[Clang] Implement CWG2813: Class member access with prvalues (#120223)

This is a rebase of #95112 with my own feedback apply as @MitalAshok has
been inactive for a while.
It's fairly important this makes clang 20 as it is a blocker for #107451

--- 

[CWG2813](https://cplusplus.github.io/CWG/issues/2813.html)

prvalue.member_fn(expression-list) now will not materialize a temporary
for prvalue if member_fn is an explicit object member function, and
prvalue will bind directly to the object parameter.

The E1 in E1.static_member is now a discarded-value expression, so if E1
was a call to a [[nodiscard]] function, there will now be a warning.
This also affects C++98 with [[gnu::warn_unused_result]] functions.

This should not affect C where TemporaryMaterializationConversion is a
no-op.

Closes #100314
Fixes #100341

---------

Co-authored-by: Mital Ashok <[email protected]>

Author: cor3ntin

clang-tools-extra/clangd/unittests/DumpASTTests.cpp

@@ -49,7 +49,7 @@ declaration: Function - root
       )"},
       {R"cpp(
 namespace root {
-struct S { static const int x = 0; };
+struct S { static const int x = 0; ~S(); };
 int y = S::x + root::S().x;
 }
       )cpp",
@@ -60,10 +60,12 @@ declaration: Namespace - root
       type: Qualified - const
         type: Builtin - int
       expression: IntegerLiteral - 0
+    declaration: CXXDestructor
+      type: Record - S
+      type: FunctionProto
+        type: Builtin - void
     declaration: CXXConstructor
     declaration: CXXConstructor
-    declaration: CXXConstructor
-    declaration: CXXDestructor
   declaration: Var - y
     type: Builtin - int
     expression: ExprWithCleanups
@@ -74,14 +76,45 @@ declaration: Namespace - root
               type: Record - S
         expression: ImplicitCast - LValueToRValue
           expression: Member - x
-            expression: MaterializeTemporary - rvalue
+            expression: CXXBindTemporary
               expression: CXXTemporaryObject - S
                 type: Elaborated
                   specifier: Namespace - root::
                   type: Record - S
       )"},
       {R"cpp(
 namespace root {
+struct S { static const int x = 0; };
+int y = S::x + root::S().x;
+}
+      )cpp",
+       R"(
+declaration: Namespace - root
+  declaration: CXXRecord - S
+    declaration: Var - x
+      type: Qualified - const
+        type: Builtin - int
+      expression: IntegerLiteral - 0
+    declaration: CXXConstructor
+    declaration: CXXConstructor
+    declaration: CXXConstructor
+    declaration: CXXDestructor
+  declaration: Var - y
+    type: Builtin - int
+    expression: BinaryOperator - +
+      expression: ImplicitCast - LValueToRValue
+        expression: DeclRef - x
+          specifier: TypeSpec
+            type: Record - S
+      expression: ImplicitCast - LValueToRValue
+        expression: Member - x
+          expression: CXXTemporaryObject - S
+            type: Elaborated
+              specifier: Namespace - root::
+              type: Record - S
+      )"},
+      {R"cpp(
+namespace root {
 template <typename T> int tmpl() {
   (void)tmpl<unsigned>();
   return T::value;

clang/docs/ReleaseNotes.rst

@@ -321,6 +321,11 @@ Resolutions to C++ Defect Reports
 - Fix name lookup for a dependent base class that is the current instantiation.
   (`CWG591: When a dependent base class is the current instantiation <https://cplusplus.github.io/CWG/issues/591.html>`_).
 
+- Clang now allows calling explicit object member functions directly with prvalues
+  instead of always materializing a temporary, meaning by-value explicit object parameters
+  do not need to move from a temporary.
+  (`CWG2813: Class member access with prvalues <https://cplusplus.github.io/CWG/issues/2813.html>`_).
+
 C Language Changes
 ------------------
 

clang/include/clang/Sema/Sema.h

@@ -10659,6 +10659,11 @@ class Sema final : public SemaBase {
                            SourceLocation EndLoc);
   void ActOnForEachDeclStmt(DeclGroupPtrTy Decl);
 
