Index: lib/Format/FormatToken.h
===================================================================
--- lib/Format/FormatToken.h
+++ lib/Format/FormatToken.h
@@ -478,6 +478,7 @@
     if (is(TT_TemplateString) && opensScope())
       return true;
     return is(TT_ArrayInitializerLSquare) ||
+           is(TT_ProtoExtensionLSquare) ||
            (is(tok::l_brace) &&
             (BlockKind == BK_Block || is(TT_DictLiteral) ||
              (!Style.Cpp11BracedListStyle && NestingLevel == 0))) ||
Index: unittests/Format/FormatTestTextProto.cpp
===================================================================
--- unittests/Format/FormatTestTextProto.cpp
+++ unittests/Format/FormatTestTextProto.cpp
@@ -389,6 +389,14 @@
                "    keyyyyyyyyyyyyyy: valuuuuuuuuuuuuuuuuuuuuuuuuue\n"
                "  }\n"
                "}");
+  verifyFormat(
+      "aaaaaaaaaaaaaaa {\n"
+      "  bbbbbb {\n"
+      "    [a.b/cy] {\n"
+      "      eeeeeeeeeeeee: \"The lazy coo cat jumps over the lazy hot dog\"\n"
+      "    }\n"
+      "  }\n"
+      "}");
 }
 
 TEST_F(FormatTestTextProto, NoSpaceAfterPercent) {