[llvm] [ADT] Fix alignment check in unique_function constructor (PR #99403)

[Dmitry Yanovsky via llvm-commits]

https://github.com/kerambyte created https://github.com/llvm/llvm-project/pull/99403

Right now the check fails for any state-capturing lambda since this expression - `alignof(decltype(StorageUnion.InlineStorage))` - returns 1 for the alignment value and not 4/8 as expected ([MSVC|Clang|GCC](https://godbolt.org/z/eTEdq4xjM)). So this check fails for pretty much any state-capturing callable we try to store into a `unique_function` and we take the out-of-line storage path:

```
if (sizeof(CallableT) > InlineStorageSize ||
    alignof(CallableT) > alignof(decltype(StorageUnion.InlineStorage))) {
  // ...
}
```

The fix is simply to use an explicit const variable to store the alignment value.

There is no easy way to unit-test the fix since inline storage is considered to be an implementation detail so we shouldn't assume how the lambda ends up being stored.

>From 9506c5f340b3ac49477a7a61684def1ecf6e57c2 Mon Sep 17 00:00:00 2001
From: kerambyte <kerambyte at gmail.com>
Date: Thu, 18 Jul 2024 00:14:54 +0100
Subject: [PATCH] initial commit

---
 llvm/include/llvm/ADT/FunctionExtras.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/llvm/include/llvm/ADT/FunctionExtras.h b/llvm/include/llvm/ADT/FunctionExtras.h
index 49e0e8ab0db40..ad5c89cbc8bda 100644
--- a/llvm/include/llvm/ADT/FunctionExtras.h
+++ b/llvm/include/llvm/ADT/FunctionExtras.h
@@ -80,6 +80,7 @@ using EnableIfCallable = std::enable_if_t<std::disjunction<
 template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
 protected:
   static constexpr size_t InlineStorageSize = sizeof(void *) * 3;
+  static constexpr size_t InlineStorageAlign = alignof(void *);
 
   template <typename T, class = void>
   struct IsSizeLessThanThresholdT : std::false_type {};
@@ -161,7 +162,8 @@ template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
     // provide three pointers worth of storage here.
     // This is mutable as an inlined `const unique_function<void() const>` may
     // still modify its own mutable members.
-    alignas(void *) mutable std::byte InlineStorage[InlineStorageSize];
+    alignas(InlineStorageAlign) mutable std::byte
+        InlineStorage[InlineStorageSize];
   } StorageUnion;
 
   // A compressed pointer to either our dispatching callback or our table of
@@ -262,7 +264,7 @@ template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
     bool IsInlineStorage = true;
     void *CallableAddr = getInlineStorage();
     if (sizeof(CallableT) > InlineStorageSize ||
-        alignof(CallableT) > alignof(decltype(StorageUnion.InlineStorage))) {
+        alignof(CallableT) > InlineStorageAlign) {
       IsInlineStorage = false;
       // Allocate out-of-line storage. FIXME: Use an explicit alignment
       // parameter in C++17 mode.