+  /// DiagnoseDiscardedExprMarkedNodiscard - Given an expression that is
+  /// semantically a discarded-value expression, diagnose if any [[nodiscard]]
+  /// value has been discarded.
+  void DiagnoseDiscardedExprMarkedNodiscard(const Expr *E);
+
   /// DiagnoseUnusedExprResult - If the statement passed in is an expression
   /// whose result is unused, warn.
   void DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID);

clang/lib/AST/Expr.cpp

@@ -2990,6 +2990,9 @@ bool Expr::isUnusedResultAWarning(const Expr *&WarnE, SourceLocation &Loc,
   case ExprWithCleanupsClass:
     return cast<ExprWithCleanups>(this)->getSubExpr()
                ->isUnusedResultAWarning(WarnE, Loc, R1, R2, Ctx);
+  case OpaqueValueExprClass:
+    return cast<OpaqueValueExpr>(this)->getSourceExpr()->isUnusedResultAWarning(
+        WarnE, Loc, R1, R2, Ctx);
   }
 }
 

clang/lib/Sema/SemaExprMember.cpp

@@ -1003,15 +1003,6 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
               : !isDependentScopeSpecifier(SS) || computeDeclContext(SS)) &&
          "dependent lookup context that isn't the current instantiation?");
 
-  // C++1z [expr.ref]p2:
-  //   For the first option (dot) the first expression shall be a glvalue [...]
-  if (!IsArrow && BaseExpr && BaseExpr->isPRValue()) {
-    ExprResult Converted = TemporaryMaterializationConversion(BaseExpr);
-    if (Converted.isInvalid())
-      return ExprError();
-    BaseExpr = Converted.get();
-  }
-
   const DeclarationNameInfo &MemberNameInfo = R.getLookupNameInfo();
   DeclarationName MemberName = MemberNameInfo.getName();
   SourceLocation MemberLoc = MemberNameInfo.getLoc();
@@ -1128,26 +1119,68 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
     BaseExpr = BuildCXXThisExpr(Loc, BaseExprType, /*IsImplicit=*/true);
   }
 
+  // C++17 [expr.ref]p2, per CWG2813:
+  //   For the first option (dot), if the id-expression names a static member or
+  //   an enumerator, the first expression is a discarded-value expression; if
+  //   the id-expression names a non-static data member, the first expression
+  //   shall be a glvalue.
+  auto ConvertBaseExprToDiscardedValue = [&] {
+    assert(getLangOpts().CPlusPlus &&
+           "Static member / member enumerator outside of C++");
+    if (IsArrow)
+      return false;
+    ExprResult Converted = IgnoredValueConversions(BaseExpr);
+    if (Converted.isInvalid())
+      return true;
+    BaseExpr = Converted.get();
+    DiagnoseDiscardedExprMarkedNodiscard(BaseExpr);
+    return false;
+  };
+  auto ConvertBaseExprToGLValue = [&] {
+    if (IsArrow || !BaseExpr->isPRValue())
+      return false;
+    ExprResult Converted = TemporaryMaterializationConversion(BaseExpr);
+    if (Converted.isInvalid())
+      return true;
+    BaseExpr = Converted.get();
+    return false;
+  };
+
   // Check the use of this member.
   if (DiagnoseUseOfDecl(MemberDecl, MemberLoc))
     return ExprError();
 
-  if (FieldDecl *FD = dyn_cast<FieldDecl>(MemberDecl))
+  if (FieldDecl *FD = dyn_cast<FieldDecl>(MemberDecl)) {
+    if (ConvertBaseExprToGLValue())
+      return ExprError();
     return BuildFieldReferenceExpr(BaseExpr, IsArrow, OpLoc, SS, FD, FoundDecl,
                                    MemberNameInfo);
+  }
 
-  if (MSPropertyDecl *PD = dyn_cast<MSPropertyDecl>(MemberDecl))
+  if (MSPropertyDecl *PD = dyn_cast<MSPropertyDecl>(MemberDecl)) {
+    // No temporaries are materialized for property references yet.
+    // They might be materialized when this is transformed into a member call.
+    // Note that this is slightly different behaviour from MSVC which doesn't
+    // implement CWG2813 yet: MSVC might materialize an extra temporary if the
+    // getter or setter function is an explicit object member function.
     return BuildMSPropertyRefExpr(*this, BaseExpr, IsArrow, SS, PD,
                                   MemberNameInfo);
+  }
 
