[llvm] Re-enable MSVC C4091 diagnostic; NFC (PR #182500)

[Aaron Ballman via llvm-commits]

https://github.com/AaronBallman created https://github.com/llvm/llvm-project/pull/182500

>From MSDN:
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4091?view=msvc-170

> 'keyword': ignored on left of 'type' when no variable is declared

This diagnostic was disabled in 0cba642a05ad0675f313e55fce29ce5273340a59 because the diagnostic was being hit in system headers under MSVC 2015. That no longer appears to be the case from my local testing; this diagnostic does not appear to be emitted.

>From 7ef9f259925a8e1840fd2400b69e4715aea8b581 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Fri, 20 Feb 2026 08:51:21 -0500
Subject: [PATCH] Re-enable MSVC C4091 diagnostic; NFC

>From MSDN:
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4091?view=msvc-170

> 'keyword': ignored on left of 'type' when no variable is declared

This diagnostic was disabled in 0cba642a05ad0675f313e55fce29ce5273340a59
because the diagnostic was being hit in system headers under MSVC 2015.
That no longer appears to be the case from my local testing; this
diagnostic does not appear to be emitted.
---
 llvm/cmake/modules/HandleLLVMOptions.cmake | 1 -
 1 file changed, 1 deletion(-)

diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index c9c720d84ed62..e14c30993a112 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -754,7 +754,6 @@ if (MSVC)
       -wd4805 # Suppress 'unsafe mix of type <type> and type <type> in operation'
       -wd4204 # Suppress 'nonstandard extension used : non-constant aggregate initializer'
       -wd4577 # Suppress 'noexcept used with no exception handling mode specified; termination on exception is not guaranteed'
-      -wd4091 # Suppress 'typedef: ignored on left of '' when no variable is declared'
           # C4592 is disabled because of false positives in Visual Studio 2015
           # Update 1. Re-evaluate the usefulness of this diagnostic with Update 2.
       -wd4592 # Suppress ''var': symbol will be dynamically initialized (implementation limitation)