llvm-project/commit/d052a578de58cbbb638cbe2dba05242d1ff443b9.diff
>>
>> LOG: [c++2a] Allow comparison functions to be explicitly defaulted.
>>
>> This adds some initial syntactic checking that only the appropriate
>> function signatures can be defaulted. No implicit definitions are
>> generated yet.
>>
>> Added:
>>     clang/test/CXX/class/class.compare/class.compare.default/p1.cpp
>>     clang/test/CXX/class/class.compare/class.eq/p1.cpp
>>     clang/test/CXX/class/class.compare/class.rel/p1.cpp
>>
>> Modified:
>>     clang/include/clang/AST/Decl.h
>>     clang/include/clang/Basic/DiagnosticCommonKinds.td
>>     clang/include/clang/Basic/DiagnosticSemaKinds.td
>>     clang/include/clang/Sema/Sema.h
>>     clang/lib/AST/Decl.cpp
>>     clang/lib/Parse/ParseDecl.cpp
>>     clang/lib/Parse/ParseDeclCXX.cpp
>>     clang/lib/Sema/SemaDecl.cpp
>>     clang/lib/Sema/SemaDeclCXX.cpp
>>     clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
>>     clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p1.cpp
>>     clang/test/Parser/cxx0x-decl.cpp
>>     clang/test/SemaCXX/cxx0x-defaulted-functions.cpp
>>     clang/test/SemaCXX/cxx17-compat.cpp
>>
>> Removed:
>>
>>
>>
>>
>> ################################################################################
>> diff  --git a/clang/include/clang/AST/Decl.h
>> b/clang/include/clang/AST/Decl.h
>> index ce674e09c44d..b3e7a570fd6d 100644
>> --- a/clang/include/clang/AST/Decl.h
>> +++ b/clang/include/clang/AST/Decl.h
>> @@ -59,6 +59,7 @@ class EnumDecl;
>>  class Expr;
>>  class FunctionTemplateDecl;
>>  class FunctionTemplateSpecializationInfo;
>> +class FunctionTypeLoc;
>>  class LabelStmt;
>>  class MemberSpecializationInfo;
>>  class Module;
>> @@ -2362,6 +2363,12 @@ class FunctionDecl : public DeclaratorDecl,
>>    /// parameters have default arguments (in C++).
>>    unsigned getMinRequiredArguments() const;
>>
>> +  /// Find the source location information for how the type of this
>> function
>> +  /// was written. May be absent (for example if the function was
>> declared via
>> +  /// a typedef) and may contain a
>> diff erent type from that of the function
>> +  /// (for example if the function type was adjusted by an attribute).
>> +  FunctionTypeLoc getFunctionTypeLoc() const;
>> +
>>    QualType getReturnType() const {
>>      return getType()->castAs<FunctionType>()->getReturnType();
>>    }
>>
>> diff  --git a/clang/include/clang/Basic/DiagnosticCommonKinds.td
>> b/clang/include/clang/Basic/DiagnosticCommonKinds.td
>> index 6018c1417789..7a416c282e3d 100644
>> --- a/clang/include/clang/Basic/DiagnosticCommonKinds.td
>> +++ b/clang/include/clang/Basic/DiagnosticCommonKinds.td
>> @@ -87,7 +87,8 @@ def warn_cxx98_compat_variadic_templates :
>>    Warning<"variadic templates are incompatible with C++98">,
>>    InGroup<CXX98Compat>, DefaultIgnore;
>>  def err_default_special_members : Error<
>> -  "only special member functions may be defaulted">;
>> +  "only special member functions %select{|and comparison operators }0"
>> +  "may be defaulted">;
>>  def err_deleted_non_function : Error<
>>    "only functions can have deleted definitions">;
>>  def err_module_not_found : Error<"module '%0' not found">, DefaultFatal;
>>
>> diff  --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td
>> b/clang/include/clang/Basic/DiagnosticSemaKinds.td
>> index d802a92c42c0..f7b98bb9ea86 100644
>> --- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
>> +++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
>> @@ -8099,6 +8099,31 @@ def note_vbase_moved_here : Note<
>>    "%select{%1 is a virtual base class of base class %2 declared here|"
>>    "virtual base class %1 declared here}0">;
>>
>> +// C++20 defaulted comparisons
>> +// This corresponds to values of Sema::DefaultedComparisonKind.
>> +def select_defaulted_comparison_kind : TextSubstitution<
>> +  "%select{<ERROR>|equality|three-way|equality|relational}0 comparison "
>> +  "operator">;
>> +def ext_defaulted_comparison : ExtWarn<
>> +  "defaulted comparison operators are a C++20 extension">,
>> InGroup<CXX2a>;
>> +def warn_cxx17_compat_defaulted_comparison : Warning<
>> +  "defaulted comparison operators are incompatible with C++ standards "
>> +  "before C++20">, InGroup<CXXPre2aCompat>, DefaultIgnore;
>> +def err_defaulted_comparison_template : Error<
>> +  "comparison operator template cannot be defaulted">;
>> +def err_defaulted_comparison_out_of_class : Error<
>> +  "%sub{select_defaulted_comparison_kind}0 can only be defaulted in a
>> class "
>> +  "definition">;
>> +def err_defaulted_comparison_param : Error<
>> +  "invalid parameter type for defaulted
>> %sub{select_defaulted_comparison_kind}0"
>> +  "%
>> diff {; found $, expected $|}1,2">;
>> +def err_defaulted_comparison_non_const : Error<
>> +  "defaulted member %sub{select_defaulted_comparison_kind}0 must be "
>> +  "const-qualified">;
>> +def err_defaulted_comparison_return_type_not_bool : Error<
>> +  "return type for defaulted %sub{select_defaulted_comparison_kind}0 "
>> +  "must be 'bool', not %1">;
>> +
>>  def ext_implicit_exception_spec_mismatch : ExtWarn<
>>    "function previously declared with an %select{explicit|implicit}0
>> exception "
>>    "specification redeclared with an %select{implicit|explicit}0
>> exception "
>>
>> diff  --git a/clang/include/clang/Sema/Sema.h
>> b/clang/include/clang/Sema/Sema.h
>> index a911c61a07f8..3058f862c6ec 100644
>> --- a/clang/include/clang/Sema/Sema.h
>> +++ b/clang/include/clang/Sema/Sema.h
>> @@ -1237,6 +1237,24 @@ class Sema {
>>    /// same special member, we should act as if it is not yet declared.
