Replace compiler with language flag.

Currently, if we have a header file for which its corresponding C++ file exists
in the compilation database, we forget that it's supposed to be a C++ header by
stripping away the compiler. Instead of just stripping it away, we should
replace the compiler with the corresponding language flag.

ycmd/completers/cpp/flags.py

@@ -137,6 +137,7 @@ def _CallExtraConfFlagsForFile( module, filename, client_data ):
 
 
 def PrepareFlagsForClang( flags, filename ):
+  flags = _CompilerToLanguageFlag( flags )
   flags = _RemoveXclangFlags( flags )
   flags = _RemoveUnusedFlags( flags, filename )
   flags = _SanitizeFlags( flags )
@@ -187,25 +188,37 @@ def _SanitizeFlags( flags ):
   return vector
 
 
+def _CompilerToLanguageFlag( flags ):
+  """When flags come from the compile_commands.json file, the first flag is
+  usually the path to the compiler that should be invoked. We want to replace
+  it with a corresponding language flag.
+  E.g., -x c for gcc and -x c++ for g++."""
+
+  # First flag doesn't start with a '-', so it's probably a compiler.
+  if not flags[ 0 ].startswith( '-' ):
+
+    # If the compiler ends with '++', it's probably a C++ compiler
+    # (E.g., c++, g++, clang++, etc).
+    if flags[ 0 ].endswith( '++' ):
+        language = 'c++'
+    else:
+        language = 'c'
+
+    flags = [ '-x', language ] + flags[ 1: ]
+
+  return flags
+
+
 def _RemoveUnusedFlags( flags, filename ):
   """Given an iterable object that produces strings (flags for Clang), removes
   the '-c' and '-o' options that Clang does not like to see when it's producing
   completions for a file. Same for '-MD' etc.
 
-  Also removes the first flag in the list if it does not
-  start with a '-' (it's highly likely to be the compiler name/path).
-
   We also try to remove any stray filenames in the flags that aren't include
   dirs."""
 
   new_flags = []
 
-  # When flags come from the compile_commands.json file, the first flag is
-  # usually the path to the compiler that should be invoked. We want to strip
-  # that.
-  if not flags[ 0 ].startswith( '-' ):
-    flags = flags[ 1: ]
-
   skip_next = False
   previous_flag_is_include = False
   previous_flag_starts_with_dash = False

ycmd/completers/cpp/tests/flags_test.py

@@ -46,20 +46,6 @@ def RemoveUnusedFlags_Passthrough_test():
        flags._RemoveUnusedFlags( [ '-foo', '-bar' ], 'file' ) )
 
 
-def RemoveUnusedFlags_RemoveCompilerPathIfFirst_test():
-  def tester( path ):
-    eq_( expected,
-        flags._RemoveUnusedFlags( [ path ] + expected, filename ) )
-
-  compiler_paths = [ 'c++', 'c', 'gcc', 'g++', 'clang', 'clang++',
-                     '/usr/bin/c++', '/some/other/path', 'some_command' ]
-  expected = [ '-foo', '-bar' ]
-  filename = 'file'
-
-  for compiler in compiler_paths:
-    yield tester, compiler
-
-
 def RemoveUnusedFlags_RemoveDashC_test():
   expected = [ '-foo', '-bar' ]
   to_remove = [ '-c' ]
@@ -189,3 +175,34 @@ def RemoveXclangFlags_test():
 
   eq_( expected + expected,
        flags._RemoveXclangFlags( expected + to_remove + expected ) )
+
+
+def CompilerToLanguageFlag_Passthrough_test():
+  eq_( [ '-foo', '-bar' ],
+       flags._CompilerToLanguageFlag( [ '-foo', '-bar' ] ) )
+
+
+def CompilerToLanguageFlag_ReplaceCCompiler_test():
+  def tester( path ):
+    eq_( [ '-x', 'c' ] + expected,
+        flags._CompilerToLanguageFlag( [ path ] + expected ) )
+
+  compiler_paths = [ 'cc', 'gcc', 'clang', '/usr/bin/cc',
+                     '/some/other/path', 'some_command' ]
+  expected = [ '-foo', '-bar' ]
+
+  for compiler in compiler_paths:
+    yield tester, compiler
+
+
+def CompilerToLanguageFlag_ReplaceCppCompiler_test():
+  def tester( path ):
+    eq_( [ '-x', 'c++' ] + expected,
+        flags._CompilerToLanguageFlag( [ path ] + expected ) )
+
+  compiler_paths = [ 'c++', 'g++', 'clang++', '/usr/bin/c++',
+                     '/some/other/path++', 'some_command++' ]
+  expected = [ '-foo', '-bar' ]
+
+  for compiler in compiler_paths:
+    yield tester, compiler