use clang as a doxygen preprocessor and turn requires clauses into \p…

…re blocks

.clang-format

@@ -46,7 +46,7 @@
   ForEachMacros: ['RANGES_FOR',],
   IncludeBlocks: Regroup,
   IncludeCategories: [
-    { Regex:           '^<range/v3/detail/disable_warnings.hpp',
+    { Regex:           '^<range/v3/detail/prologue.hpp',
       Priority:        8},
     { Regex:           '^<range/v3/range_fwd.hpp',
       Priority:        6},

doc/CMakeLists.txt

@@ -11,6 +11,7 @@ endif()
 set(CMAKE_FOLDER "doc")
 
 configure_file(Doxyfile.in Doxyfile @ONLY)
+configure_file(preprocess.sh.in preprocess.sh @ONLY)
 add_custom_target(doc.check
     COMMAND ${DOXYGEN_EXECUTABLE} Doxyfile
     COMMENT "Running Doxygen to validate the documentation"

doc/Doxyfile.in

@@ -65,6 +65,7 @@ EXAMPLE_PATH            = @Range-v3_SOURCE_DIR@/example \
                           @Range-v3_SOURCE_DIR@/test
 EXAMPLE_RECURSIVE       = YES
 IMAGE_PATH              = @Range-v3_BINARY_DIR@/image
+FILTER_PATTERNS         = *.hpp="\"@Range-v3_BINARY_DIR@/doc/preprocess.sh\""
 WARN_IF_UNDOCUMENTED    = NO
 
 SHOW_GROUPED_MEMB_INC   = YES
@@ -86,7 +87,6 @@ INLINE_SIMPLE_STRUCTS   = NO
 GENERATE_HTML           = YES
 GENERATE_LATEX          = NO
 
-
 GENERATE_TODOLIST       = YES
 GENERATE_TESTLIST       = YES
 GENERATE_BUGLIST        = YES

doc/ignore_errors.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+$* 2>/dev/null
+exit 0

doc/preprocess.sh.in

@@ -0,0 +1,3 @@
+#!/bin/bash
+@Range-v3_SOURCE_DIR@/doc/ignore_errors.sh @CMAKE_CXX_COMPILER@ -x c++ -std=c++2a -DRANGES_DOXYGEN_INVOKED=1 -DMETA_DOXYGEN_INVOKED=1 -DCPP_DOXYGEN_INVOKED=1 -I @Range-v3_SOURCE_DIR@/include -E -CC $1 | @Range-v3_SOURCE_DIR@/doc/unpreprocess.pl
+exit 0

doc/unpreprocess.pl

@@ -0,0 +1,19 @@
+#!/usr/bin/perl
+
+use strict;
+
+my $file = "";
+my $first = 1;
+my $emit = 0;
+
+while(<>) {
+    if ($first) {
+        $_ =~ m/^#\s*\d+\s+"(.*)"/;
+        $file = $1;
+        $first = 0;
+    } elsif ($_ =~ m/^#\s*\d+\s+"(.*)"/) {
+        $emit = ($1 eq $file);
+    } elsif ($emit) {
+        print $_;
+    }
+}