>>    llvm::SmallPtrSet<SpecialMemberDecl, 4> SpecialMembersBeingDeclared;
>>
>> +  /// Kinds of defaulted comparison operator functions.
>> +  enum class DefaultedComparisonKind {
>> +    /// This is not a defaultable comparison operator.
>> +    None,
>> +    /// This is an operator== that should be implemented as a series of
>> +    /// subobject comparisons.
>> +    Equal,
>> +    /// This is an operator<=> that should be implemented as a series of
>> +    /// subobject comparisons.
>> +    ThreeWay,
>> +    /// This is an operator!= that should be implemented as a rewrite in
>> terms
>> +    /// of a == comparison.
>> +    NotEqual,
>> +    /// This is an <, <=, >, or >= that should be implemented as a
>> rewrite in
>> +    /// terms of a <=> comparison.
>> +    Relational,
>> +  };
>> +
>>    /// The function definitions which were renamed as part of
>> typo-correction
>>    /// to match their respective declarations. We want to keep track of
>> them
>>    /// to ensure that we don't emit a "redefinition" error if we
>> encounter a
>> @@ -2541,7 +2559,52 @@ class Sema {
>>    bool SpecialMemberIsTrivial(CXXMethodDecl *MD, CXXSpecialMember CSM,
>>                                TrivialABIHandling TAH =
>> TAH_IgnoreTrivialABI,
>>                                bool Diagnose = false);
>> -  CXXSpecialMember getSpecialMember(const CXXMethodDecl *MD);
>> +
>> +  /// For a defaulted function, the kind of defaulted function that it
>> is.
>> +  class DefaultedFunctionKind {
>> +    CXXSpecialMember SpecialMember : 8;
>> +    DefaultedComparisonKind Comparison : 8;
>> +
>> +  public:
>> +    DefaultedFunctionKind()
>> +        : SpecialMember(CXXInvalid),
>> Comparison(DefaultedComparisonKind::None) {
>> +    }
>> +    DefaultedFunctionKind(CXXSpecialMember CSM)
>> +        : SpecialMember(CSM), Comparison(DefaultedComparisonKind::None)
>> {}
>> +    DefaultedFunctionKind(DefaultedComparisonKind Comp)
>> +        : SpecialMember(CXXInvalid), Comparison(Comp) {}
>> +
>> +    bool isSpecialMember() const { return SpecialMember != CXXInvalid; }
>> +    bool isComparison() const {
>> +      return Comparison != DefaultedComparisonKind::None;
>> +    }
>> +
>> +    explicit operator bool() const {
>> +      return isSpecialMember() || isComparison();
>> +    }
>> +
>> +    CXXSpecialMember asSpecialMember() const { return SpecialMember; }
>> +    DefaultedComparisonKind asComparison() const { return Comparison; }
>> +
>> +    /// Get the index of this function kind for use in diagnostics.
>> +    unsigned getDiagnosticIndex() const {
>> +      static_assert(CXXInvalid > CXXDestructor,
>> +                    "invalid should have highest index");
>> +      static_assert((unsigned)DefaultedComparisonKind::None == 0,
>> +                    "none should be equal to zero");
>> +      return SpecialMember + (unsigned)Comparison;
>> +    }
>> +  };
>> +
>> +  DefaultedFunctionKind getDefaultedFunctionKind(const FunctionDecl *FD);
>> +
>> +  CXXSpecialMember getSpecialMember(const CXXMethodDecl *MD) {
>> +    return getDefaultedFunctionKind(MD).asSpecialMember();
>> +  }
>> +  DefaultedComparisonKind getDefaultedComparisonKind(const FunctionDecl
>> *FD) {
>> +    return getDefaultedFunctionKind(FD).asComparison();
>> +  }
>> +
>>    void ActOnLastBitfield(SourceLocation DeclStart,
>>                           SmallVectorImpl<Decl *> &AllIvarDecls);
>>    Decl *ActOnIvar(Scope *S, SourceLocation DeclStart,
>> @@ -6361,9 +6424,15 @@ class Sema {
>>                                       StorageClass &SC);
>>    void CheckDeductionGuideTemplate(FunctionTemplateDecl *TD);
>>
>> -  void CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD);
>> +  void CheckExplicitlyDefaultedFunction(FunctionDecl *MD);
>> +
>> +  bool CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
>> +                                             CXXSpecialMember CSM);
>>    void CheckDelayedMemberExceptionSpecs();
>>
>> +  bool CheckExplicitlyDefaultedComparison(FunctionDecl *MD,
>> +                                          DefaultedComparisonKind DCK);
>> +
>>
>>  //===--------------------------------------------------------------------===//
>>    // C++ Derived Classes
>>    //
>>
>> diff  --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
>> index 80235d8496d2..dae4af8bb249 100644
>> --- a/clang/lib/AST/Decl.cpp
>> +++ b/clang/lib/AST/Decl.cpp
>> @@ -3322,12 +3322,14 @@ bool
>> FunctionDecl::doesDeclarationForceExternallyVisibleDefinition() const {
>>    return FoundBody;
>>  }
>>
>> -SourceRange FunctionDecl::getReturnTypeSourceRange() const {
>> +FunctionTypeLoc FunctionDecl::getFunctionTypeLoc() const {
>>    const TypeSourceInfo *TSI = getTypeSourceInfo();
>> -  if (!TSI)
>> -    return SourceRange();
>> -  FunctionTypeLoc FTL =
>> -      TSI->getTypeLoc().IgnoreParens().getAs<FunctionTypeLoc>();
>> +  return TSI ? TSI->getTypeLoc().IgnoreParens().getAs<FunctionTypeLoc>()
>> +             : FunctionTypeLoc();
>> +}
>> +
>> +SourceRange FunctionDecl::getReturnTypeSourceRange() const {
>> +  FunctionTypeLoc FTL = getFunctionTypeLoc();
>>    if (!FTL)
>>      return SourceRange();
>>
>> @@ -3343,15 +3345,8 @@ SourceRange
>> FunctionDecl::getReturnTypeSourceRange() const {
>>  }
>>
