Fix pr31836 on Windows too, and correctly handle repeated separators.

The approach in D30000 assumes that the '/' returned by path::begin()
is the first element for absolute paths, but that's not true on
Windows.

Also, on Windows backslashes in include lines often end up escaped
so that there are two of them. Having backslashes in include lines
is undefined behavior in most cases and implementation-defined
behavior in C++20, but since clang treats it as normal repeated
path separators, the diagnostic should too.

Unbreaks -Wnonportable-include-path for absolute paths on Windows,
and unbreaks it on non-Windows in the case of absolute paths with
repeated directory separators.

This affects e.g. the `#include __FILE__` technique if the file
passed to clang has the wrong case for the drive letter. Before:

C:\src\llvm-project>bin\clang-cl.exe c:\src\llvm-project\test.cc
c:\\src\\llvm-project\\test.cc(4,10): warning: non-portable path to file
    '"c\\srccllvm-projectctest.cc.'; specified path differs in case from
    file name on disk [-Wnonportable-include-path]
         ^

Now:

C:\src\llvm-project> out\gn\bin\clang-cl c:\src\llvm-project\test.cc
c:\\src\\llvm-project\\test.cc(4,10): warning: non-portable path to file
    '"C:\\src\\llvm-project\\test.cc"'; specified path differs in case from
    file name on disk [-Wnonportable-include-path]
         ^

Differential Revision: https://reviews.llvm.org/D79223

clang/lib/Lex/PPDirectives.cpp

@@ -1915,14 +1915,18 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
   SourceLocation FilenameLoc = FilenameTok.getLocation();
   StringRef LookupFilename = Filename;
 
-#ifndef _WIN32
+#ifdef _WIN32
+  llvm::sys::path::Style BackslashStyle = llvm::sys::path::Style::windows;
+#else
   // Normalize slashes when compiling with -fms-extensions on non-Windows. This
   // is unnecessary on Windows since the filesystem there handles backslashes.
   SmallString<128> NormalizedPath;
+  llvm::sys::path::Style BackslashStyle = llvm::sys::path::Style::posix;
   if (LangOpts.MicrosoftExt) {
     NormalizedPath = Filename.str();
     llvm::sys::path::native(NormalizedPath);
     LookupFilename = NormalizedPath;
+    BackslashStyle = llvm::sys::path::Style::windows;
   }
 #endif
 
@@ -2105,21 +2109,44 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
       SmallString<128> Path;
       Path.reserve(Name.size()+2);
       Path.push_back(isAngled ? '<' : '"');
-      bool isLeadingSeparator = llvm::sys::path::is_absolute(Name);
+
+      const auto IsSep = [BackslashStyle](char c) {
+        return llvm::sys::path::is_separator(c, BackslashStyle);
+      };
+
       for (auto Component : Components) {
-        if (isLeadingSeparator)
-          isLeadingSeparator = false;
-        else
+        // On POSIX, Components will contain a single '/' as first element
+        // exactly if Name is an absolute path.
+        // On Windows, it will contain "C:" followed by '\' for absolute paths.
+        // The drive letter is optional for absolute paths on Windows, but
+        // clang currently cannot process absolute paths in #include lines that
+        // don't have a drive.
+        // If the first entry in Components is a directory separator,
+        // then the code at the bottom of this loop that keeps the original
+        // directory separator style copies it. If the second entry is
+        // a directory separator (the C:\ case), then that separator already
+        // got copied when the C: was processed and we want to skip that entry.
+        if (!(Component.size() == 1 && IsSep(Component[0])))
           Path.append(Component);
-        // Append the separator the user used, or the close quote
-        Path.push_back(
-          Path.size() <= Filename.size() ? Filename[Path.size()-1] :
-            (isAngled ? '>' : '"'));
+        else if (!Path.empty())
+          continue;
+
+        // Append the separator(s) the user used, or the close quote
+        if (Path.size() > Filename.size()) {
+          Path.push_back(isAngled ? '>' : '"');
+          continue;
+        }
+        assert(IsSep(Filename[Path.size()-1]));
+        do
+          Path.push_back(Filename[Path.size()-1]);
+        while (Path.size() <= Filename.size() && IsSep(Filename[Path.size()-1]));
       }
-      // For user files and known standard headers, by default we issue a diagnostic.
-      // For other system headers, we don't. They can be controlled separately.
-      auto DiagId = (FileCharacter == SrcMgr::C_User || warnByDefaultOnWrongCase(Name)) ?
-          diag::pp_nonportable_path : diag::pp_nonportable_system_path;
+      // For user files and known standard headers, issue a diagnostic.
+      // For other system headers, don't. They can be controlled separately.
+      auto DiagId =
+          (FileCharacter == SrcMgr::C_User || warnByDefaultOnWrongCase(Name))
+              ? diag::pp_nonportable_path
+              : diag::pp_nonportable_system_path;
       Diag(FilenameTok, DiagId) << Path <<
         FixItHint::CreateReplacement(FilenameRange, Path);
     }

