diff --git a/clang-tools-extra/clangd/unittests/SymbolCollectorTests.cpp b/clang-tools-extra/clangd/unittests/SymbolCollectorTests.cpp
--- a/clang-tools-extra/clangd/unittests/SymbolCollectorTests.cpp
+++ b/clang-tools-extra/clangd/unittests/SymbolCollectorTests.cpp
@@ -1838,6 +1838,20 @@
   EXPECT_THAT(TU.headerSymbols(), Not(Contains(QName("X"))));
 }
 
+TEST_F(SymbolCollectorTest, NoCrashOnObjCMethodCStyleParam) {
+  auto TU = TestTU::withCode(R"objc(
+    @interface Foo
+    - (void)fun:(bool)foo, bool bar;
+    @end
+  )objc");
+  TU.ExtraArgs.push_back("-xobjective-c++");
+
+  TU.build();
+  // We mostly care about not crashing.
+  EXPECT_THAT(TU.headerSymbols(),
+              UnorderedElementsAre(QName("Foo"), QName("Foo::fun:")));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang
diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -3529,9 +3529,11 @@
         Result.AddTypedTextChunk("");
     }
     unsigned Idx = 0;
+    // The extra Idx < Sel.getNumArgs() check is needed due to legacy C-style
+    // method parameters.
     for (ObjCMethodDecl::param_const_iterator P = Method->param_begin(),
                                               PEnd = Method->param_end();
-         P != PEnd; (void)++P, ++Idx) {
+         P != PEnd && Idx < Sel.getNumArgs(); (void)++P, ++Idx) {
       if (Idx > 0) {
         std::string Keyword;
         if (Idx > StartParameter)