From 7beaabc934e13d7bf32e3d46ec438885eefdefe2 Mon Sep 17 00:00:00 2001
From: Jon Meow <46229924+jonmeow@users.noreply.github.com>
Date: Tue, 21 Sep 2021 10:44:00 -0700
Subject: [PATCH] Prevent ostream << for pointers to printable types. (#837)

Co-authored-by: Geoff Romer <gromer@google.com>
---
 common/ostream.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/common/ostream.h b/common/ostream.h
index c430fbfbbfd8..1920a82493c5 100644
--- a/common/ostream.h
+++ b/common/ostream.h
@@ -18,6 +18,14 @@ auto operator<<(llvm::raw_ostream& out, const T& obj) -> llvm::raw_ostream& {
   return out;
 }
 
+// Prevents ostream << for pointers to printable types.
+template <typename T, typename std::enable_if<std::is_member_function_pointer<
+                          decltype(&T::Print)>::value>::type* = nullptr>
+__attribute__((unavailable(
+    "Received a pointer to a printable type, are you missing a `*`? "
+    "To print as a pointer, cast to void*."))) auto
+operator<<(llvm::raw_ostream& out, const T* /*obj*/) -> llvm::raw_ostream&;
+
 }  // namespace Carbon
 
 #endif  // COMMON_OSTREAM_H_