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+- Support for Compilation Database [feature]
+
+Number of CLang-based C/C++ code analyzing tools (such as clang-check,
+clang-format, clang-tidy) require compilation flags to be provided for the
+source file being analized. The information can be provided in the form of
+Compilation Database (http://clang.llvm.org/docs/JSONCompilationDatabase.html).
+
+The following (referenced by llvm.org) tools generate compile_commands.json
+file:
+
+* CMake >= 2.8.5. Command 'cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON' generates
+  compile_commands.json among other files.
+
+* Build EAR tool (https://github.com/rizsotto/Bear) creates json during build
+  process by intercepting compiler exec*() calls.
+
+* Bunch of wrapper scripts (https://pypi.python.org/pypi/scan-build) apply
+  intercepting approach as well. Command 'intercept-build b clean update' being
+  run in build2 project directory produces compile_commands.json file.
+
+* The Ninja build system (https://ninja-build.org/manual.html) produce the file
+  running a dedicated operation (ninja -t compdb rule1 rule2 ...) against
+  specified list of rules (whatever it means).
+
+
+So how we would like to support that database? Specifically the questions are:
+
+* Which moment it is get generated and updated (if ever should)?
+
+* Where the json file should be placed?
+
+* Which files it should reference?
+
+Seems there is not much sense to have any long-living database being updated
+by consecutive build2 runs. The different source files with inconsistent
+compilation flags can be present in such a database which makes it pretty much
+useless (if not to say harmfull). So probably generating new database by an
+explicit request is the right approach.
+
+Probably generating the file as a part of update operation but being explicitly
+asked for that would be the most intuitive way. Especially in the light of
+build2 ability to change compilation options on the fly based on command line
+variables. Meanwhile the resulted file should not dependend on the fact if
+files it references were indeed compiled. So running update on cleaned up and
+up to date project directory should result in the same database file. [That btw
+not true about all mentioned interception-based tools which doesn't tell in
+their favor.] For a symmetry clean operation could remove the file if
+requested.
+
+The location of the resulted database file can be specified as a part of that
+explicit request.
+
+The resulted database file can reference source files that are prerequisites
+of the targets being built. It probably make sense to reduce the set to source
+files belonging to the projects of the targets. We could even try to narrow
+down further based on the assumption that user most likelly expect only the
+files in the current working directory (and below) to be present in the
+database. While maybe having some "extra" files is not an issue (as not very
+likelly somebody will read them through too often) so project-based filtering
+is probably a good compromise.
+
+Implementation-wise we could invent configuration variable(s)
+(config.cc.cdbase=<path>) not inventing any new build2 option(s). It could be
+convenient to add the variable to the project configuration so the json file
+reflects the last update operation and get removed with a clean operation. If
+<path> is an existing directory then <path>/compile_commands.json file path is
+assumed.
+
+Should be possible to run like 'b config.cc.cdbase=. bpkg/ brep/' so
+compile_commands.json common for both projects is created in the current
+directory.
+
+
+Example of generating/using compilation database file:
+
+$ mkdir hello
+$ cd hello
+$ cat > hello.cpp <<EOF
+namespace abc
+{
+  const int a (10);
+}
+
+void
+test(int z)
+{
+  if (z == 0)
+    int x (1 / z);
+}
+
+int
+main()
+{
+  int b (0);
+  b += 1;
+  return 0;
+}
+EOF
+
+$ clang-tidy hello.cpp
+LLVM ERROR: Could not auto-detect compilation database for file "hello.cpp"
+No compilation database found in /home/karen/projects/hello or any parent directory
+json-compilation-database: Error while opening JSON database: No such file or directory
+
+$ cat > CMakeLists.txt <<EOF
+cmake_minimum_required(VERSION 2.8.5)
+add_executable(hello hello.cpp)
+EOF
+
+$ cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
+$ cat compile_commands.json
+[
+{
+  "directory": "/home/karen/projects/hello",
+  "command": "/usr/bin/c++      -o CMakeFiles/hello.dir/hello.cpp.o -c /home/karen/projects/hello/hello.cpp",
+  "file": "/home/karen/projects/hello/hello.cpp"
+}
+]
+
+$ clang-tidy hello.cpp
+/home/karen/projects/hello/hello.cpp:1:11: warning: namespace not terminated with a closing comment [llvm-namespace-comment]
+namespace abc
+          ^
+/home/karen/projects/hello/hello.cpp:10:9: warning: Value stored to 'x' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
+    int x (1 / z);
+        ^
+/home/karen/projects/hello/hello.cpp:10:9: note: Value stored to 'x' during its initialization is never read
+    int x (1 / z);
+        ^
+/home/karen/projects/hello/hello.cpp:10:14: warning: Division by zero [clang-analyzer-core.DivideZero]
+    int x (1 / z);
+             ^
+/home/karen/projects/hello/hello.cpp:9:7: note: Assuming 'z' is equal to 0
+  if (z == 0)
+      ^
+/home/karen/projects/hello/hello.cpp:9:3: note: Taking true branch
+  if (z == 0)
+  ^
+/home/karen/projects/hello/hello.cpp:10:14: note: Division by zero
+    int x (1 / z);
+             ^
+/home/karen/projects/hello/hello.cpp:17:3: warning: Value stored to 'b' is never read [clang-analyzer-deadcode.DeadStores]
+  b += 1;
+  ^
+/home/karen/projects/hello/hello.cpp:17:3: note: Value stored to 'b' is never read
+  b += 1;
+  ^