>>  SourceRange FunctionDecl::getExceptionSpecSourceRange() const {
>> -  const TypeSourceInfo *TSI = getTypeSourceInfo();
>> -  if (!TSI)
>> -    return SourceRange();
>> -  FunctionTypeLoc FTL =
>> -    TSI->getTypeLoc().IgnoreParens().getAs<FunctionTypeLoc>();
>> -  if (!FTL)
>> -    return SourceRange();
>> -
>> -  return FTL.getExceptionSpecRange();
>> +  FunctionTypeLoc FTL = getFunctionTypeLoc();
>> +  return FTL ? FTL.getExceptionSpecRange() : SourceRange();
>>  }
>>
>>  /// For an inline function definition in C, or for a gnu_inline function
>>
>> diff  --git a/clang/lib/Parse/ParseDecl.cpp
>> b/clang/lib/Parse/ParseDecl.cpp
>> index b248d7582d84..c41eb74a9cf3 100644
>> --- a/clang/lib/Parse/ParseDecl.cpp
>> +++ b/clang/lib/Parse/ParseDecl.cpp
>> @@ -2349,7 +2349,8 @@ Decl
>> *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
>>          Diag(ConsumeToken(),
>> diag::err_default_delete_in_multiple_declaration)
>>            << 0 /* default */;
>>        else
>> -        Diag(ConsumeToken(), diag::err_default_special_members);
>> +        Diag(ConsumeToken(), diag::err_default_special_members)
>> +            << getLangOpts().CPlusPlus2a;
>>      } else {
>>        InitializerScopeRAII InitScope(*this, D, ThisDecl);
>>
>>
>> diff  --git a/clang/lib/Parse/ParseDeclCXX.cpp
>> b/clang/lib/Parse/ParseDeclCXX.cpp
>> index b98ce3e66292..6d4a1a4a4e87 100644
>> --- a/clang/lib/Parse/ParseDeclCXX.cpp
>> +++ b/clang/lib/Parse/ParseDeclCXX.cpp
>> @@ -2978,7 +2978,8 @@ ExprResult Parser::ParseCXXMemberInitializer(Decl
>> *D, bool IsFunction,
>>          Diag(Tok, diag::err_default_delete_in_multiple_declaration)
>>            << 0 /* default */;
>>        else
>> -        Diag(ConsumeToken(), diag::err_default_special_members);
>> +        Diag(ConsumeToken(), diag::err_default_special_members)
>> +            << getLangOpts().CPlusPlus2a;
>>        return ExprError();
>>      }
>>    }
>>
>> diff  --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
>> index 62ec83967bff..6202391ee0b8 100644
>> --- a/clang/lib/Sema/SemaDecl.cpp
>> +++ b/clang/lib/Sema/SemaDecl.cpp
>> @@ -2993,28 +2993,6 @@ struct GNUCompatibleParamWarning {
>>
>>  } // end anonymous namespace
>>
>> -/// getSpecialMember - get the special member enum for a method.
>> -Sema::CXXSpecialMember Sema::getSpecialMember(const CXXMethodDecl *MD) {
>> -  if (const CXXConstructorDecl *Ctor = dyn_cast<CXXConstructorDecl>(MD))
>> {
>> -    if (Ctor->isDefaultConstructor())
>> -      return Sema::CXXDefaultConstructor;
>> -
>> -    if (Ctor->isCopyConstructor())
>> -      return Sema::CXXCopyConstructor;
>> -
>> -    if (Ctor->isMoveConstructor())
>> -      return Sema::CXXMoveConstructor;
>> -  } else if (isa<CXXDestructorDecl>(MD)) {
>> -    return Sema::CXXDestructor;
>> -  } else if (MD->isCopyAssignmentOperator()) {
>> -    return Sema::CXXCopyAssignment;
>> -  } else if (MD->isMoveAssignmentOperator()) {
>> -    return Sema::CXXMoveAssignment;
>> -  }
>> -
>> -  return Sema::CXXInvalid;
>> -}
>> -
>>  // Determine whether the previous declaration was a definition, implicit
>>  // declaration, or a declaration.
>>  template <typename T>
>>
>> diff  --git a/clang/lib/Sema/SemaDeclCXX.cpp
>> b/clang/lib/Sema/SemaDeclCXX.cpp
>> index ff90b9548e29..0201d014e6f2 100644
>> --- a/clang/lib/Sema/SemaDeclCXX.cpp
>> +++ b/clang/lib/Sema/SemaDeclCXX.cpp
>> @@ -6084,6 +6084,67 @@ void Sema::propagateDLLAttrToBaseClassTemplate(
>>    }
>>  }
>>
>> +/// Determine the kind of defaulting that would be done for a given
>> function.
>> +///
>> +/// If the function is both a default constructor and a copy / move
>> constructor
>> +/// (due to having a default argument for the first parameter), this
>> picks
>> +/// CXXDefaultConstructor.
>> +///
>> +/// FIXME: Check that case is properly handled by all callers.
>> +Sema::DefaultedFunctionKind
>> +Sema::getDefaultedFunctionKind(const FunctionDecl *FD) {
>> +  if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
>> +    if (const CXXConstructorDecl *Ctor =
>> dyn_cast<CXXConstructorDecl>(FD)) {
>> +      if (Ctor->isDefaultConstructor())
>> +        return Sema::CXXDefaultConstructor;
>> +
>> +      if (Ctor->isCopyConstructor())
>> +        return Sema::CXXCopyConstructor;
>> +
>> +      if (Ctor->isMoveConstructor())
>> +        return Sema::CXXMoveConstructor;
>> +    }
>> +
>> +    if (MD->isCopyAssignmentOperator())
>> +      return Sema::CXXCopyAssignment;
>> +
>> +    if (MD->isMoveAssignmentOperator())
>> +      return Sema::CXXMoveAssignment;
>> +
>> +    if (isa<CXXDestructorDecl>(FD))
>> +      return Sema::CXXDestructor;
>> +  }
>> +
>> +  switch (FD->getDeclName().getCXXOverloadedOperator()) {
>> +  case OO_EqualEqual:
>> +    return DefaultedComparisonKind::Equal;
>> +
>> +  case OO_ExclaimEqual:
>> +    return DefaultedComparisonKind::NotEqual;
>> +
>> +  case OO_Spaceship:
>> +    // No point allowing this if <=> doesn't exist in the current
>> language mode.
>> +    if (!getLangOpts().CPlusPlus2a)
>> +      break;
>> +    return DefaultedComparisonKind::ThreeWay;
>> +
>> +  case OO_Less:
>> +  case OO_LessEqual:
>> +  case OO_Greater:
>> +  case OO_GreaterEqual:
>> +    // No point allowing this if <=> doesn't exist in the current
>> language mode.
