Include the error in the detailed error message in the GCC handler

autoload/ale/handlers/gcc.vim

@@ -92,9 +92,12 @@ function! ale#handlers#gcc#HandleGCCFormat(buffer, lines) abort
         " the previous error parsed in output
         if l:match[4] is# 'note'
             if !empty(l:output)
-                let l:output[-1]['detail'] =
+                if !has_key(l:output[-1], 'detail')
-                \   get(l:output[-1], 'detail', '')
+                    let l:output[-1].detail = l:output[-1].text
-                \   . s:RemoveUnicodeQuotes(l:match[0]) . "\n"
+                endif
+
+                let l:output[-1].detail = l:output[-1].detail . "\n"
+                \   . s:RemoveUnicodeQuotes(l:match[0])
             endif
 
             continue

test/handler/test_gcc_handler.vader

@@ -174,6 +174,22 @@ Execute(The GCC handler should handle notes with no previous message):
   \   '<stdin>:1:1: note: x',
   \ ])
 
+Execute(The GCC handler should attach notes to previous messages):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 6,
+  \     'col': 12,
+  \     'type': 'E',
+  \     'text': 'Some error',
+  \     'detail': "Some error\n<stdin>:1:1: note: x",
+  \   },
+  \ ],
+  \ ale#handlers#gcc#HandleGCCFormatWithIncludes(347, [
+  \   '-:6:12: error: Some error',
+  \   '<stdin>:1:1: note: x',
+  \ ])
+
 Execute(The GCC handler should interpret - as being the current file):
   AssertEqual
   \ [