False positive `-Wempty-translation-unit` warning

[0f-0b]

clangd reports an -Wempty-translation-unit warning in this C code,

int main() {}

…under the following configuration.

CompileFlags:
  Add: -Wempty-translation-unit

I think this is a bug in clangd because clang -Wempty-translation-unit on the same code doesn't print any warning.

Logs

I[19:38:40.000] clangd version 21.0.0git (https://github.com/llvm/llvm-project a2ca2f3f10002da61e9860d0ce11e0272482baba)
I[19:38:40.002] Features: mac+grpc+xpc
I[19:38:40.002] PID: 485
I[19:38:40.002] Working directory: /Users/ud2/repro
I[19:38:40.002] argv[0]: /Users/ud2/Downloads/clangd_snapshot_20250401/bin/clangd
I[19:38:40.002] argv[1]: --log=verbose
V[19:38:40.002] User config file is /Users/ud2/Library/Preferences/clangd/config.yaml
I[19:38:40.002] Starting LSP over stdin/stdout
V[19:38:40.003] <<< {"id":0,"jsonrpc":"2.0","method":"initialize","params":{"capabilities":{"general":{"markdown":{"parser":"marked","version":"1.1.0"},"positionEncodings":["utf-16"],"regularExpressions":{"engine":"ECMAScript","version":"ES2020"},"staleRequestSupport":{"cancel":true,"retryOnContentModified":["textDocument/semanticTokens/full","textDocument/semanticTokens/range","textDocument/semanticTokens/full/delta"]}},"notebookDocument":{"synchronization":{"dynamicRegistration":true,"executionSummarySupport":true}},"textDocument":{"callHierarchy":{"dynamicRegistration":true},"codeAction":{"codeActionLiteralSupport":{"codeActionKind":{"valueSet":["","quickfix","refactor","refactor.extract","refactor.inline","refactor.rewrite","source","source.organizeImports"]}},"dataSupport":true,"disabledSupport":true,"dynamicRegistration":true,"honorsChangeAnnotations":false,"isPreferredSupport":true,"resolveSupport":{"properties":["edit"]}},"codeLens":{"dynamicRegistration":true},"colorProvider":{"dynamicRegistration":true},"completion":{"completionItem":{"commitCharactersSupport":true,"deprecatedSupport":true,"documentationFormat":["markdown","plaintext"],"insertReplaceSupport":true,"insertTextModeSupport":{"valueSet":[1,2]},"labelDetailsSupport":true,"preselectSupport":true,"resolveSupport":{"properties":["documentation","detail","additionalTextEdits"]},"snippetSupport":true,"tagSupport":{"valueSet":[1]}},"completionItemKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]},"completionList":{"itemDefaults":["commitCharacters","editRange","insertTextFormat","insertTextMode"]},"contextSupport":true,"dynamicRegistration":true,"editsNearCursor":true,"insertTextMode":2},"declaration":{"dynamicRegistration":true,"linkSupport":true},"definition":{"dynamicRegistration":true,"linkSupport":true},"diagnostic":{"dynamicRegistration":true,"relatedDocumentSupport":false},"documentHighlight":{"dynamicRegistration":true},"documentLink":{"dynamicRegistration":true,"tooltipSupport":true},"documentSymbol":{"dynamicRegistration":true,"hierarchicalDocumentSymbolSupport":true,"labelSupport":true,"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]},"tagSupport":{"valueSet":[1]}},"foldingRange":{"dynamicRegistration":true,"foldingRange":{"collapsedText":false},"foldingRangeKind":{"valueSet":["comment","imports","region"]},"lineFoldingOnly":true,"rangeLimit":5000},"formatting":{"dynamicRegistration":true},"hover":{"contentFormat":["markdown","plaintext"],"dynamicRegistration":true},"implementation":{"dynamicRegistration":true,"linkSupport":true},"inactiveRegionsCapabilities":{"inactiveRegions":true},"inlayHint":{"dynamicRegistration":true,"resolveSupport":{"properties":["tooltip","textEdits","label.tooltip","label.location","label.command"]}},"inlineValue":{"dynamicRegistration":true},"linkedEditingRange":{"dynamicRegistration":true},"onTypeFormatting":{"dynamicRegistration":true},"publishDiagnostics":{"codeDescriptionSupport":true,"dataSupport":true,"relatedInformation":true,"tagSupport":{"valueSet":[1,2]},"versionSupport":false},"rangeFormatting":{"dynamicRegistration":true},"references":{"dynamicRegistration":true},"rename":{"dynamicRegistration":true,"honorsChangeAnnotations":true,"prepareSupport":true,"prepareSupportDefaultBehavior":1},"selectionRange":{"dynamicRegistration":true},"semanticTokens":{"augmentsSyntaxTokens":true,"dynamicRegistration":true,"formats":["relative"],"multilineTokenSupport":false,"overlappingTokenSupport":false,"requests":{"full":{"delta":true},"range":true},"serverCancelSupport":true,"tokenModifiers":["declaration","definition","readonly","static","deprecated","abstract","async","modification","documentation","defaultLibrary"],"tokenTypes":["namespace","type","class","enum","interface","struct","typeParameter","parameter","variable","property","enumMember","event","function","method","macro","keyword","modifier","comment","string","number","regexp","operator","decorator"]},"signatureHelp":{"contextSupport":true,"dynamicRegistration":true,"signatureInformation":{"activeParameterSupport":true,"documentationFormat":["markdown","plaintext"],"parameterInformation":{"labelOffsetSupport":true}}},"synchronization":{"didSave":true,"dynamicRegistration":true,"willSave":true,"willSaveWaitUntil":true},"typeDefinition":{"dynamicRegistration":true,"linkSupport":true},"typeHierarchy":{"dynamicRegistration":true}},"window":{"showDocument":{"support":true},"showMessage":{"messageActionItem":{"additionalPropertiesSupport":true}},"workDoneProgress":true},"workspace":{"applyEdit":true,"codeLens":{"refreshSupport":true},"configuration":true,"diagnostics":{"refreshSupport":true},"didChangeConfiguration":{"dynamicRegistration":true},"didChangeWatchedFiles":{"dynamicRegistration":true,"relativePatternSupport":true},"executeCommand":{"dynamicRegistration":true},"fileOperations":{"didCreate":true,"didDelete":true,"didRename":true,"dynamicRegistration":true,"willCreate":true,"willDelete":true,"willRename":true},"inlayHint":{"refreshSupport":true},"inlineValue":{"refreshSupport":true},"semanticTokens":{"refreshSupport":true},"symbol":{"dynamicRegistration":true,"resolveSupport":{"properties":["location.range"]},"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]},"tagSupport":{"valueSet":[1]}},"workspaceEdit":{"changeAnnotationSupport":{"groupsOnLabel":true},"documentChanges":true,"failureHandling":"textOnlyTransactional","normalizesLineEndings":true,"resourceOperations":["create","rename","delete"]},"workspaceFolders":true}},"clientInfo":{"name":"Visual Studio Code","version":"1.99.0"},"initializationOptions":{"clangdFileStatus":true,"fallbackFlags":[]},"locale":"en","processId":449,"rootPath":"/Users/ud2/repro","rootUri":"file:///Users/ud2/repro","trace":"off","workspaceFolders":[{"name":"repro","uri":"file:///Users/ud2/repro"}]}}

