[clang-tidy] Fix misc-unused-using-decls false positives in presence of compile errors

llvm-svn: 294578

clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp

@@ -48,6 +48,9 @@ void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
 }
 
 void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
+  if (Result.Context->getDiagnostics().hasUncompilableErrorOccurred())
+    return;
+
   if (const auto *Using = Result.Nodes.getNodeAs<UsingDecl>("using")) {
     // Ignores using-declarations defined in macros.
     if (Using->getLocation().isMacroID())

clang-tools-extra/test/clang-tidy/misc-unused-using-decls-errors.cpp

@@ -0,0 +1,12 @@
+// RUN: %check_clang_tidy %s misc-unused-using-decls %t
+
+namespace n {
+class C;
+}
+
+using n::C;
+
+void f() {
+  for (C *p : unknown()) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: error: use of undeclared identifier 'unknown' [clang-diagnostic-error]
+}