>> +    if (!getLangOpts().CPlusPlus2a)
>> +      break;
>> +    return DefaultedComparisonKind::Relational;
>> +
>> +  default:
>> +    break;
>> +  }
>> +
>> +  // Not defaultable.
>> +  return DefaultedFunctionKind();
>> +}
>> +
>>  static void DefineImplicitSpecialMember(Sema &S, CXXMethodDecl *MD,
>>                                          SourceLocation DefaultLoc) {
>>    switch (S.getSpecialMember(MD)) {
>> @@ -6331,9 +6392,9 @@ void Sema::CheckCompletedCXXClass(CXXRecordDecl
>> *Record) {
>>      Record->setHasTrivialSpecialMemberForCall();
>>
>>    auto CompleteMemberFunction = [&](CXXMethodDecl *M) {
>> -    // Check whether the explicitly-defaulted special members are valid.
>> +    // Check whether the explicitly-defaulted members are valid.
>>      if (!M->isInvalidDecl() && M->isExplicitlyDefaulted())
>> -      CheckExplicitlyDefaultedSpecialMember(M);
>> +      CheckExplicitlyDefaultedFunction(M);
>>
>>      // For an explicitly defaulted or deleted special member, we defer
>>      // determining triviality until the class is complete. That time is
>> now!
>> @@ -6413,6 +6474,15 @@ void Sema::CheckCompletedCXXClass(CXXRecordDecl
>> *Record) {
>>        DiagnoseAbsenceOfOverrideControl(M);
>>    }
>>
>> +  // Process any defaulted friends in the member-specification.
>> +  if (!Record->isDependentType()) {
>> +    for (FriendDecl *D : Record->friends()) {
>> +      auto *FD = dyn_cast_or_null<FunctionDecl>(D->getFriendDecl());
>> +      if (FD && !FD->isInvalidDecl() && FD->isExplicitlyDefaulted())
>> +        CheckExplicitlyDefaultedFunction(FD);
>> +    }
>> +  }
>> +
>>    // ms_struct is a request to use the same ABI rules as MSVC.  Check
>>    // whether this class uses any C++ features that are implemented
>>    // completely
>> diff erently in MSVC, and if so, emit a diagnostic.
>> @@ -6766,9 +6836,22 @@ void
>> Sema::EvaluateImplicitExceptionSpec(SourceLocation Loc, CXXMethodDecl *MD)
>>      UpdateExceptionSpec(MD->getCanonicalDecl(), ESI);
>>  }
>>
>> -void Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
>> +void Sema::CheckExplicitlyDefaultedFunction(FunctionDecl *FD) {
>> +  assert(FD->isExplicitlyDefaulted() && "not explicitly-defaulted");
>> +
>> +  DefaultedFunctionKind DefKind = getDefaultedFunctionKind(FD);
>> +  assert(DefKind && "not a defaultable function");
>> +
>> +  if (DefKind.isSpecialMember()
>> +          ?
>> CheckExplicitlyDefaultedSpecialMember(cast<CXXMethodDecl>(FD),
>> +
>> DefKind.asSpecialMember())
>> +          : CheckExplicitlyDefaultedComparison(FD,
>> DefKind.asComparison()))
>> +    FD->setInvalidDecl();
>> +}
>> +
>> +bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
>> +                                                 CXXSpecialMember CSM) {
>>    CXXRecordDecl *RD = MD->getParent();
>> -  CXXSpecialMember CSM = getSpecialMember(MD);
>>
>>    assert(MD->isExplicitlyDefaulted() && CSM != CXXInvalid &&
>>           "not an explicitly-defaulted special member");
>> @@ -6781,7 +6864,7 @@ void
>> Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
>>
>>    // C++11 [dcl.fct.def.default]p1:
>>    //   A function that is explicitly defaulted shall
>> -  //     -- be a special member function (checked elsewhere),
>> +  //     -- be a special member function [...] (checked elsewhere),
>>    //     -- have the same type (except for ref-qualifiers, and except
>> that a
>>    //        copy operation can take a non-const reference) as an implicit
>>    //        declaration, and
>> @@ -6960,8 +7043,87 @@ void
>> Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
>>      }
>>    }
>>
>> -  if (HadError)
>> -    MD->setInvalidDecl();
>> +  return HadError;
>> +}
>> +
>> +bool Sema::CheckExplicitlyDefaultedComparison(FunctionDecl *FD,
>> +                                              DefaultedComparisonKind
>> DCK) {
>> +  assert(DCK != DefaultedComparisonKind::None && "not a defaulted
>> comparison");
>> +
>> +  // C++2a [class.compare.default]p1:
>> +  //   A defaulted comparison operator function for some class C shall
>> be a
>> +  //   non-template function declared in the member-specification of C
>> that is
>> +  //    -- a non-static const member of C having one parameter of type
>> +  //       const C&, or
>> +  //    -- a friend of C having two parameters of type const C&.
>> +  CXXRecordDecl *RD =
>> dyn_cast<CXXRecordDecl>(FD->getLexicalDeclContext());
>> +  assert(RD && "defaulted comparison is not defaulted in a class");
>> +
>> +  QualType ExpectedParmType =
>> +
>> Context.getLValueReferenceType(Context.getRecordType(RD).withConst());
>> +  for (const ParmVarDecl *Param : FD->parameters()) {
>> +    if (!Context.hasSameType(Param->getType(), ExpectedParmType)) {
>> +      Diag(FD->getLocation(), diag::err_defaulted_comparison_param)
>> +          << (int)DCK << Param->getType() << ExpectedParmType
>> +          << Param->getSourceRange();
>> +      return true;
>> +    }
>> +  }
>> +
>> +  // ... non-static const member ...
>> +  if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
>> +    assert(!MD->isStatic() && "comparison function cannot be a static
>> member");
>> +    if (!MD->isConst()) {
>> +      SourceLocation InsertLoc;
>> +      if (FunctionTypeLoc Loc = MD->getFunctionTypeLoc())
>> +        InsertLoc = getLocForEndOfToken(Loc.getRParenLoc());
>> +      Diag(MD->getLocation(), diag::err_defaulted_comparison_non_const)
>> +        << (int)DCK << FixItHint::CreateInsertion(InsertLoc, " const");
>> +
>> +      // Add the 'const' to the type to recover.