I[19:38:40.003] <-- initialize(0)
V[19:38:40.003] Invoking /usr/bin/xcrun to find clang installation
V[19:38:40.021] Invoking /usr/bin/xcrun to find clang installation
I[19:38:40.032] --> reply:initialize(0) 29 ms
V[19:38:40.032] >>> {"id":0,"jsonrpc":"2.0","result":{"capabilities":{"astProvider":true,"callHierarchyProvider":true,"clangdInlayHintsProvider":true,"codeActionProvider":{"codeActionKinds":["quickfix","refactor","info"]},"compilationDatabase":{"automaticReload":true},"completionProvider":{"resolveProvider":false,"triggerCharacters":[".","<",">",":","\"","/","*"]},"declarationProvider":true,"definitionProvider":true,"documentFormattingProvider":true,"documentHighlightProvider":true,"documentLinkProvider":{"resolveProvider":false},"documentOnTypeFormattingProvider":{"firstTriggerCharacter":"\n","moreTriggerCharacter":[]},"documentRangeFormattingProvider":true,"documentSymbolProvider":true,"executeCommandProvider":{"commands":["clangd.applyFix","clangd.applyRename","clangd.applyTweak"]},"foldingRangeProvider":true,"hoverProvider":true,"implementationProvider":true,"inactiveRegionsProvider":true,"inlayHintProvider":true,"memoryUsageProvider":true,"referencesProvider":true,"renameProvider":{"prepareProvider":true},"selectionRangeProvider":true,"semanticTokensProvider":{"full":{"delta":true},"legend":{"tokenModifiers":["declaration","definition","deprecated","deduced","readonly","static","abstract","virtual","dependentName","defaultLibrary","usedAsMutableReference","usedAsMutablePointer","constructorOrDestructor","userDefined","functionScope","classScope","fileScope","globalScope"],"tokenTypes":["variable","variable","parameter","function","method","function","property","variable","class","interface","enum","enumMember","type","type","unknown","namespace","typeParameter","concept","type","macro","modifier","operator","bracket","label","comment"]},"range":false},"signatureHelpProvider":{"triggerCharacters":["(",")","{","}","<",">",","]},"standardTypeHierarchyProvider":true,"textDocumentSync":{"change":2,"openClose":true,"save":true},"typeDefinitionProvider":true,"typeHierarchyProvider":true,"workspaceSymbolProvider":true},"serverInfo":{"name":"clangd","version":"clangd version 21.0.0git (https://github.com/llvm/llvm-project a2ca2f3f10002da61e9860d0ce11e0272482baba) mac+grpc+xpc arm64-apple-darwin24.3.0; target=x86_64-apple-darwin24.3.0"}}}

