[Clang][Sema] Don't build CXXDependentScopeMemberExprs for potentiall…

…y implicit class member access expressions (llvm#92318)

According to [expr.prim.id.general] p2:
> If an _id-expression_ `E` denotes a non-static non-type member of some
class `C` at a point where the current class is `X` and
> - `E` is potentially evaluated or `C` is `X` or a base class of `X`,
and
> - `E` is not the _id-expression_ of a class member access expression,
and
> - if `E` is a _qualified-id_, `E` is not the un-parenthesized operand
of the unary `&` operator,
>
> the _id-expression_ is transformed into a class member access
expression using `(*this)` as the object expression.

Consider the following:
```
struct A
{
    void f0();

    template<typename T>
    void f1();
};

template<typename T>
struct B : T
{
    auto g0() -> decltype(T::f0()); // ok

    auto g1() -> decltype(T::template f1<int>()); // error: call to non-static member function without an object argument
};

template struct B<A>;
```

Clang incorrectly rejects the call to `f1` in the _trailing-return-type_
of `g1`. Furthermore, the following snippet results in a crash during
codegen:
```
struct A
{
    void f();
};

template<typename T>
struct B : T
{
    template<typename U>
    static void g();
    
    template<>
    void g<int>()
    {
        return T::f(); // crash here
    }
};

template struct B<A>;
```
This happens because we unconditionally build a
`CXXDependentScopeMemberExpr` (with an implicit object expression) for
`T::f` when parsing the template definition, even though we don't know
whether `g` is an implicit object member function yet.

This patch fixes these issues by instead building
`DependentScopeDeclRefExpr`s for such expressions, and only transforming
them into implicit class member access expressions during instantiation.
Since we implemented the MS "unqualified lookup into dependent bases"
extension by building an implicit class member access (and relying on
the first component name of the _nested-name-specifier_ to be looked up
in the context of the object expression during instantiation), we
instead pre-append a fake _nested-name-specifier_ that refers to the
injected-class-name of the enclosing class. This patch also refactors
`Sema::BuildQualifiedDeclarationNameExpr` and
`Sema::BuildQualifiedTemplateIdExpr`, streamlining their implementation
and removing any redundant checks.

clang/docs/ReleaseNotes.rst

@@ -744,6 +744,8 @@ Bug Fixes to C++ Support
   explicit object argument member functions. Fixes (#GH92188).
 - Fix a C++11 crash when a non-const non-static member function is defined out-of-line with
   the ``constexpr`` specifier. Fixes (#GH61004).
+- Clang no longer transforms dependent qualified names into implicit class member access expressions
+  until it can be determined whether the name is that of a non-static member.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -5375,11 +5375,9 @@ class Sema final : public SemaBase {
   bool UseArgumentDependentLookup(const CXXScopeSpec &SS, const LookupResult &R,
                                   bool HasTrailingLParen);
 
-  ExprResult
-  BuildQualifiedDeclarationNameExpr(CXXScopeSpec &SS,
-                                    const DeclarationNameInfo &NameInfo,
-                                    bool IsAddressOfOperand, const Scope *S,
-                                    TypeSourceInfo **RecoveryTSI = nullptr);
+  ExprResult BuildQualifiedDeclarationNameExpr(
+      CXXScopeSpec &SS, const DeclarationNameInfo &NameInfo,
+      bool IsAddressOfOperand, TypeSourceInfo **RecoveryTSI = nullptr);
 
   ExprResult BuildDeclarationNameExpr(const CXXScopeSpec &SS, LookupResult &R,
                                       bool NeedsADL,
@@ -8991,7 +8989,8 @@ class Sema final : public SemaBase {
   ExprResult
   BuildQualifiedTemplateIdExpr(CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
                                const DeclarationNameInfo &NameInfo,
-                               const TemplateArgumentListInfo *TemplateArgs);
+                               const TemplateArgumentListInfo *TemplateArgs,
+                               bool IsAddressOfOperand);
 
   TemplateNameKind ActOnTemplateName(Scope *S, CXXScopeSpec &SS,
                                      SourceLocation TemplateKWLoc,

