[clang] [Clang] [Sema] Handle placeholders in '.*' expressions (PR #83103)

[via cfe-commits]

https://github.com/Sirraide created https://github.com/llvm/llvm-project/pull/83103

When analysing whether we should handle a binary expression as an overloaded operator call or a builtin operator, we were calling `checkPlaceholderForOverload()`, which takes care of any placeholders that are not overload sets—which would usually make sense since those need to be handled as part of overload resolution.

Unfortunately, we were also doing that for `.*`, which is not overloadable, and then proceeding to create a builtin operator anyway, which would crash if the RHS happened to be an unresolved overload set (due hitting an assertion in `CreateBuiltinBinOp()`—specifically, in one of its callees—in the `.*` case that makes sure its arguments aren’t placeholders).

This pr instead makes it so we check for *all* placeholders early if the operator is `.*`.

It’s worth noting that,
1. In the `.*` case, we now additionally also check for *any* placeholders (not just non-overload-sets) in the LHS; this shouldn’t make a difference, however—at least I couldn’t think of a way to trigger the assertion with an overload set as the LHS of `.*`; it is worth noting that the assertion in question would also complain if the LHS happened to be of placeholder type, though.
2. There is another case in which we also don’t perform overload resolution—namely `=` if the LHS is not of class or enumeration type after handling non-overload-set placeholders—as in the `.*` case, but similarly to 1., I first couldn’t think of a way of getting this case to crash, and secondly, `CreateBuiltinBinOp()` doesn’t seem to care about placeholders in the LHS or RHS in the `=` case (from what I can tell, it, or rather one of its callees, only checks that the LHS is not a pseudo-object type, but those will have already been handled by the call to `checkPlaceholderForOverload()` by the time we get to this function), so I don’t think this case suffers from the same problem.

This fixes #53815.

>From 071b6287e89e4601d9e441978f0b4bd53e757c26 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Tue, 27 Feb 2024 06:19:05 +0100
Subject: [PATCH] [Clang] [Sema] Handle placeholders in '.*' expressions

---
 clang/lib/Sema/SemaOverload.cpp | 22 +++++++++++++++++-----
 clang/test/SemaCXX/gh53815.cpp  | 21 +++++++++++++++++++++
 2 files changed, 38 insertions(+), 5 deletions(-)
 create mode 100644 clang/test/SemaCXX/gh53815.cpp

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index c46f6338a5a125..f4b67e7b469418 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -14474,6 +14474,23 @@ ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
                                        CurFPFeatureOverrides());
   }
 
+  // If this is the .* operator, which is not overloadable, just
+  // create a built-in binary operator.
+  if (Opc == BO_PtrMemD) {
+    auto CheckPlaceholder = [&](Expr *&Arg) {
+      ExprResult Res = CheckPlaceholderExpr(Arg);
+      if (!Res.isInvalid())
+        Arg = Res.get();
+      return Res.isInvalid();
+    };
+
+    // CreateBuiltinBinOp() doesn't like it if we tell it to create a '.*'
+    // expression that contains placeholders (in either the LHS or RHS).
+    if (CheckPlaceholder(Args[0]) || CheckPlaceholder(Args[1]))
+      return ExprError();
+    return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);
+  }
+
   // Always do placeholder-like conversions on the RHS.
   if (checkPlaceholderForOverload(*this, Args[1]))
     return ExprError();
@@ -14493,11 +14510,6 @@ ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
   if (Opc == BO_Assign && !Args[0]->getType()->isOverloadableType())
     return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);
 
-  // If this is the .* operator, which is not overloadable, just
-  // create a built-in binary operator.
-  if (Opc == BO_PtrMemD)
-    return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);
-
   // Build the overload set.
   OverloadCandidateSet CandidateSet(OpLoc, OverloadCandidateSet::CSK_Operator,
                                     OverloadCandidateSet::OperatorRewriteInfo(
diff --git a/clang/test/SemaCXX/gh53815.cpp b/clang/test/SemaCXX/gh53815.cpp
new file mode 100644
index 00000000000000..326c911c7bfaf5
--- /dev/null
+++ b/clang/test/SemaCXX/gh53815.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// expected-no-diagnostics
+
+// Check that we don't crash due to forgetting to check for placeholders
+// in the RHS of '.*'.
+
+template <typename Fn>
+static bool has_explicitly_named_overload() {
+  return requires { Fn().*&Fn::operator(); };
+}
+
+int main() {
+  has_explicitly_named_overload<decltype([](auto){})>();
+}
+
+template <typename Fn>
+constexpr bool has_explicitly_named_overload_2() {
+  return requires { Fn().*&Fn::operator(); };
+}
+
+static_assert(!has_explicitly_named_overload_2<decltype([](auto){})>());