>> +      const auto *FPT = MD->getType()->castAs<FunctionProtoType>();
>> +      FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
>> +      EPI.TypeQuals.addConst();
>> +      MD->setType(Context.getFunctionType(FPT->getReturnType(),
>> +                                          FPT->getParamTypes(), EPI));
>> +    }
>> +  } else {
>> +    // A non-member function declared in a class must be a friend.
>> +    assert(FD->getFriendObjectKind() && "expected a friend declaration");
>> +  }
>> +
>> +  // C++2a [class.compare.default]p2:
>> +  //   A defaulted comparison operator function for class C is defined as
>> +  //   deleted if any non-static data member of C is of reference type
>> or C is
>> +  //   a union-like class.
>> +  // FIXME: Applying this to cases other than == and <=> is unreasonable.
>> +  // FIXME: Implement.
>> +
>> +  // C++2a [class.eq]p1, [class.rel]p1:
>> +  //   A [defaulted comparison other than <=>] shall have a declared
>> return
>> +  //   type bool.
>> +  if (DCK != DefaultedComparisonKind::ThreeWay &&
>> +      !Context.hasSameType(FD->getDeclaredReturnType(), Context.BoolTy))
>> {
>> +    Diag(FD->getLocation(),
>> diag::err_defaulted_comparison_return_type_not_bool)
>> +        << (int)DCK << FD->getDeclaredReturnType() << Context.BoolTy
>> +        << FD->getReturnTypeSourceRange();
>> +    return true;
>> +  }
>> +
>> +  // FIXME: Determine whether the function should be defined as deleted.
>> +
>> +  // C++2a [dcl.fct.def.default]p3:
>> +  //   An explicitly-defaulted function [..] may be declared constexpr or
>> +  //   consteval only if it would have been implicitly declared
>> constexpr.
>> +  // FIXME: There are no rules governing when these should be constexpr,
>> +  // except for the special case of the injected operator==, for which
>> +  // C++2a [class.compare.default]p3 says:
>> +  //   The operator is a constexpr function if its definition would
>> satisfy
>> +  //   the requirements for a constexpr function.
>> +  // FIXME: Apply this rule to all defaulted comparisons. The only way
>> this
>> +  // can fail is if the return type of a defaulted operator<=> is not a
>> literal
>> +  // type.
>> +  return false;
>>  }
>>
>>  void Sema::CheckDelayedMemberExceptionSpecs() {
>> @@ -15006,51 +15168,88 @@ void Sema::SetDeclDeleted(Decl *Dcl,
>> SourceLocation DelLoc) {
>>  }
>>
>>  void Sema::SetDeclDefaulted(Decl *Dcl, SourceLocation DefaultLoc) {
>> -  CXXMethodDecl *MD = dyn_cast_or_null<CXXMethodDecl>(Dcl);
>> +  if (!Dcl || Dcl->isInvalidDecl())
>> +    return;
>>
>> -  if (MD) {
>> -    if (MD->getParent()->isDependentType()) {
>> -      MD->setDefaulted();
>> -      MD->setExplicitlyDefaulted();
>> -      return;
>> +  auto *FD = dyn_cast<FunctionDecl>(Dcl);
>> +  if (!FD) {
>> +    if (auto *FTD = dyn_cast<FunctionTemplateDecl>(Dcl)) {
>> +      if
>> (getDefaultedFunctionKind(FTD->getTemplatedDecl()).isComparison()) {
>> +        Diag(DefaultLoc, diag::err_defaulted_comparison_template);
>> +        return;
>> +      }
>>      }
>>
>> -    CXXSpecialMember Member = getSpecialMember(MD);
>> -    if (Member == CXXInvalid) {
>> -      if (!MD->isInvalidDecl())
>> -        Diag(DefaultLoc, diag::err_default_special_members);
>> -      return;
>> -    }
>> +    Diag(DefaultLoc, diag::err_default_special_members)
>> +        << getLangOpts().CPlusPlus2a;
>> +    return;
>> +  }
>>
>> -    MD->setDefaulted();
>> -    MD->setExplicitlyDefaulted();
>> +  // Reject if this can't possibly be a defaultable function.
>> +  DefaultedFunctionKind DefKind = getDefaultedFunctionKind(FD);
>> +  if (!DefKind &&
>> +      // A dependent function that doesn't locally look defaultable can
>> +      // still instantiate to a defaultable function if it's a
>> constructor
>> +      // or assignment operator.
>> +      (!FD->isDependentContext() ||
>> +       (!isa<CXXConstructorDecl>(FD) &&
>> +        FD->getDeclName().getCXXOverloadedOperator() != OO_Equal))) {
>> +    Diag(DefaultLoc, diag::err_default_special_members)
>> +        << getLangOpts().CPlusPlus2a;
>> +    return;
>> +  }
>>
>> -    // Unset that we will have a body for this function. We might not,
>> -    // if it turns out to be trivial, and we don't need this marking now
>> -    // that we've marked it as defaulted.
>> -    MD->setWillHaveBody(false);
>> +  if (DefKind.isComparison() &&
>> +      !isa<CXXRecordDecl>(FD->getLexicalDeclContext())) {
>> +    Diag(FD->getLocation(), diag::err_defaulted_comparison_out_of_class)
>> +        << (int)DefKind.asComparison();
>> +    return;
>> +  }
>>
>> -    // If this definition appears within the record, do the checking when
>> -    // the record is complete.
>> -    const FunctionDecl *Primary = MD;
>> -    if (const FunctionDecl *Pattern =
>> MD->getTemplateInstantiationPattern())
>> -      // Ask the template instantiation pattern that actually had the
>> -      // '= default' on it.
>> -      Primary = Pattern;
>> +  // Issue compatibility warning. We already warned if the operator is
>> +  // 'operator<=>' when parsing the '<=>' token.
>> +  if (DefKind.isComparison() &&
>> +      DefKind.asComparison() != DefaultedComparisonKind::ThreeWay) {
>> +    Diag(DefaultLoc, getLangOpts().CPlusPlus2a
>> +                         ? diag::warn_cxx17_compat_defaulted_comparison
>> +                         : diag::ext_defaulted_comparison);
>> +  }
>>
>> -    // If the method was defaulted on its first declaration, we will have
>> -    // already performed the checking in CheckCompletedCXXClass. Such a
>> -    // declaration doesn't trigger an implicit definition.
