@acowley seems to have the same issue. I'll investigate what -fopenmp does

Yes, I add it as a filtered flag to the ccls startup options. The only downside is warnings about unknown openmp pragmas in my code.

With clang trunk, I cannot reproduce the spurious diagnostic with

hg clone https://bitbucket.org/eigen/eigen

git clone https://github.com/google/glog
cd glog; ./autogen.sh; ./configure; make -j

and clang -Ieigen -Iglog/src -fopenmp ...

Have you tried running clang '-###' -fsyntax-only -I/tmp/p/eigen -I/tmp/p/glog/src a.cc and updating .ccls (or compile_commands.json) with the clang -cc1 options? Delete suspicious option one by one to narrow down.

clang.excludeArgs: ["-fopenmp"]

cd eigen; hg up 3.3.5. The latest stable release version of eigen should be able to reproduce it. There should be a bug in it that frustrates ccls a lot.

Sorry I can reproduce it, with either head or 3.3.5

It is an issue of preamble building with -fopenmp.

In clang_complete.cc:DiagnosticMain, if I don't use preamble (basically longest prefix of preprocessor tokens), the spurious error goes away.

-    std::shared_ptr<PreambleData> preamble = session->GetPreamble();
     std::shared_ptr<PreambleData> preamble;

"use of undeclared identifier" is the fallout of an earlier issue.
When building the preamble in clang/lib/Parse/ParseOpenMP.cpp ParseOpenMPDeclarativeDirectiveWithExtDecl, this is reported as a diag::err_omp_unexpected_directive because Tok is clang:;tok::TokenKind::identifier.

 929|   case OMPD_target_teams_distribute_simd:
 930|     Diag(Tok, diag::err_omp_unexpected_directive)
 931|         << 1 << getOpenMPDirectiveName(DKind);
 932|     break;

(gdb) p SourceLocation::getFromRawEncoding(Tok.Loc).printToString(PP.getSourceManager())
$18 = "/tmp/p/eigen/Eigen/src/Core/products/Parallelizer.h:134:15"

To me it looks like this is actually very similar to the "-Werror leads to spurious diagnostics" case. It is an issue of preamble building with both SkipFunctionBodies and -fopenmp.

Eigen/src/Core/products/Parallelizer.h:134:15 is a pragma clause related to OpenMP, it references a local variable which is skipped because of SkipFunctionBodies, but the pragma itself is not skipped, thus the error.

@Riatre I feel this same. When I get more time I'll study more about clang's lexer/parser.

another MVE for this

#include <vector>

#include <parallel/algorithm>

int main() {
    std::vector<int> a = { 3, 2, 1 };
    __gnu_parallel::sort(std::begin(a), std::end(a));
    return 0;
}

@MaskRay @Riatre This is still an open issue with LLVM 8 and latest ccls for me. As far as I understand this is a LLVM problem and not directly related to ccls. So is there a bug report for LLVM/clang to track this?

FYI this is still very much an issue in LLVM 9.0.1, with the latest ccls at time of writing. Turning off openmp in a project that utilizes Eigen fixes all the false diagnostics.

Obviously, not a great option for development, but doable for now, would like to see a fix if possible, can help if needed.