-  if (IndirectFieldDecl *FD = dyn_cast<IndirectFieldDecl>(MemberDecl))
+  if (IndirectFieldDecl *FD = dyn_cast<IndirectFieldDecl>(MemberDecl)) {
+    if (ConvertBaseExprToGLValue())
+      return ExprError();
     // We may have found a field within an anonymous union or struct
     // (C++ [class.union]).
     return BuildAnonymousStructUnionMemberReference(SS, MemberLoc, FD,
                                                     FoundDecl, BaseExpr,
                                                     OpLoc);
+  }
 
+  // Static data member
   if (VarDecl *Var = dyn_cast<VarDecl>(MemberDecl)) {
+    if (ConvertBaseExprToDiscardedValue())
+      return ExprError();
     return BuildMemberExpr(BaseExpr, IsArrow, OpLoc,
                            SS.getWithLocInContext(Context), TemplateKWLoc, Var,
                            FoundDecl, /*HadMultipleCandidates=*/false,
@@ -1161,7 +1194,13 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
     if (MemberFn->isInstance()) {
       valueKind = VK_PRValue;
       type = Context.BoundMemberTy;
+      if (MemberFn->isImplicitObjectMemberFunction() &&
+          ConvertBaseExprToGLValue())
+        return ExprError();
     } else {
+      // Static member function
+      if (ConvertBaseExprToDiscardedValue())
+        return ExprError();
       valueKind = VK_LValue;
       type = MemberFn->getType();
     }
@@ -1174,13 +1213,17 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
   assert(!isa<FunctionDecl>(MemberDecl) && "member function not C++ method?");
 
   if (EnumConstantDecl *Enum = dyn_cast<EnumConstantDecl>(MemberDecl)) {
+    if (ConvertBaseExprToDiscardedValue())
+      return ExprError();
     return BuildMemberExpr(
         BaseExpr, IsArrow, OpLoc, SS.getWithLocInContext(Context),
         TemplateKWLoc, Enum, FoundDecl, /*HadMultipleCandidates=*/false,
         MemberNameInfo, Enum->getType(), VK_PRValue, OK_Ordinary);
   }
 
   if (VarTemplateDecl *VarTempl = dyn_cast<VarTemplateDecl>(MemberDecl)) {
+    if (ConvertBaseExprToDiscardedValue())
+      return ExprError();
     if (!TemplateArgs) {
       diagnoseMissingTemplateArguments(
           SS, /*TemplateKeyword=*/TemplateKWLoc.isValid(), VarTempl, MemberLoc);

clang/lib/Sema/SemaOverload.cpp

@@ -5933,7 +5933,9 @@ ExprResult Sema::PerformImplicitObjectArgumentInitialization(
     DestType = ImplicitParamRecordType;
     FromClassification = From->Classify(Context);
 
-    // When performing member access on a prvalue, materialize a temporary.
+    // CWG2813 [expr.call]p6:
+    //   If the function is an implicit object member function, the object
+    //   expression of the class member access shall be a glvalue [...]
     if (From->isPRValue()) {
       From = CreateMaterializeTemporaryExpr(FromRecordType, From,
                                             Method->getRefQualifier() !=
@@ -6464,11 +6466,6 @@ static Expr *GetExplicitObjectExpr(Sema &S, Expr *Obj,
                                 VK_LValue, OK_Ordinary, SourceLocation(),
                                 /*CanOverflow=*/false, FPOptionsOverride());
   }
-  if (Obj->Classify(S.getASTContext()).isPRValue()) {
-    Obj = S.CreateMaterializeTemporaryExpr(
-        ObjType, Obj,
-        !Fun->getParamDecl(0)->getType()->isRValueReferenceType());
-  }
   return Obj;
 }
 
@@ -15584,8 +15581,6 @@ ExprResult Sema::BuildCallToMemberFunction(Scope *S, Expr *MemExprE,
                          CurFPFeatureOverrides(), Proto->getNumParams());
   } else {
     // Convert the object argument (for a non-static member function call).
-    // We only need to do this if there was actually an overload; otherwise
-    // it was done at lookup.
     ExprResult ObjectArg = PerformImplicitObjectArgumentInitialization(
         MemExpr->getBase(), Qualifier, FoundDecl, Method);
     if (ObjectArg.isInvalid())