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -796,6 +796,14 @@ bool Sema::BuildCXXNestedNameSpecifier(Scope *S, NestedNameSpecInfo &IdInfo,
         Diag(IdInfo.IdentifierLoc,
              diag::ext_undeclared_unqual_id_with_dependent_base)
             << IdInfo.Identifier << ContainingClass;
+        // Fake up a nested-name-specifier that starts with the
+        // injected-class-name of the enclosing class.
+        QualType T = Context.getTypeDeclType(ContainingClass);
+        TypeLocBuilder TLB;
+        TLB.pushTrivial(Context, T, IdInfo.IdentifierLoc);
+        SS.Extend(Context, /*TemplateKWLoc=*/SourceLocation(),
+                  TLB.getTypeLocInContext(Context, T), IdInfo.IdentifierLoc);
+        // Add the identifier to form a dependent name.
         SS.Extend(Context, IdInfo.Identifier, IdInfo.IdentifierLoc,
                   IdInfo.CCLoc);
         return false;

clang/lib/Sema/SemaExpr.cpp

@@ -2718,34 +2718,6 @@ Sema::ActOnIdExpression(Scope *S, CXXScopeSpec &SS,
     return ExprError();
   }
 
-  // C++ [temp.dep.expr]p3:
-  //   An id-expression is type-dependent if it contains:
-  //     -- an identifier that was declared with a dependent type,
-  //        (note: handled after lookup)
-  //     -- a template-id that is dependent,
-  //        (note: handled in BuildTemplateIdExpr)
-  //     -- a conversion-function-id that specifies a dependent type,
-  //     -- a nested-name-specifier that contains a class-name that
-  //        names a dependent type.
-  // Determine whether this is a member of an unknown specialization;
-  // we need to handle these differently.
-  bool DependentID = false;
-  if (Name.getNameKind() == DeclarationName::CXXConversionFunctionName &&
-      Name.getCXXNameType()->isDependentType()) {
-    DependentID = true;
-  } else if (SS.isSet()) {
-    if (DeclContext *DC = computeDeclContext(SS, false)) {
-      if (RequireCompleteDeclContext(SS, DC))
-        return ExprError();
-    } else {
-      DependentID = true;
-    }
-  }
-
-  if (DependentID)
-    return ActOnDependentIdExpression(SS, TemplateKWLoc, NameInfo,
-                                      IsAddressOfOperand, TemplateArgs);
-
   // BoundsSafety: This specially handles arguments of bounds attributes
   // appertains to a type of C struct field such that the name lookup
   // within a struct finds the member name, which is not the case for other
@@ -2781,7 +2753,7 @@ Sema::ActOnIdExpression(Scope *S, CXXScopeSpec &SS,
                            &AssumedTemplate))
       return ExprError();
 
-    if (R.wasNotFoundInCurrentInstantiation())
+    if (R.wasNotFoundInCurrentInstantiation() || SS.isInvalid())
       return ActOnDependentIdExpression(SS, TemplateKWLoc, NameInfo,
                                         IsAddressOfOperand, TemplateArgs);
   } else {
@@ -2791,7 +2763,7 @@ Sema::ActOnIdExpression(Scope *S, CXXScopeSpec &SS,
 
     // If the result might be in a dependent base class, this is a dependent
     // id-expression.
-    if (R.getResultKind() == LookupResult::NotFoundInCurrentInstantiation)
+    if (R.wasNotFoundInCurrentInstantiation() || SS.isInvalid())
       return ActOnDependentIdExpression(SS, TemplateKWLoc, NameInfo,
                                         IsAddressOfOperand, TemplateArgs);
 
@@ -2946,26 +2918,14 @@ Sema::ActOnIdExpression(Scope *S, CXXScopeSpec &SS,
 /// this path.
 ExprResult Sema::BuildQualifiedDeclarationNameExpr(
     CXXScopeSpec &SS, const DeclarationNameInfo &NameInfo,
-    bool IsAddressOfOperand, const Scope *S, TypeSourceInfo **RecoveryTSI) {
-  if (NameInfo.getName().isDependentName())
-    return BuildDependentDeclRefExpr(SS, /*TemplateKWLoc=*/SourceLocation(),
-                                     NameInfo, /*TemplateArgs=*/nullptr);
-
-  DeclContext *DC = computeDeclContext(SS, false);
-  if (!DC)
-    return BuildDependentDeclRefExpr(SS, /*TemplateKWLoc=*/SourceLocation(),
-                                     NameInfo, /*TemplateArgs=*/nullptr);
-
-  if (RequireCompleteDeclContext(SS, DC))
-    return ExprError();
-
+    bool IsAddressOfOperand, TypeSourceInfo **RecoveryTSI) {
   LookupResult R(*this, NameInfo, LookupOrdinaryName);
-  LookupQualifiedName(R, DC);
+  LookupParsedName(R, /*S=*/nullptr, &SS, /*ObjectType=*/QualType());
 
   if (R.isAmbiguous())
     return ExprError();
 
