Fix #2525 - Convert Windows paths in a Unix environment

autoload/ale/path.vim

@@ -3,13 +3,20 @@
 
 " simplify a path, and fix annoying issues with paths on Windows.
 "
-" Forward slashes are changed to back slashes so path equality works better.
+" Forward slashes are changed to back slashes so path equality works better
+" on Windows. Back slashes are changed to forward slashes on Unix.
+"
+" Unix paths can technically contain back slashes, but in practice no path
+" should, and replacing back slashes with forward slashes makes linters work
+" in environments like MSYS.
 "
 " Paths starting with more than one forward slash are changed to only one
 " forward slash, to prevent the paths being treated as special MSYS paths.
 function! ale#path#Simplify(path) abort
     if has('unix')
-        return substitute(simplify(a:path), '^//\+', '/', 'g') " no-custom-checks
+        let l:unix_path = substitute(a:path, '\\', '/', 'g')
+
+        return substitute(simplify(l:unix_path), '^//\+', '/', 'g') " no-custom-checks
     endif
 
     let l:win_path = substitute(a:path, '/', '\\', 'g')

test/handler/test_mypy_handler.vader

@@ -78,7 +78,7 @@ Execute(The mypy handler should handle Windows names with spaces):
   \   {
   \     'lnum': 4,
   \     'col': 0,
-  \     'filename': 'C:\something\with spaces.py',
+  \     'filename': ale#path#Simplify('C:\something\with spaces.py'),
   \     'type': 'E',
   \     'text': 'No library stub file for module ''django.db''',
   \   },

test/test_path_equality.vader

@@ -6,6 +6,22 @@ Before:
 After:
   delfunction CheckPath
 
+Execute(ale#path#Simplify should adjust paths correctly):
+  if has('unix')
+    " Multiple slashes should be removed correctly.
+    AssertEqual '/foo/bar/baz', ale#path#Simplify('////foo///bar///baz')
+    " Back slashes should be converted to forward slashes.
+    " This means some valid filenames are adjusted incorrectly, but in practice
+    " no filenames for code should contain back slashes, and adjusting slashes
+    " like this makes ALE work in MSYS.
+    AssertEqual 'foo/bar/baz', ale#path#Simplify('foo\bar\baz')
+  else
+    " Multiple slashes should be removed correctly.
+    AssertEqual '\foo\bar\baz', ale#path#Simplify('\\\foo\bar\baz')
+    " Forward slashes should be replaced with back slashes.
+    AssertEqual 'foo\bar\baz', ale#path#Simplify('foo/bar/baz')
+  endif
+
 Execute(ale#path#IsBufferPath should match simple relative paths):
   call ale#test#SetFilename('app/foo.txt')
 
@@ -53,3 +69,14 @@ Execute(ale#path#IsBufferPath should match files in /tmp):
   call ale#test#SetFilename('app/test.ts')
 
   Assert ale#path#IsBufferPath(bufnr(''), tempname() . '/test.ts')
+
+Execute(ale#path#IsBufferPath should match Windows paths on Unix):
+  " This test should pass Unix.
+  "
+  " Back slashes in paths should be replaced with forward slashes, even though
+  " back slashes are valid in filenames on Unix.
+  if has('unix')
+    call ale#test#SetFilename('app/foo/test.ts')
+
+    Assert ale#path#IsBufferPath(bufnr(''), 'foo\test.ts')
+  endif