[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 <mital@mitalashok.co.uk>

1 files changed, 55 insertions, 12 deletions

diff --git a/clang/lib/Sema/SemaExprMember.cpp b/clang/lib/Sema/SemaExprMember.cpp
index 85d5dfcb..bcc1b92 100644
--- a/clang/lib/Sema/SemaExprMember.cpp
+++ b/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();
     }
@@ -1174,6 +1213,8 @@ 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,
@@ -1181,6 +1222,8 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
   }
 
   if (VarTemplateDecl *VarTempl = dyn_cast<VarTemplateDecl>(MemberDecl)) {
+    if (ConvertBaseExprToDiscardedValue())
+      return ExprError();
     if (!TemplateArgs) {
       diagnoseMissingTemplateArguments(
           SS, /*TemplateKeyword=*/TemplateKWLoc.isValid(), VarTempl, MemberLoc);