-  if (R.getResultKind() == LookupResult::NotFoundInCurrentInstantiation)
+  if (R.wasNotFoundInCurrentInstantiation() || SS.isInvalid())
     return BuildDependentDeclRefExpr(SS, /*TemplateKWLoc=*/SourceLocation(),
                                      NameInfo, /*TemplateArgs=*/nullptr);
 
@@ -2974,6 +2934,7 @@ ExprResult Sema::BuildQualifiedDeclarationNameExpr(
     // diagnostic during template instantiation is likely bogus, e.g. if a class
     // is invalid because it's derived from an invalid base class, then missing
     // members were likely supposed to be inherited.
+    DeclContext *DC = computeDeclContext(SS);
     if (const auto *CD = dyn_cast<CXXRecordDecl>(DC))
       if (CD->isInvalidDecl())
         return ExprError();
@@ -3017,16 +2978,14 @@ ExprResult Sema::BuildQualifiedDeclarationNameExpr(
     return ExprEmpty();
   }
 
-  // Defend against this resolving to an implicit member access. We usually
-  // won't get here if this might be a legitimate a class member (we end up in
-  // BuildMemberReferenceExpr instead), but this can be valid if we're forming
-  // a pointer-to-member or in an unevaluated context in C++11.
-  if (!R.empty() && (*R.begin())->isCXXClassMember() && !IsAddressOfOperand)
+  // If necessary, build an implicit class member access.
+  if (isPotentialImplicitMemberAccess(SS, R, IsAddressOfOperand))
     return BuildPossibleImplicitMemberExpr(SS,
                                            /*TemplateKWLoc=*/SourceLocation(),
-                                           R, /*TemplateArgs=*/nullptr, S);
+                                           R, /*TemplateArgs=*/nullptr,
+                                           /*S=*/nullptr);
 
-  return BuildDeclarationNameExpr(SS, R, /* ADL */ false);
+  return BuildDeclarationNameExpr(SS, R, /*ADL=*/false);
 }
 
 /// Cast a base object to a member's actual type.
@@ -3190,7 +3149,7 @@ bool Sema::UseArgumentDependentLookup(const CXXScopeSpec &SS,
     return false;
 
   // Never if a scope specifier was provided.
-  if (SS.isSet())
+  if (SS.isNotEmpty())
     return false;
 
   // Only in C++ or ObjC++.

clang/lib/Sema/SemaLookup.cpp

@@ -2771,16 +2771,19 @@ bool Sema::LookupParsedName(LookupResult &R, Scope *S, CXXScopeSpec *SS,
             ObjectType->castAs<TagType>()->isBeingDefined()) &&
            "Caller should have completed object type");
   } else if (SS && SS->isNotEmpty()) {
-    if (NestedNameSpecifier *NNS = SS->getScopeRep();
-        NNS->getKind() == NestedNameSpecifier::Super)
-      return LookupInSuper(R, NNS->getAsRecordDecl());
     // This nested-name-specifier occurs after another nested-name-specifier,
     // so long into the context associated with the prior nested-name-specifier.
     if ((DC = computeDeclContext(*SS, EnteringContext))) {
       // The declaration context must be complete.
       if (!DC->isDependentContext() && RequireCompleteDeclContext(*SS, DC))
         return false;
       R.setContextRange(SS->getRange());
+      // FIXME: '__super' lookup semantics could be implemented by a
+      // LookupResult::isSuperLookup flag which skips the initial search of
+      // the lookup context in LookupQualified.
+      if (NestedNameSpecifier *NNS = SS->getScopeRep();
+          NNS->getKind() == NestedNameSpecifier::Super)
+        return LookupInSuper(R, NNS->getAsRecordDecl());
     }
     IsDependent = !DC && isDependentScopeSpecifier(*SS);
   } else {

