[clang] [clang-tools-extra] [Clang] Implement CWG2813: Class member access with prvalues (PR #95112)

[Mital Ashok via cfe-commits]

================
@@ -1140,26 +1131,68 @@ 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 MakeDiscardedValue = [&BaseExpr, IsArrow, this] {
+    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();
+    DiagnoseUnusedExprResult(BaseExpr,
+                             diag::warn_discarded_class_member_access);
+    return false;
+  };
+  auto MakeGLValue = [&BaseExpr, IsArrow, this] {
+    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 (MakeGLValue())
+      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)) {
+    // Properties treated as non-static data members for the purpose of
+    // temporary materialization
+    if (MakeGLValue())
----------------
MitalAshok wrote:

(Tested locally on v19.41.33923) MSVC does not seem to implement CWG2813 so this:

```c++
struct S {
    S(const S&) = delete;
    S() = default;
    int getX(this S) { return 42; }
    __declspec(property(get = getX))
    int x;
};

int main() {
    return S().x;
}
```

... doesn't compile (attempts to call `S::S(const S&)` on the temporary to construct the explicit object argument).

I assume MSVC will eventually allow this, so we should wait to materialize the temporary here? Especially since https://learn.microsoft.com/en-gb/cpp/cpp/property-cpp?view=msvc-170 describes properties directly as a transformation from data member access -> member function call. I'll get this changed

https://github.com/llvm/llvm-project/pull/95112