diff --git a/libcxx/.clang-format b/libcxx/.clang-format
--- a/libcxx/.clang-format
+++ b/libcxx/.clang-format
@@ -4,10 +4,24 @@
 Language: Cpp
 Standard: Cpp03
 
+# Lots of code uses indent of 4, but that's not consistent.
+# AccessModifierOffset: -4
+# IndentWidth: 4
+AlignConsecutiveAssignments: true
+AlignTrailingComments: true
+# BreakBeforeBraces: Allman # Usage is not consistent.
+# AlwaysBreakAfterReturnType: All # Do we want it only in templates? We could request and addition of AlwaysBreakBeforeReturnType as well.
+
 AlwaysBreakTemplateDeclarations: true
 PointerAlignment: Left
+IndentPPDirectives: AfterHash
+# SpacesBeforeTrailingComments: 2 # Already inconsistent
 
 # Disable formatting options which may break tests.
 SortIncludes: false
 ReflowComments: false
+# Alternative solution: we can have a different .clang-format in test/. E.g. with:
+# CommentPragmas: '^ expected-' # Alternative to disabling ReflowComments.
+
+# Caveat currently released versions of clang-format mishandle concepts and consider boolean operators as rvalue references, hence using PointerAlignment.
 ---
diff --git a/libcxx/include/variant b/libcxx/include/variant
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -1663,8 +1663,8 @@
   using argument_type = monostate;
   using result_type = size_t;
 
-  inline _LIBCPP_INLINE_VISIBILITY
-  result_type operator()(const argument_type&) const _NOEXCEPT {
+  inline _LIBCPP_INLINE_VISIBILITY result_type
+  operator()(const argument_type&) const _NOEXCEPT {
     return 66740831; // return a fundamentally attractive random value.
   }
 };
diff --git a/libcxx/include/vector b/libcxx/include/vector
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -3403,7 +3403,7 @@
 erase_if(vector<_Tp, _Allocator>& __c, _Predicate __pred) {
   auto __old_size = __c.size();
   __c.erase(_VSTD::remove_if(__c.begin(), __c.end(), __pred), __c.end());
-  return __old_size - __c.size();
+    return __old_size - __c.size();
 }
 #endif
 
diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -15,6 +15,19 @@
 #
 
 steps:
+  - label: "Format"
+    command: "libcxx/utils/ci/run-buildbot check-format"
+    artifact_paths:
+      - "**/clang-format.patch"
+    agents:
+      queue: "libcxx-builders"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+
+  - wait
+
   - label: "C++03"
     command: "libcxx/utils/ci/run-buildbot generic-cxx03"
     artifact_paths:
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -8,6 +8,7 @@
 #===----------------------------------------------------------------------===##
 
 set -ex
+set -o pipefail
 
 PROGNAME="$(basename "${0}")"
 
@@ -105,6 +106,15 @@
 }
 
 case "${BUILDER}" in
+check-format)
+    clean
+    echo "+++ Checking formatting"
+    # We need to set --extensions so that clang-format checks extensionless files.
+    mkdir -p ${BUILD_DIR}
+    git-clang-format --diff --extensions ',h,hh,hpp,hxx,c,cc,cxx,cpp' HEAD~1 -- libcxx/ libcxxabi/ | tee ${BUILD_DIR}/clang-format.patch
+    # Check if the diff is empty, fail otherwise.
+    ! grep -q '^--- a' ${BUILD_DIR}/clang-format.patch
+;;
 generic-cxx03)
     export CC=clang
     export CXX=clang++
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -564,14 +564,14 @@
   for tc in feature_test_macros:
     if std not in tc["values"]:
       continue
-    inner_indent = 1
+    inner_indent = 2
     if 'depends' in tc.keys():
       assert 'internal_depends' in tc.keys()
-      result += "# if %s\n" % tc["internal_depends"]
+      result += "#  if %s\n" % tc["internal_depends"]
       inner_indent += 2
     if get_value_before(tc["values"], std) is not None:
       assert 'depends' not in tc.keys()
-      result += "# undef  %s\n" % tc["name"]
+      result += "#  undef %s\n" % tc["name"]
     line = "#%sdefine %s" % ((" " * inner_indent), tc["name"])
     line += " " * (indent - len(line))
     line += "%sL" % tc["values"][std]
@@ -580,7 +580,7 @@
     result += line
     result += "\n"
     if 'depends' in tc.keys():
-      result += "# endif\n"
+      result += "#  endif\n"
   return result
 
 def chunks(l, n):
@@ -645,9 +645,11 @@
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
+#  pragma GCC system_header
 #endif
 
+// clang-format off
+
 #if _LIBCPP_STD_VER > 11
 {cxx14_macros}
 #endif
@@ -660,6 +662,8 @@
 {cxx2a_macros}
 #endif
 
+// clang-format on
+
 #endif // _LIBCPP_VERSIONH
 """
 
@@ -680,48 +684,48 @@
 
 test_types = {
   "undefined": """
-# ifdef {name}
-#   error "{name} should not be defined before {std_first}"
-# endif
+#  ifdef {name}
+#    error "{name} should not be defined before {std_first}"
+#  endif
 """,
 
   "depends": """
-# if {depends}
-#   ifndef {name}
-#     error "{name} should be defined in {std}"
-#   endif
-#   if {name} != {value}
-#     error "{name} should have the value {value} in {std}"
-#   endif
-# else
-#   ifdef {name}
-#     error "{name} should not be defined when {depends} is not defined!"
-#   endif
-# endif
+#  if {depends}
+#    ifndef {name}
+#      error "{name} should be defined in {std}"
+#    endif
+#    if {name} != {value}
+#      error "{name} should have the value {value} in {std}"
+#    endif
+#  else
+#    ifdef {name}
+#      error "{name} should not be defined when {depends} is not defined!"
+#    endif
+#  endif
 """,
 
   "unimplemented": """
-# if !defined(_LIBCPP_VERSION)
-#   ifndef {name}
-#     error "{name} should be defined in {std}"
-#   endif
-#   if {name} != {value}
-#     error "{name} should have the value {value} in {std}"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef {name}
-#     error "{name} should not be defined because it is unimplemented in libc++!"
-#   endif
-# endif
+#  if !defined(_LIBCPP_VERSION)
+#    ifndef {name}
+#      error "{name} should be defined in {std}"
+#    endif
+#    if {name} != {value}
+#      error "{name} should have the value {value} in {std}"
+#    endif
+#  else // _LIBCPP_VERSION
+#    ifdef {name}
+#      error "{name} should not be defined because it is unimplemented in libc++!"
+#    endif
+#  endif
 """,
 
   "defined":"""
-# ifndef {name}
-#   error "{name} should be defined in {std}"
-# endif
-# if {name} != {value}
-#   error "{name} should have the value {value} in {std}"
-# endif
+#  ifndef {name}
+#    error "{name} should be defined in {std}"
+#  endif
+#  if {name} != {value}
+#    error "{name} should have the value {value} in {std}"
+#  endif
 """
 }
 
@@ -787,6 +791,8 @@
 #include <{header}>
 #include "test_macros.h"
 
+// clang-format off
+
 #if TEST_STD_VER < 14
 
 {cxx11_tests}
@@ -805,6 +811,8 @@
 
 #endif // TEST_STD_VER > 17
 
+// clang-format on
+
 int main(int, char**) {{ return 0; }}
 """.format(script_name=script_name,
            header=h,