Clang-tidy asserts on Clang fixes, which were not enabled

[EugeneZelenko]

Bugzilla Link	34130
Version	unspecified
OS	Linux

Extended Description

I experienced Clang-tidy crashes on my work code base:

Fix conflicts with existing fix! The new insertion has the same insert location as an existing replacement. New replacement: ./../obj/linux-x86/debug/Hash.h: 4912:+0:"break; " Existing replacement: ./../obj/linux-x86/debug/Hash.h: 4912:+0:"[[clang::fallthrough]]; " clang-tidy: tools/clang/tools/extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp:86: virtual void (anonymous namespace)::ClangTidyDiagnosticRenderer::emitCodeContext(clang::FullSourceLoc, DiagnosticsEngine::Level, SmallVectorImpl &, ArrayRef): Assertion `false && "Fix conflicts with existing fix!"' failed.

But I run only modernize checks, not Clang diagnostics:

-checks="-,modernize-deprecated-headers" -header-filter="." -checks="-,modernize-use-default-member-init" -header-filter="."

Code in question was generated by gperf and contain next switch:

switch (hval) { default: hval += asso_values[static_cast(str[6])]; /FALLTHROUGH/ case 6: hval += asso_values[static_cast(str[5])]; /FALLTHROUGH/ case 5: hval += asso_values[static_cast(str[4])]; /FALLTHROUGH/ case 4: hval += asso_values[static_cast(str[3])]; /FALLTHROUGH/ case 3: hval += asso_values[static_cast(str[2])]; /FALLTHROUGH/ case 2: case 1: hval += asso_values[static_cast(str[0])]; break; }

[llvmbot]

I replaced warning options with -w.

-w is special, since it can't be overridden. Sometimes there's a valid need to enable certain compiler diagnostics via clang-tidy (using -extra-arg=-Wsomething + -checks=clang-diagnostic-something), and then -w would be an obstacle.

Anyway, I think it may be reasonable to disable diagnostics coming from the compilation database. At least I've not seen use cases where it would be wrong.

A step further in that direction might be to make clang diagnostics first-class citizen (i.e. only enable them when requested by the checks filter). In this case we'd need to get the list of diagnostics from Clang, so that we can list them with -list-checks and add appropriate -W flag when requested by the checks filter.

[EugeneZelenko]

I replaced warning options with -w.

[llvmbot]

You mean implicitly adding -Wno-everything (and maybe -Wno-error) to the options received from the compilation database?

[EugeneZelenko]

I also found workaround to disable warnings in compile database. But may be Clang-tidy should not envoke Clang diagnostics unless it explicitly requested in -checks?

[llvmbot]

I suppose the file you're analyzing gets the -Wimplicit-fallthrough compiler argument from a compilation database. The easiest way to work around this is to add -extra-arg=-Wno-implicit-fallthrough (or -Wno-everything for that matter). As for why this diagnostic causes an assertion failure, it's because it has two alternative fixes, which clang-tidy doesn't handle correctly.