Avoid overriding parsed C/C++ -std=* flag

ALE appends flags from {c,cpp}_{clang,gcc}_options after those found by
parsing compile_commands.json or Makefile output. If -std=* flags are
present in both the ALE flags and parsed flags, the last one present
(i.e., ALE's -std=* flag) will determine the mode the compiler works in.
This can result in errors showing up in vim but not in the actual build
or vice-versa.

For example, say you have foo.cpp:

    #include <type_traits>
    int main() {
        return std::is_same_v<float, int>;
    }

If cpp_clang_options contains -std=c++17 and -std=c++14 is parsed from
compile_commands.json, then ALE would end up running something like:

    clang++ -S -x c++ -fsyntax-only -std=c++14 -std=c++17 - < foo.cpp

This would result in no errors showing up in Vim, but the actual build
would fail with:

    <stdin>:3:14: error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?
            return std::is_same_v<float, int>;
                   ~~~~~^~~~~~~~~
                        is_same
    /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/type_traits:872:61: note: 'is_same' declared here
    template <class _Tp, class _Up> struct _LIBCPP_TEMPLATE_VIS is_same           : public false_type {};
                                                                ^
    <stdin>:3:35: error: expected '(' for function-style cast or type construction
            return std::is_same_v<float, int>;
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~^
    2 errors generated.

as the actual build would not have the -std=c++17 flag added by ALE.

If cpp_clang_options contains -std=c++14 and -std=c++17 is parsed from
compile_commands.json, then the opposite problem would occur. ALE would
end up running something like:

    clang++ -S -x c++ -fsyntax-only -std=c++17 -std=c++14 - < foo.cpp

and would show an error on line 3 of foo.cpp:

    [clang] No template named 'is_same_v' in namespace 'std'; did you mean 'is_same'? (fix available)

The actual build, on the other hand, would succeed without any
complaints.

Removing -std=* from ALE's flags if it is already present in the parsed
flags ensures that the wrong -std=* flag is not used.

An alternative would have been to switch the order in which parsed flags
and ALE flags were concatenated when producing the command to execute,
but that could prevent a user from intentionally using ALE's flags to
override some other flags, e.g.  -W* flags to enable/disable warnings in
a project whose flags are not under the developer's control.

-std=* flags are also present in cuda/nvcc.vim, objc/clang.vim,
objcpp/clang.vim, and vhdl/ghdl.vim, but none of those linters appear to
parse compile_commands.json or `make` output.

2 files changed, 20 insertions, 2 deletions

diff --git a/ale_linters/c/clang.vim b/ale_linters/c/clang.vim
index 681101fc..84c105de 100644
--- a/ale_linters/c/clang.vim
+++ b/ale_linters/c/clang.vim
@@ -6,13 +6,22 @@ call ale#Set('c_clang_options', '-std=c11 -Wall')
 
 function! ale_linters#c#clang#GetCommand(buffer, output) abort
     let l:cflags = ale#c#GetCFlags(a:buffer, a:output)
+    let l:ale_flags = ale#Var(a:buffer, 'c_clang_options')
+
+    if l:cflags =~# '-std='
+        let l:ale_flags = substitute(
+        \   l:ale_flags,
+        \   '-std=\(c\|gnu\)[0-9]\{2\}',
+        \   '',
+        \   'g')
+    endif
 
     " -iquote with the directory the file is in makes #include work for
     "  headers in the same directory.
     return '%e -S -x c -fsyntax-only'
     \   . ' -iquote ' . ale#Escape(fnamemodify(bufname(a:buffer), ':p:h'))
     \   . ale#Pad(l:cflags)
-    \   . ale#Pad(ale#Var(a:buffer, 'c_clang_options')) . ' -'
+    \   . ale#Pad(l:ale_flags) . ' -'
 endfunction
 
 call ale#linter#Define('c', {
diff --git a/ale_linters/c/gcc.vim b/ale_linters/c/gcc.vim
index 1df1018e..252285a3 100644
--- a/ale_linters/c/gcc.vim
+++ b/ale_linters/c/gcc.vim
@@ -6,6 +6,15 @@ call ale#Set('c_gcc_options', '-std=c11 -Wall')
 
 function! ale_linters#c#gcc#GetCommand(buffer, output) abort
     let l:cflags = ale#c#GetCFlags(a:buffer, a:output)
+    let l:ale_flags = ale#Var(a:buffer, 'c_gcc_options')
+
+    if l:cflags =~# '-std='
+        let l:ale_flags = substitute(
+        \   l:ale_flags,
+        \   '-std=\(c\|gnu\)[0-9]\{2\}',
+        \   '',
+        \   'g')
+    endif
 
     " -iquote with the directory the file is in makes #include work for
     "  headers in the same directory.
@@ -16,7 +25,7 @@ function! ale_linters#c#gcc#GetCommand(buffer, output) abort
     \   . ' -o ' . g:ale#util#nul_file
     \   . ' -iquote ' . ale#Escape(fnamemodify(bufname(a:buffer), ':p:h'))
     \   . ale#Pad(l:cflags)
-    \   . ale#Pad(ale#Var(a:buffer, 'c_gcc_options')) . ' -'
+    \   . ale#Pad(l:ale_flags) . ' -'
 endfunction
 
 call ale#linter#Define('c', {