[libc++] Remove libc++'s <float.h> (#175849)

The existence of <float.h> in libc++ leads to difficulties with modules,
especially when it comes to supporting the various __need macros
supported by the Clang builtin headers.

Fortunately, libc++'s <float.h> only defines FLT_EVAL_METHOD and
DECIMAL_DIG, which are already defined by the Clang builtin headers in
C++11 mode. Hence, removing the header entirely should have no impact,
apart from not defining FLT_EVAL_METHOD and DECIMAL_DIG in C++03 mode.
However, that is arguably something that libc++ shouldn't be doing: if
the underlying C library doesn't want to provide these definitions, then
libc++ shouldn't force it to.

libcxx/include/CMakeLists.txt

@@ -1027,7 +1027,6 @@ set(files
   filesystem
   flat_map
   flat_set
-  float.h
   format
   forward_list
   fstream

libcxx/include/__cxx03/cfloat

@@ -71,14 +71,12 @@ Macros:
 
 #include <__cxx03/__config>
 
-#include <float.h>
-
-#ifndef _LIBCPP_FLOAT_H
-#   error <cfloat> tried including <float.h> but didn't find libc++'s <float.h> header. \
-          This usually means that your header search paths are not configured properly. \
-          The header search paths should contain the C++ Standard Library headers before \
-          any C Standard Library, and you are probably using compiler flags that make that \
-          not be the case.
+// <float.h> is not provided by libc++
+#if __has_include(<float.h>)
+#  include <float.h>
+#  ifdef _LIBCPP_FLOAT_H
+#    error "If libc++ starts defining <float.h>, the __has_include check should move to libc++'s <float.h>"
+#  endif
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

libcxx/include/__cxx03/module.modulemap

@@ -443,10 +443,6 @@ module cxx03_std_fenv_h [system] {
   header "fenv.h"
   export *
 }
-module cxx03_std_float_h [system] {
-  header "float.h"
-  export *
-}
 module cxx03_std_inttypes_h [system] {
   header "inttypes.h"
   export *

libcxx/include/cfloat

@@ -74,14 +74,12 @@ Macros:
 #else
 #  include <__config>
 
-#  include <float.h>
-
-#  ifndef _LIBCPP_FLOAT_H
-#   error <cfloat> tried including <float.h> but didn't find libc++'s <float.h> header. \
-          This usually means that your header search paths are not configured properly. \
-          The header search paths should contain the C++ Standard Library headers before \
-          any C Standard Library, and you are probably using compiler flags that make that \
-          not be the case.
+// <float.h> is not provided by libc++
+#  if __has_include(<float.h>)
+#    include <float.h>
+#    ifdef _LIBCPP_FLOAT_H
+#      error "If libc++ starts defining <float.h>, the __has_include check should move to libc++'s <float.h>"
+#    endif
 #  endif
 
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

libcxx/include/float.h

@@ -1,99 +0,0 @@
-// -*- C++ -*-
-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef _LIBCPP_FLOAT_H
-#define _LIBCPP_FLOAT_H
-
-/*
-    float.h synopsis
-
-Macros:
-
-    FLT_ROUNDS
-    FLT_EVAL_METHOD     // C99
-    FLT_RADIX
-
-    FLT_MANT_DIG
-    DBL_MANT_DIG
-    LDBL_MANT_DIG
-
-    FLT_HAS_SUBNORM     // C11
-    DBL_HAS_SUBNORM     // C11
-    LDBL_HAS_SUBNORM    // C11
-
-    DECIMAL_DIG         // C99
-    FLT_DECIMAL_DIG     // C11
-    DBL_DECIMAL_DIG     // C11
-    LDBL_DECIMAL_DIG    // C11
-
-    FLT_DIG
-    DBL_DIG
-    LDBL_DIG
-
-    FLT_MIN_EXP
-    DBL_MIN_EXP
-    LDBL_MIN_EXP
-
-    FLT_MIN_10_EXP
-    DBL_MIN_10_EXP
-    LDBL_MIN_10_EXP
-
-    FLT_MAX_EXP
-    DBL_MAX_EXP
-    LDBL_MAX_EXP
-
-    FLT_MAX_10_EXP
-    DBL_MAX_10_EXP
-    LDBL_MAX_10_EXP
-
-    FLT_MAX
-    DBL_MAX
-    LDBL_MAX
-
-    FLT_EPSILON
-    DBL_EPSILON
-    LDBL_EPSILON
-
-    FLT_MIN
-    DBL_MIN
-    LDBL_MIN
-
-    FLT_TRUE_MIN        // C11
-    DBL_TRUE_MIN        // C11
-    LDBL_TRUE_MIN       // C11
-
-*/
-
-#if defined(__cplusplus) && __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
-#  include <__cxx03/__config>
-#else
-#  include <__config>
-#endif
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-#if __has_include_next(<float.h>)
-#  include_next <float.h>
-#endif
-
-#ifdef __cplusplus
-
-#  ifndef FLT_EVAL_METHOD
-#    define FLT_EVAL_METHOD __FLT_EVAL_METHOD__
-#  endif
-
-#  ifndef DECIMAL_DIG
-#    define DECIMAL_DIG __DECIMAL_DIG__
-#  endif
-
-#endif // __cplusplus
-
-#endif // _LIBCPP_FLOAT_H

libcxx/include/module.modulemap.in

@@ -2428,10 +2428,6 @@ module std_fenv_h [system] {
   header "fenv.h"
   export *
 }
-module std_float_h [system] {
-  header "float.h"
-  export *
-}
 module std_inttypes_h [system] {
   header "inttypes.h"
   export *

libcxx/test/std/depr/depr.c.headers/float_h.compile.pass.cpp

@@ -7,6 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 // test <float.h>
+//
+// Even though <float.h> is not provided by libc++, we still test that using it
+// with libc++ on the search path will work, and that it provides the necessary
+// content.
 
 #include <float.h>
 
@@ -16,9 +20,11 @@
 #error FLT_ROUNDS not defined
 #endif
 
+#if TEST_STD_VER >= 11
 #ifndef FLT_EVAL_METHOD
 #error FLT_EVAL_METHOD not defined
 #endif
+#endif
 
 #ifndef FLT_RADIX
 #error FLT_RADIX not defined
@@ -50,9 +56,11 @@
 #error LDBL_MANT_DIG not defined
 #endif
 
+#if TEST_STD_VER >= 11
 #ifndef DECIMAL_DIG
 #error DECIMAL_DIG not defined
 #endif
+#endif
 
 #if TEST_STD_VER > 14
 #ifndef FLT_DECIMAL_DIG

libcxx/test/std/language.support/support.limits/c.limits/cfloat.pass.cpp

@@ -16,9 +16,11 @@
 #error FLT_ROUNDS not defined
 #endif
 
+#if TEST_STD_VER >= 11
 #ifndef FLT_EVAL_METHOD
 #error FLT_EVAL_METHOD not defined
 #endif
+#endif
 
 #ifndef FLT_RADIX
 #error FLT_RADIX not defined
@@ -50,9 +52,11 @@
 #error LDBL_MANT_DIG not defined
 #endif
 
+#if TEST_STD_VER >= 11
 #ifndef DECIMAL_DIG
 #error DECIMAL_DIG not defined
 #endif
+#endif
 
 #if TEST_STD_VER > 14
 #ifndef FLT_DECIMAL_DIG