>> -    if (Primary->getCanonicalDecl()->isDefaulted())
>> -      return;
>> +  FD->setDefaulted();
>> +  FD->setExplicitlyDefaulted();
>>
>> -    CheckExplicitlyDefaultedSpecialMember(MD);
>> +  // Defer checking functions that are defaulted in a dependent context.
>> +  if (FD->isDependentContext())
>> +    return;
>>
>> -    if (!MD->isInvalidDecl())
>> -      DefineImplicitSpecialMember(*this, MD, DefaultLoc);
>> -  } else {
>> -    Diag(DefaultLoc, diag::err_default_special_members);
>> -  }
>> +  // Unset that we will have a body for this function. We might not,
>> +  // if it turns out to be trivial, and we don't need this marking now
>> +  // that we've marked it as defaulted.
>> +  FD->setWillHaveBody(false);
>> +
>> +  // If this definition appears within the record, do the checking when
>> +  // the record is complete. This is always the case for a defaulted
>> +  // comparison.
>> +  if (DefKind.isComparison())
>> +    return;
>> +  auto *MD = cast<CXXMethodDecl>(FD);
>> +
>> +  const FunctionDecl *Primary = FD;
>> +  if (const FunctionDecl *Pattern =
>> FD->getTemplateInstantiationPattern())
>> +    // Ask the template instantiation pattern that actually had the
>> +    // '= default' on it.
>> +    Primary = Pattern;
>> +
>> +  // If the method was defaulted on its first declaration, we will have
>> +  // already performed the checking in CheckCompletedCXXClass. Such a
>> +  // declaration doesn't trigger an implicit definition.
>> +  if (Primary->getCanonicalDecl()->isDefaulted())
>> +    return;
>> +
>> +  if (CheckExplicitlyDefaultedSpecialMember(MD,
>> DefKind.asSpecialMember()))
>> +    MD->setInvalidDecl();
>> +  else
>> +    DefineImplicitSpecialMember(*this, MD, DefaultLoc);
>>  }
>>
>>  static void SearchForReturnInStmt(Sema &Self, Stmt *S) {
>>
>> diff  --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
>> b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
>> index d1ad304e62e4..31a4302ba826 100644
>> --- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
>> +++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
>> @@ -2049,6 +2049,11 @@ Decl
>> *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D,
>>      }
>>    }
>>
>> +  if (D->isExplicitlyDefaulted())
>> +    SemaRef.SetDeclDefaulted(Function, D->getLocation());
>> +  if (D->isDeleted())
>> +    SemaRef.SetDeclDeleted(Function, D->getLocation());
>> +
>>    if (Function->isLocalExternDecl() && !Function->getPreviousDecl())
>>      DC->makeDeclVisibleInContext(PrincipalDecl);
>>
>> @@ -2056,7 +2061,6 @@ Decl
>> *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D,
>>        PrincipalDecl->isInIdentifierNamespace(Decl::IDNS_Ordinary))
>>      PrincipalDecl->setNonMemberOperator();
>>
>> -  assert(!D->isDefaulted() && "only methods should be defaulted");
>>    return Function;
>>  }
>>
>> @@ -4016,9 +4020,6 @@ void Sema::InstantiateExceptionSpec(SourceLocation
>> PointOfInstantiation,
>>  bool
>>  TemplateDeclInstantiator::InitFunctionInstantiation(FunctionDecl *New,
>>                                                      FunctionDecl *Tmpl) {
>> -  if (Tmpl->isDeleted())
>> -    New->setDeletedAsWritten();
>> -
>>    New->setImplicit(Tmpl->isImplicit());
>>
>>    // Forward the mangling number from the template to the instantiated
>> decl.
>>
>> diff  --git
>> a/clang/test/CXX/class/class.compare/class.compare.default/p1.cpp
>> b/clang/test/CXX/class/class.compare/class.compare.default/p1.cpp
>> new file mode 100644
>> index 000000000000..1f8d6a2a7cff
>> --- /dev/null
>> +++ b/clang/test/CXX/class/class.compare/class.compare.default/p1.cpp
>> @@ -0,0 +1,46 @@
>> +// RUN: %clang_cc1 -std=c++2a -verify %s
>> +
>> +struct B {};
>> +bool operator==(const B&, const B&) = default; // expected-error
>> {{equality comparison operator can only be defaulted in a class definition}}
>> +bool operator<=>(const B&, const B&) = default; // expected-error
>> {{three-way comparison operator can only be defaulted in a class
>> definition}}
>> +
>> +template<typename T = void>
>> +  bool operator<(const B&, const B&) = default; // expected-error
>> {{comparison operator template cannot be defaulted}}
>> +
>> +struct A {
>> +  friend bool operator==(const A&, const A&) = default;
>> +  friend bool operator!=(const A&, const B&) = default; //
>> expected-error {{invalid parameter type for defaulted equality comparison}}
>> +  friend bool operator!=(const B&, const B&) = default; //
>> expected-error {{invalid parameter type for defaulted equality comparison}}
>> +  friend bool operator<(const A&, const A&);
>> +  friend bool operator<(const B&, const B&) = default; // expected-error
>> {{invalid parameter type for defaulted relational comparison}}
>> +  friend bool operator>(A, A) = default; // expected-error {{invalid
>> parameter type for defaulted relational comparison}}
>> +
>> +  bool operator<(const A&) const;
>> +  bool operator<=(const A&) const = default;
>> +  bool operator==(const A&) const volatile && = default; //
>> surprisingly, OK
>> +  bool operator<=>(const A&) = default; // expected-error {{defaulted
>> member three-way comparison operator must be const-qualified}}
>> +  bool operator>=(const B&) const = default; // expected-error {{invalid
>> parameter type for defaulted relational comparison}}
>> +  static bool operator>(const B&) = default; // expected-error
>> {{overloaded 'operator>' cannot be a static member function}}
>> +
>> +  template<typename T = void>
>> +    friend bool operator==(const A&, const A&) = default; //
>> expected-error {{comparison operator template cannot be defaulted}}
>> +  template<typename T = void>
>> +    bool operator==(const A&) const = default; // expected-error
>> {{comparison operator template cannot be defaulted}}
>> +};
>> +
>> +// FIXME: The wording is not clear as to whether these are valid, but the
>> +// intention is that they are not.