V[19:38:40.033] <<< {"jsonrpc":"2.0","method":"initialized","params":{}}

I[19:38:40.033] <-- initialized
V[19:38:41.934] <<< {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"languageId":"c","text":"int main() {}\n","uri":"file:///Users/ud2/repro/main.c","version":1}}}

I[19:38:41.934] <-- textDocument/didOpen
V[19:38:41.934] config note at /Users/ud2/repro/.clangd:1:0: Parsing config fragment
V[19:38:41.934] config note at /Users/ud2/repro/.clangd:1:0: Parsed 1 fragments from file
V[19:38:41.934] Config fragment: compiling /Users/ud2/repro/.clangd:1 -> 0x0000000121004FB8 (trusted=false)
I[19:38:41.935] --> textDocument/publishDiagnostics
V[19:38:41.935] >>> {"jsonrpc":"2.0","method":"textDocument/publishDiagnostics","params":{"diagnostics":[],"uri":"file:///Users/ud2/repro/.clangd"}}

I[19:38:41.935] Failed to find compilation database for /Users/ud2/repro/main.c
I[19:38:41.935] ASTWorker building file /Users/ud2/repro/main.c version 1 with command clangd fallback
[/Users/ud2/repro]
/Library/Developer/CommandLineTools/usr/bin/clang -Wempty-translation-unit -resource-dir=/Users/ud2/Downloads/clangd_snapshot_20250401/lib/clang/21 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -- /Users/ud2/repro/main.c
V[19:38:41.937] Driver produced command: cc1 -cc1 -triple x86_64-apple-macosx15.0.0 -Wundef-prefix=TARGET_OS_ -Werror=undef-prefix -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -fsyntax-only -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name main.c -mrelocation-model pic -pic-level 2 -mframe-pointer=all -ffp-contract=on -fno-rounding-math -funwind-tables=2 -target-sdk-version=15.4 -fcompatibility-qualified-id-block-type-checking -fvisibility-inlines-hidden-static-local-var -fdefine-target-os-macros -fno-modulemap-allow-subdirectory-search -target-cpu penryn -tune-cpu generic -debugger-tuning=lldb -fdebug-compilation-dir=/Users/ud2/repro -target-linker-version 1022.1 -fcoverage-compilation-dir=/Users/ud2/repro -resource-dir /Users/ud2/Downloads/clangd_snapshot_20250401/lib/clang/21 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include -internal-isystem /Users/ud2/Downloads/clangd_snapshot_20250401/lib/clang/21/include -internal-externc-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -Wempty-translation-unit -ferror-limit 19 -stack-protector 1 -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fmax-type-align=16 -no-round-trip-args -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c /Users/ud2/repro/main.c
I[19:38:41.937] --> textDocument/clangd.fileStatus
V[19:38:41.937] >>> {"jsonrpc":"2.0","method":"textDocument/clangd.fileStatus","params":{"state":"parsing includes, running Update","uri":"file:///Users/ud2/repro/main.c"}}

V[19:38:41.937] Building first preamble for /Users/ud2/repro/main.c version 1
I[19:38:41.944] Built preamble of size 247864 for file /Users/ud2/repro/main.c version 1 in 0.01 seconds
I[19:38:41.945] --> workspace/semanticTokens/refresh(0)
V[19:38:41.945] >>> {"id":0,"jsonrpc":"2.0","method":"workspace/semanticTokens/refresh","params":null}