clang/test/Lexer/case-insensitive-include-ms.c

@@ -6,6 +6,8 @@
 // RUN: %clang_cc1 -fsyntax-only -fms-compatibility %s -include %s -I %t/Output -verify
 // RUN: %clang_cc1 -fsyntax-only -fms-compatibility -fdiagnostics-parseable-fixits %s -include %s -I %t/Output 2>&1 | FileCheck %s
 
+// FIXME: Add a test with repeated backslashes once clang can handle that
+// in ms-compat mode on non-Windows hosts.
 #include "..\Output\.\case-insensitive-include.h"
 #include "..\Output\.\Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
 // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:50}:"\"..\\Output\\.\\case-insensitive-include.h\""

clang/test/Lexer/case-insensitive-include-pr31836.sh

@@ -1,5 +1,4 @@
 // REQUIRES: case-insensitive-filesystem
-// UNSUPPORTED: system-windows
 
 // RUN: mkdir -p %t
 // RUN: touch %t/case-insensitive-include-pr31836.h

clang/test/Lexer/case-insensitive-include.c

@@ -16,11 +16,11 @@
 #include "Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
 // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:38}:"\"case-insensitive-include.h\""
 
-#include "../Output/./case-insensitive-include.h"
-#include "../Output/./Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
-// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:50}:"\"../Output/./case-insensitive-include.h\""
-#include "../output/./case-insensitive-include.h" // expected-warning {{non-portable path}}
-// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:50}:"\"../Output/./case-insensitive-include.h\""
+#include "../Output/.//case-insensitive-include.h"
+#include "../Output/.//Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:51}:"\"../Output/.//case-insensitive-include.h\""
+#include "../output/.//case-insensitive-include.h" // expected-warning {{non-portable path}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:51}:"\"../Output/.//case-insensitive-include.h\""
 
 #include "apath/.././case-insensitive-include.h"
 #include "apath/.././Case-Insensitive-Include.h" // expected-warning {{non-portable path}}

llvm/include/llvm/Support/Path.h

@@ -47,7 +47,7 @@ enum class Style { windows, posix, native };
 ///   foo/       => foo,.
 ///   /foo/bar   => /,foo,bar
 ///   ../        => ..,.
-///   C:\foo\bar => C:,/,foo,bar
+///   C:\foo\bar => C:,\,foo,bar
 /// @endcode
 class const_iterator
     : public iterator_facade_base<const_iterator, std::input_iterator_tag,

llvm/unittests/Support/Path.cpp

@@ -1176,6 +1176,7 @@ TEST_F(FileSystemTest, FileMapping) {
 }
 
 TEST(Support, NormalizePath) {
+  //                           Input,        Expected Win, Expected Posix
   using TestTuple = std::tuple<const char *, const char *, const char *>;
   std::vector<TestTuple> Tests;
   Tests.emplace_back("a", "a", "a");