ASTUnit *AU = static_cast<ASTUnit *>(TU->TUData);
if (TU->Diagnostics && checkIfChanged) {
+ // In normal use, ASTUnit's diagnostics should not change unless we reparse.
+ // Currently they can only change by using the internal testing flag
+ // '-error-on-deserialized-decl' which will error during deserialization of
+ // a declaration. What will happen is:
+ //
+ // -c-index-test gets a CXTranslationUnit
+ // -checks the diagnostics, the diagnostics set is lazily created,
+ // no errors are reported
+ // -later does an operation, like annotation of tokens, that triggers
+ // -error-on-deserialized-decl, that will emit a diagnostic error,
+ // that ASTUnit will catch and add to its stored diagnostics vector.
+ // -c-index-test wants to check whether an error occurred after performing
+ // the operation but can only query the lazily created set.
+ //
+ // We check here if a new diagnostic was appended since the last time the
+ // diagnostic set was created, in which case we reset it.
+
CXDiagnosticSetImpl *
Set = static_cast<CXDiagnosticSetImpl*>(TU->Diagnostics);
if (AU->stored_diag_size() != Set->getNumDiagnostics()) {