V[19:38:41.945] indexed preamble AST for /Users/ud2/repro/main.c version 1:
  symbol slab: 0 symbols, 120 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
V[19:38:41.948] <<< {"id":1,"jsonrpc":"2.0","method":"textDocument/documentSymbol","params":{"textDocument":{"uri":"file:///Users/ud2/repro/main.c"}}}

I[19:38:41.948] <-- textDocument/documentSymbol(1)
V[19:38:41.948] <<< {"id":2,"jsonrpc":"2.0","method":"textDocument/semanticTokens/full","params":{"textDocument":{"uri":"file:///Users/ud2/repro/main.c"}}}

I[19:38:41.948] <-- textDocument/semanticTokens/full(2)
I[19:38:41.948] Indexing c17 standard library in the context of /Users/ud2/repro/main.c
V[19:38:41.948] <<< {"id":0,"jsonrpc":"2.0","result":null}

I[19:38:41.948] <-- reply(0)
V[19:38:41.959] indexed file AST for /Users/ud2/repro/main.c version 1:
  symbol slab: 1 symbols, 4448 bytes
  ref slab: 1 symbols, 1 refs, 4248 bytes
  relations slab: 0 relations, 24 bytes
V[19:38:41.959] Build dynamic index for main-file symbols with estimated memory usage of 11520 bytes
I[19:38:41.959] --> textDocument/publishDiagnostics
V[19:38:41.959] >>> {"jsonrpc":"2.0","method":"textDocument/publishDiagnostics","params":{"diagnostics":[{"code":"-Wempty-translation-unit","message":"ISO C requires a translation unit to contain at least one declaration","range":{"end":{"character":0,"line":0},"start":{"character":0,"line":0}},"relatedInformation":[],"severity":2,"source":"clang"}],"uri":"file:///Users/ud2/repro/main.c","version":1}}

I[19:38:41.959] --> textDocument/inactiveRegions
V[19:38:41.959] >>> {"jsonrpc":"2.0","method":"textDocument/inactiveRegions","params":{"regions":[],"textDocument":{"uri":"file:///Users/ud2/repro/main.c"}}}

V[19:38:41.959] ASTWorker running DocumentSymbols on version 1 of /Users/ud2/repro/main.c
I[19:38:41.959] --> reply:textDocument/documentSymbol(1) 10 ms
V[19:38:41.959] >>> {"id":1,"jsonrpc":"2.0","result":[{"detail":"int ()","kind":12,"name":"main","range":{"end":{"character":13,"line":0},"start":{"character":0,"line":0}},"selectionRange":{"end":{"character":8,"line":0},"start":{"character":4,"line":0}}}]}

V[19:38:41.959] ASTWorker running SemanticHighlights on version 1 of /Users/ud2/repro/main.c
I[19:38:41.959] --> reply:textDocument/semanticTokens/full(2) 10 ms
V[19:38:41.959] >>> {"id":2,"jsonrpc":"2.0","result":{"data":[0,4,4,3,131075],"resultId":"1"}}

I[19:38:41.959] --> textDocument/clangd.fileStatus
V[19:38:41.959] >>> {"jsonrpc":"2.0","method":"textDocument/clangd.fileStatus","params":{"state":"idle","uri":"file:///Users/ud2/repro/main.c"}}

System information

Output of clangd --version:

clangd version 21.0.0git (https://github.com/llvm/llvm-project a2ca2f3f10002da61e9860d0ce11e0272482baba)
Features: mac+grpc+xpc
Platform: arm64-apple-darwin24.3.0; target=x86_64-apple-darwin24.3.0

Editor/LSP plugin: vscode-clangd 0.1.33

Operating system: macOS 15.3.2

[HighCommander4]

The diagnostic is coming from the preamble --> this is an artifact of clangd's preamble optimization.

Clangd is basically processing the file in two steps that are roughly equivalent to:

clang -cc1 preamble.h -emit-pch -o preamble.h.pch -Wempty-translation-unit
clang -cc1 -include-pch preamble.h.pch main.c  -Wempty-translation-unit

where preamble.h is an invented file containing "all of the includes at the top of main.c". In this case, preamble.h is empty.

[QBos07]

Can confirm this. Starting the file with this worked for me as a workaround:

#ifdef __clang__
#pragma clang diagnostic ignored "-Wempty-translation-unit" // bug?
#endif

/* your normal code here */

(my target only has gcc as a possible compiler, thats why i can make the destinction like this)