clang/lib/Sema/SemaTemplate.cpp

@@ -726,44 +726,22 @@ Sema::ActOnDependentIdExpression(const CXXScopeSpec &SS,
                                  const DeclarationNameInfo &NameInfo,
                                  bool isAddressOfOperand,
                            const TemplateArgumentListInfo *TemplateArgs) {
-  DeclContext *DC = getFunctionLevelDeclContext();
-
-  // C++11 [expr.prim.general]p12:
-  //   An id-expression that denotes a non-static data member or non-static
-  //   member function of a class can only be used:
-  //   (...)
-  //   - if that id-expression denotes a non-static data member and it
-  //     appears in an unevaluated operand.
-  //
-  // If this might be the case, form a DependentScopeDeclRefExpr instead of a
-  // CXXDependentScopeMemberExpr. The former can instantiate to either
-  // DeclRefExpr or MemberExpr depending on lookup results, while the latter is
-  // always a MemberExpr.
-  bool MightBeCxx11UnevalField =
-      getLangOpts().CPlusPlus11 && isUnevaluatedContext();
-
-  // Check if the nested name specifier is an enum type.
-  bool IsEnum = false;
-  if (NestedNameSpecifier *NNS = SS.getScopeRep())
-    IsEnum = isa_and_nonnull<EnumType>(NNS->getAsType());
-
-  if (!MightBeCxx11UnevalField && !isAddressOfOperand && !IsEnum &&
-      isa<CXXMethodDecl>(DC) &&
-      cast<CXXMethodDecl>(DC)->isImplicitObjectMemberFunction()) {
-    QualType ThisType =
-        cast<CXXMethodDecl>(DC)->getThisType().getNonReferenceType();
-
-    // Since the 'this' expression is synthesized, we don't need to
-    // perform the double-lookup check.
-    NamedDecl *FirstQualifierInScope = nullptr;
+  if (SS.isEmpty()) {
+    // FIXME: This codepath is only used by dependent unqualified names
+    // (e.g. a dependent conversion-function-id, or operator= once we support
+    // it). It doesn't quite do the right thing, and it will silently fail if
+    // getCurrentThisType() returns null.
+    QualType ThisType = getCurrentThisType();
+    if (ThisType.isNull())
+      return ExprError();
 
     return CXXDependentScopeMemberExpr::Create(
-        Context, /*This=*/nullptr, ThisType,
+        Context, /*Base=*/nullptr, ThisType,
         /*IsArrow=*/!Context.getLangOpts().HLSL,
-        /*Op=*/SourceLocation(), SS.getWithLocInContext(Context), TemplateKWLoc,
-        FirstQualifierInScope, NameInfo, TemplateArgs);
+        /*OperatorLoc=*/SourceLocation(),
+        /*QualifierLoc=*/NestedNameSpecifierLoc(), TemplateKWLoc,
+        /*FirstQualifierFoundInScope=*/nullptr, NameInfo, TemplateArgs);
   }
-
   return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
 }
 
@@ -772,13 +750,15 @@ Sema::BuildDependentDeclRefExpr(const CXXScopeSpec &SS,
                                 SourceLocation TemplateKWLoc,
                                 const DeclarationNameInfo &NameInfo,
                                 const TemplateArgumentListInfo *TemplateArgs) {
-  // DependentScopeDeclRefExpr::Create requires a valid QualifierLoc
-  NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
-  if (!QualifierLoc)
-    return ExprError();
+  // DependentScopeDeclRefExpr::Create requires a valid NestedNameSpecifierLoc
+  if (!SS.isValid())
+    return CreateRecoveryExpr(
+        SS.getBeginLoc(),
+        TemplateArgs ? TemplateArgs->getRAngleLoc() : NameInfo.getEndLoc(), {});
 
   return DependentScopeDeclRefExpr::Create(
-      Context, QualifierLoc, TemplateKWLoc, NameInfo, TemplateArgs);
+      Context, SS.getWithLocInContext(Context), TemplateKWLoc, NameInfo,
+      TemplateArgs);
 }
 
 
@@ -5747,50 +5727,36 @@ ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
 }
 
 // We actually only call this from template instantiation.
-ExprResult
-Sema::BuildQualifiedTemplateIdExpr(CXXScopeSpec &SS,
-                                   SourceLocation TemplateKWLoc,
-                                   const DeclarationNameInfo &NameInfo,
-                             const TemplateArgumentListInfo *TemplateArgs) {
-
+ExprResult Sema::BuildQualifiedTemplateIdExpr(
+    CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
+    const DeclarationNameInfo &NameInfo,
+    const TemplateArgumentListInfo *TemplateArgs, bool IsAddressOfOperand) {
   assert(TemplateArgs || TemplateKWLoc.isValid());
-  DeclContext *DC;
-  if (!(DC = computeDeclContext(SS, false)) ||
-      DC->isDependentContext() ||
-      RequireCompleteDeclContext(SS, DC))
-    return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
 