>> +bool operator<(const A&, const A&) = default; // expected-error
>> {{relational comparison operator can only be defaulted in a class
>> definition}}
>> +bool A::operator<(const A&) const = default; // expected-error {{can
>> only be defaulted in a class definition}}
>> +
>> +template<typename T> struct Dependent {
>> +  using U = typename T::type;
>> +  bool operator==(U) const = default; // expected-error {{found
>> 'Dependent<Bad>::U'}}
>> +  friend bool operator==(U, U) = default; // expected-error {{found
>> 'Dependent<Bad>::U'}}
>> +};
>> +
>> +struct Good { using type = const Dependent<Good>&; };
>> +template struct Dependent<Good>;
>> +
>> +struct Bad { using type = Dependent<Bad>&; };
>> +template struct Dependent<Bad>; // expected-note {{in instantiation of}}
>>
>> diff  --git a/clang/test/CXX/class/class.compare/class.eq/p1.cpp
>> b/clang/test/CXX/class/class.compare/class.eq/p1.cpp
>> new file mode 100644
>> index 000000000000..622f66cf9281
>> --- /dev/null
>> +++ b/clang/test/CXX/class/class.compare/class.eq/p1.cpp
>> @@ -0,0 +1,25 @@
>> +// RUN: %clang_cc1 -std=c++2a -verify %s
>> +
>> +struct Good {
>> +  bool operator==(const Good&) const = default;
>> +  bool operator!=(const Good&) const = default;
>> +  friend bool operator==(const Good&, const Good&) = default;
>> +  friend bool operator!=(const Good&, const Good&) = default;
>> +};
>> +
>> +enum Bool : bool {};
>> +struct Bad {
>> +  bool &operator==(const Bad&) const = default; // expected-error
>> {{return type for defaulted equality comparison operator must be 'bool',
>> not 'bool &'}}
>> +  const bool operator!=(const Bad&) const = default; // expected-error
>> {{return type for defaulted equality comparison operator must be 'bool',
>> not 'const bool'}}
>> +  friend Bool operator==(const Bad&, const Bad&) = default; //
>> expected-error {{return type for defaulted equality comparison operator
>> must be 'bool', not 'Bool'}}
>> +  friend int operator!=(const Bad&, const Bad&) = default; //
>> expected-error {{return type for defaulted equality comparison operator
>> must be 'bool', not 'int'}}
>> +};
>> +
>> +template<typename T> struct Ugly {
>> +  T operator==(const Ugly&) const = default; // expected-error {{return
>> type}}
>> +  T operator!=(const Ugly&) const = default; // expected-error {{return
>> type}}
>> +  friend T operator==(const Ugly&, const Ugly&) = default; //
>> expected-error {{return type}}
>> +  friend T operator!=(const Ugly&, const Ugly&) = default; //
>> expected-error {{return type}}
>> +};
>> +template struct Ugly<bool>;
>> +template struct Ugly<int>; // expected-note {{in instantiation of}}
>>
>> diff  --git a/clang/test/CXX/class/class.compare/class.rel/p1.cpp
>> b/clang/test/CXX/class/class.compare/class.rel/p1.cpp
>> new file mode 100644
>> index 000000000000..3797d5f81f56
>> --- /dev/null
>> +++ b/clang/test/CXX/class/class.compare/class.rel/p1.cpp
>> @@ -0,0 +1,25 @@
>> +// RUN: %clang_cc1 -std=c++2a -verify %s
>> +
>> +struct Good {
>> +  bool operator<(const Good&) const = default;
>> +  bool operator>(const Good&) const = default;
>> +  friend bool operator<=(const Good&, const Good&) = default;
>> +  friend bool operator>=(const Good&, const Good&) = default;
>> +};
>> +
>> +enum Bool : bool {};
>> +struct Bad {
>> +  bool &operator<(const Bad&) const = default; // expected-error
>> {{return type for defaulted relational comparison operator must be 'bool',
>> not 'bool &'}}
>> +  const bool operator>(const Bad&) const = default; // expected-error
>> {{return type for defaulted relational comparison operator must be 'bool',
>> not 'const bool'}}
>> +  friend Bool operator<=(const Bad&, const Bad&) = default; //
>> expected-error {{return type for defaulted relational comparison operator
>> must be 'bool', not 'Bool'}}
>> +  friend int operator>=(const Bad&, const Bad&) = default; //
>> expected-error {{return type for defaulted relational comparison operator
>> must be 'bool', not 'int'}}
>> +};
>> +
>> +template<typename T> struct Ugly {
>> +  T operator<(const Ugly&) const = default; // expected-error {{return
>> type}}
>> +  T operator>(const Ugly&) const = default; // expected-error {{return
>> type}}
>> +  friend T operator<=(const Ugly&, const Ugly&) = default; //
>> expected-error {{return type}}
>> +  friend T operator>=(const Ugly&, const Ugly&) = default; //
>> expected-error {{return type}}
>> +};
>> +template struct Ugly<bool>;
>> +template struct Ugly<int>; // expected-note {{in instantiation of}}
>>
>> diff  --git
>> a/clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p1.cpp
>> b/clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p1.cpp
>> index 3f2bc569edf6..6e9b45903d39 100644
>> --- a/clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p1.cpp
>> +++ b/clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p1.cpp
>> @@ -1,12 +1,28 @@
>> -// RUN: %clang_cc1 -verify %s -std=c++11
>> -// RUN: %clang_cc1 -verify %s -std=c++17
>> -// RUN: %clang_cc1 -verify %s -std=c++2a
>> +// RUN: %clang_cc1 -verify=expected,pre2a %s -std=c++11
>> +// RUN: %clang_cc1 -verify=expected,pre2a %s -std=c++17
>> +// RUN: %clang_cc1 -verify=expected %s -std=c++2a
>>
>>  // A function that is explicitly defaulted shall
>>  struct A {
>> -  // -- be a special member function,
>> -  A(int) = default; // expected-error {{only special member functions
>> may be defaulted}}
>> +  // -- be a special member function [C++2a: or a comparison operator
>> function],
>> +  A(int) = default;
>> +#if __cplusplus <= 201703L
>> +  // expected-error at -2 {{only special member functions may be
>> defaulted}}
>> +#else
>> +  // expected-error at -4 {{only special member functions and comparison
>> operators may be defaulted}}
>> +#endif
>>    A(A) = default; // expected-error {{must pass its first argument by
>> reference}}
>> +  void f(A) = default; // expected-error-re {{only special member
