From 8fbfc82cc2a0c95a7fd10fa3051ecad235cdaa66 Mon Sep 17 00:00:00 2001
From: Kaleb Barrett <ktbarrett@hudson-trading.com>
Date: Sat, 19 Sep 2026 19:34:30 +0200
Subject: [PATCH] Fix issues with configuring flex during build on MacOS
 (#8412)

---
 CMakeLists.txt               | 31 ++++++++++++++++++++++++++++++-
 ci/ci-build.bash             |  7 +++++++
 configure.ac                 | 12 ++++++++++++
 docs/guide/install-cmake.rst |  2 +-
 src/flexfix                  | 11 -----------
 5 files changed, 50 insertions(+), 13 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2d356722a..dceca27bc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -93,7 +93,36 @@ if(OBJCACHE_ENABLED)
 endif()
 
 find_package(BISON)
-find_package(FLEX)
+# Prefer the header installed with the selected generator, including through
+# symlinks such as Homebrew's bin/flex. Explicit cache entries take precedence.
+find_program(
+    FLEX_EXECUTABLE
+    NAMES flex win-flex win_flex
+    DOC "Path to the flex executable"
+)
+if(NOT FLEX_EXECUTABLE)
+    message(
+        FATAL_ERROR
+        "Cannot find Flex; set FLEX_EXECUTABLE to its executable path"
+    )
+endif()
+get_filename_component(flex_real_executable "${FLEX_EXECUTABLE}" REALPATH)
+get_filename_component(flex_bin_dir "${flex_real_executable}" DIRECTORY)
+find_path(
+    FLEX_INCLUDE_DIR
+    FlexLexer.h
+    PATHS "${flex_bin_dir}/../include" "${flex_bin_dir}"
+    NO_DEFAULT_PATH
+    DOC "Path to the flex headers"
+)
+find_path(FLEX_INCLUDE_DIR FlexLexer.h)
+if(NOT FLEX_INCLUDE_DIR)
+    message(
+        FATAL_ERROR
+        "Cannot find FlexLexer.h; set FLEX_INCLUDE_DIR to its directory"
+    )
+endif()
+mark_as_advanced(FLEX_EXECUTABLE FLEX_INCLUDE_DIR)
 
 # Build
 #set_property(GLOBAL PROPERTY JOB_POOLS one_job=1)
diff --git a/ci/ci-build.bash b/ci/ci-build.bash
index f5e384950..008338f9a 100755
--- a/ci/ci-build.bash
+++ b/ci/ci-build.bash
@@ -79,6 +79,13 @@ fi
 if [ "$OPT_LIGHT_DEBUG" = 1 ]; then
   CONFIGURE_ARGS="$CONFIGURE_ARGS --enable-light-debug"
 fi
+if [ "$(uname -s)" = Darwin ]; then
+  # Force the use of the Homebrew flex over the one included as part of the
+  # Apple SDK that's preinstalled on the MacOS Github Action runners.
+  FLEX_PREFIX=$(brew --prefix flex)
+  export LEX="$FLEX_PREFIX/bin/flex"
+  export CPPFLAGS="${CPPFLAGS:+$CPPFLAGS }-I$FLEX_PREFIX/include"
+fi
 autoconf
 ./configure $CONFIGURE_ARGS CXX="$CXX"
 
diff --git a/configure.ac b/configure.ac
index 164d5b304..80f51e09a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -264,6 +264,7 @@ fi
 python3_version=$($PYTHON3 --version | head -1)
 AC_MSG_RESULT([$PYTHON3 --version = $python3_version])
 
+AC_ARG_VAR([LEX], [Flex executable])
 AC_CHECK_PROG(LEX,flex,flex)
 if test "x$LEX" = "x" ; then
    AC_MSG_ERROR([Cannot find "flex" in your PATH, please install it])
@@ -271,6 +272,17 @@ fi
 flex_version=$($LEX --version | head -1)
 AC_MSG_RESULT([$LEX --version = $flex_version])
 
+# Use the header installed with the selected generator.
+flex_include=$($PYTHON3 -c '
+import pathlib
+import shutil
+import sys
+print(pathlib.Path(shutil.which(sys.argv@<:@1@:>@) or sys.argv@<:@1@:>@).resolve().parent.parent / "include")
+' "$LEX")
+if test -f "$flex_include/FlexLexer.h"; then
+   CPPFLAGS="$CPPFLAGS -I$flex_include"
+fi
+
 AC_CHECK_PROG(YACC,bison,bison)
 if test "x$YACC" = "x" ; then
    AC_MSG_ERROR([Cannot find "bison" in your PATH, please install it])
diff --git a/docs/guide/install-cmake.rst b/docs/guide/install-cmake.rst
index 6d840a552..f7ce15acf 100644
--- a/docs/guide/install-cmake.rst
+++ b/docs/guide/install-cmake.rst
@@ -28,7 +28,7 @@ Quick Install
    and install.
 #. For build on Windows using MSVC set environment variable WIN_FLEX_BISON
    to install directory. For build on Windows/Linux/OS-X using ninja set
-   the environment variable FLEX_INCLUDE to the directory containing
+   the CMake variable FLEX_INCLUDE_DIR to the directory containing
    FlexLexer.h and ensure that flex/bison is available within the PATH.
 
 To obtain Verilator sources download
diff --git a/src/flexfix b/src/flexfix
index 0a64beb0c..24b52514c 100755
--- a/src/flexfix
+++ b/src/flexfix
@@ -14,7 +14,6 @@
 
 import re
 import sys
-import platform
 
 for line in sys.stdin:
     # Fix flex 2.6.1 warning
@@ -48,14 +47,4 @@ for line in sys.stdin:
     line = re.sub(r'(#line \d+ ".*)_pretmp', r'\1', line)
     # Fix 'register' storage class specifier is deprecated and incompatible with C++17
     line = re.sub(r'register ', '', line)
-    # Change int to size_t on LexerInput/LexerOutput when using macOS Tahoe
-    if platform.system() == "Darwin":
-        try:
-            osv = platform.mac_ver()[0]  # Example "26.2"
-            if len(osv) > 2 and osv[2] == "." and int(osv[:2]) >= 26:
-                if "::LexerInput(" in line or "::LexerOutput(" in line:
-                    line = re.sub(r'int ', r'size_t ', line)
-        except IndexError as e:
-            print(f"Unexpected index error {e}")
-
     print(line, end='')