   LookupResult R(*this, NameInfo, LookupOrdinaryName);
-  if (LookupTemplateName(R, (Scope *)nullptr, SS, QualType(),
-                         /*Entering*/ false, TemplateKWLoc))
+  if (LookupTemplateName(R, /*S=*/nullptr, SS, /*ObjectType=*/QualType(),
+                         /*EnteringContext=*/false, TemplateKWLoc))
     return ExprError();
 
   if (R.isAmbiguous())
     return ExprError();
 
+  if (R.wasNotFoundInCurrentInstantiation() || SS.isInvalid())
+    return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
+
   if (R.empty()) {
+    DeclContext *DC = computeDeclContext(SS);
     Diag(NameInfo.getLoc(), diag::err_no_member)
       << NameInfo.getName() << DC << SS.getRange();
     return ExprError();
   }
 
-  auto DiagnoseTypeTemplateDecl = [&](TemplateDecl *Temp,
-                                      bool isTypeAliasTemplateDecl) {
-    Diag(NameInfo.getLoc(), diag::err_template_kw_refers_to_type_template)
-        << SS.getScopeRep() << NameInfo.getName().getAsString() << SS.getRange()
-        << isTypeAliasTemplateDecl;
-    Diag(Temp->getLocation(), diag::note_referenced_type_template)
-        << isTypeAliasTemplateDecl;
-    return CreateRecoveryExpr(NameInfo.getBeginLoc(), NameInfo.getEndLoc(), {});
-  };
-
-  if (ClassTemplateDecl *Temp = R.getAsSingle<ClassTemplateDecl>())
-    return DiagnoseTypeTemplateDecl(Temp, false);
-
-  if (TypeAliasTemplateDecl *Temp = R.getAsSingle<TypeAliasTemplateDecl>())
-    return DiagnoseTypeTemplateDecl(Temp, true);
+  // If necessary, build an implicit class member access.
+  if (isPotentialImplicitMemberAccess(SS, R, IsAddressOfOperand))
+    return BuildPossibleImplicitMemberExpr(SS, TemplateKWLoc, R, TemplateArgs,
+                                           /*S=*/nullptr);
 
-  return BuildTemplateIdExpr(SS, TemplateKWLoc, R, /*ADL*/ false, TemplateArgs);
+  return BuildTemplateIdExpr(SS, TemplateKWLoc, R, /*ADL=*/false, TemplateArgs);
 }
 
 /// Form a template name from a name that is syntactically required to name a
@@ -5982,8 +5948,7 @@ bool Sema::CheckTemplateTypeArgument(
       LookupParsedName(Result, CurScope, &SS, /*ObjectType=*/QualType());
 
       if (Result.getAsSingle<TypeDecl>() ||
-          Result.getResultKind() ==
-              LookupResult::NotFoundInCurrentInstantiation) {
+          Result.wasNotFoundInCurrentInstantiation()) {
         assert(SS.getScopeRep() && "dependent scope expr must has a scope!");
         // Suggest that the user add 'typename' before the NNS.
         SourceLocation Loc = AL.getSourceRange().getBegin();

clang/lib/Sema/TreeTransform.h

@@ -3478,11 +3478,11 @@ class TreeTransform {
     SS.Adopt(QualifierLoc);
 
     if (TemplateArgs || TemplateKWLoc.isValid())
-      return getSema().BuildQualifiedTemplateIdExpr(SS, TemplateKWLoc, NameInfo,
-                                                    TemplateArgs);
+      return getSema().BuildQualifiedTemplateIdExpr(
+          SS, TemplateKWLoc, NameInfo, TemplateArgs, IsAddressOfOperand);
 
     return getSema().BuildQualifiedDeclarationNameExpr(
-        SS, NameInfo, IsAddressOfOperand, /*S*/nullptr, RecoveryTSI);
+        SS, NameInfo, IsAddressOfOperand, RecoveryTSI);
   }
 
   /// Build a new template-id expression.