>> functions{{( and comparison operators)?}} may be defaulted}}
>> +
>> +  bool operator==(const A&) const = default; // pre2a-warning
>> {{defaulted comparison operators are a C++20 extension}}
>> +  bool operator!=(const A&) const = default; // pre2a-warning
>> {{defaulted comparison operators are a C++20 extension}}
>> +  bool operator<(const A&) const = default; // pre2a-error {{only
>> special member functions may be defaulted}}
>> +  bool operator>(const A&) const = default; // pre2a-error {{only
>> special member functions may be defaulted}}
>> +  bool operator<=(const A&) const = default; // pre2a-error {{only
>> special member functions may be defaulted}}
>> +  bool operator>=(const A&) const = default; // pre2a-error {{only
>> special member functions may be defaulted}}
>> +  bool operator<=>(const A&) const = default; // pre2a-error 1+{{}}
>> pre2a-warning {{'<=>' is a single token in C++2a}}
>> +
>> +  A operator+(const A&) const = default; // expected-error-re {{only
>> special member functions{{( and comparison operators)?}} may be defaulted}}
>>
>>    // -- have the same declared function type as if it had been implicitly
>>    //    declared
>>
>> diff  --git a/clang/test/Parser/cxx0x-decl.cpp
>> b/clang/test/Parser/cxx0x-decl.cpp
>> index 2f219ac87fb8..3c1c3602691b 100644
>> --- a/clang/test/Parser/cxx0x-decl.cpp
>> +++ b/clang/test/Parser/cxx0x-decl.cpp
>> @@ -39,7 +39,7 @@ static_assert(something, ""); // expected-error
>> {{undeclared identifier}}
>>
>>  // PR9903
>>  struct SS {
>> -  typedef void d() = default; // expected-error {{function definition
>> declared 'typedef'}} expected-error {{only special member functions may be
>> defaulted}}
>> +  typedef void d() = default; // expected-error {{function definition
>> declared 'typedef'}} expected-error {{only special member functions and
>> comparison operators may be defaulted}}
>>  };
>>
>>  using PR14855 = int S::; // expected-error {{expected ';' after alias
>> declaration}}
>>
>> diff  --git a/clang/test/SemaCXX/cxx0x-defaulted-functions.cpp
>> b/clang/test/SemaCXX/cxx0x-defaulted-functions.cpp
>> index 45a65440d599..c68b7d67932e 100644
>> --- a/clang/test/SemaCXX/cxx0x-defaulted-functions.cpp
>> +++ b/clang/test/SemaCXX/cxx0x-defaulted-functions.cpp
>> @@ -175,7 +175,7 @@ namespace PR14577 {
>>    Outer<T>::Inner1<T>::~Inner1() = delete; // expected-error {{nested
>> name specifier 'Outer<T>::Inner1<T>::' for declaration does not refer into
>> a class, class template or class template partial specialization}}
>> expected-error {{only functions can have deleted definitions}}
>>
>>    template<typename T>
>> -  Outer<T>::Inner2<T>::~Inner2() = default; // expected-error {{nested
>> name specifier 'Outer<T>::Inner2<T>::' for declaration does not refer into
>> a class, class template or class template partial specialization}}
>> expected-error {{only special member functions may be defaulted}}
>> +  Outer<T>::Inner2<T>::~Inner2() = default; // expected-error {{nested
>> name specifier 'Outer<T>::Inner2<T>::' for declaration does not refer into
>> a class, class template or class template partial specialization}}
>>  }
>>
>>  extern "C" { // expected-note {{extern "C" language linkage
>> specification begins here}}
>>
>> diff  --git a/clang/test/SemaCXX/cxx17-compat.cpp
>> b/clang/test/SemaCXX/cxx17-compat.cpp
>> index 3d5420fa0637..e063b1fc1807 100644
>> --- a/clang/test/SemaCXX/cxx17-compat.cpp
>> +++ b/clang/test/SemaCXX/cxx17-compat.cpp
>> @@ -88,3 +88,36 @@ void f() {
>>      // expected-warning at -4 {{decomposition declaration declared with
>> 'static thread_local' specifiers is incompatible with C++ standards before
>> C++2a}}
>>  #endif
>>  }
>> +
>> +struct DefaultedComparisons {
>> +  bool operator==(const DefaultedComparisons&) const = default;
>> +  bool operator!=(const DefaultedComparisons&) const = default;
>> +#if __cplusplus <= 201703L
>> +  // expected-warning at -3 {{defaulted comparison operators are a C++20
>> extension}}
>> +  // expected-warning at -3 {{defaulted comparison operators are a C++20
>> extension}}
>> +#else
>> +  // expected-warning at -6 {{defaulted comparison operators are
>> incompatible with C++ standards before C++20}}
>> +  // expected-warning at -6 {{defaulted comparison operators are
>> incompatible with C++ standards before C++20}}
>> +#endif
>> +  bool operator<=>(const DefaultedComparisons&) const = default;
>> +#if __cplusplus <= 201703L
>> +  // expected-error at -2 {{'operator<=' cannot be the name of a variable
>> or data member}} expected-error at -2 0+{{}} expected-warning at -2 {{}}
>> +#else
>> +  // expected-warning at -4 {{'<=>' operator is incompatible with C++
>> standards before C++2a}}
>> +#endif
>> +  bool operator<(const DefaultedComparisons&) const = default;
>> +  bool operator<=(const DefaultedComparisons&) const = default;
>> +  bool operator>(const DefaultedComparisons&) const = default;
>> +  bool operator>=(const DefaultedComparisons&) const = default;
>> +#if __cplusplus <= 201703L
>> +  // expected-error at -5 {{only special member functions}}
>> +  // expected-error at -5 {{only special member functions}}
>> +  // expected-error at -5 {{only special member functions}}
>> +  // expected-error at -5 {{only special member functions}}
>> +#else
>> +  // expected-warning at -10 {{defaulted comparison operators are
>> incompatible with C++ standards before C++20}}
>> +  // expected-warning at -10 {{defaulted comparison operators are
>> incompatible with C++ standards before C++20}}
>> +  // expected-warning at -10 {{defaulted comparison operators are
>> incompatible with C++ standards before C++20}}
>> +  // expected-warning at -10 {{defaulted comparison operators are
>> incompatible with C++ standards before C++20}}
>> +#endif
>> +};
>>
>>
>>
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