Index: lldb/include/lldb/DataFormatters/FormattersContainer.h
===================================================================
--- lldb/include/lldb/DataFormatters/FormattersContainer.h
+++ lldb/include/lldb/DataFormatters/FormattersContainer.h
@@ -296,14 +296,24 @@
     llvm::StringRef key_str = key.GetStringRef();
     std::lock_guard<std::recursive_mutex> guard(m_format_map.mutex());
     MapIterator pos, end = m_format_map.map().end();
+    MapIterator found = end;
     for (pos = m_format_map.map().begin(); pos != end; pos++) {
       lldb::RegularExpressionSP regex = pos->first;
       if (regex->Execute(key_str)) {
-        value = pos->second;
-        return true;
+        if (found != end) {
+          llvm::errs() << "Two regexes (\"" << found->first->GetText()
+                       << "\" and \"" << regex->GetText()
+                       << "\") ambiguously match the same string \"" << key_str
+                       << "\".\n";
+          return false;
+        }
+        found = pos;
       }
     }
-    return false;
+    if (found == end)
+      return false;
+    value = found->second;
+    return true;
   }
 
   bool GetExact_Impl(ConstString key, MapValueType &value,
Index: lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-advanced/TestDataFormatterAdv.py
===================================================================
--- lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-advanced/TestDataFormatterAdv.py
+++ lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-advanced/TestDataFormatterAdv.py
@@ -130,6 +130,10 @@
         self.expect("frame variable int_array",
                     substrs=['1,2'])
 
+        # "int []" gets converted by FixArrayTypeNameWithRegex into "int \[[0-9]+\]"
+        # which becomes ambiguous for some string against "int \[[0-9]\]".
+        self.runCmd("type summary clear")
+
         self.runCmd(
             'type summary add --summary-string \"${var[0-1]}\" "int []"')
 
Index: lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp
===================================================================
--- lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp
+++ lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp
@@ -486,8 +486,11 @@
       cpp_category_sp,
       lldb_private::formatters::LibcxxStdListSyntheticFrontEndCreator,
       "libc++ std::list synthetic children",
-      ConstString("^std::__[[:alnum:]]+::list<.+>(( )?&)?$"), stl_deref_flags,
-      true);
+      // A POSIX variant of: "^std::__(?!cxx11:)[[:alnum:]]+::list<.+>(( )?&)?$"
+      // so that it does not clash with: "^std::(__cxx11::)?list<.+>(( )?&)?$"
+      ConstString("^std::__([A-Zabd-z0-9]|cx?[A-Za-wyz0-9]|cxx1?[A-Za-z02-9]|"
+                  "cxx11[[:alnum:]])[[:alnum:]]*::list<.+>(( )?&)?$"),
+      stl_deref_flags, true);
   AddCXXSynthetic(
       cpp_category_sp,
       lldb_private::formatters::LibcxxStdMapSyntheticFrontEndCreator,
@@ -583,11 +586,14 @@
                 "libc++ std::list summary provider",
                 ConstString("^std::__[[:alnum:]]+::forward_list<.+>(( )?&)?$"),
                 stl_summary_flags, true);
-  AddCXXSummary(cpp_category_sp,
-                lldb_private::formatters::LibcxxContainerSummaryProvider,
-                "libc++ std::list summary provider",
-                ConstString("^std::__[[:alnum:]]+::list<.+>(( )?&)?$"),
-                stl_summary_flags, true);
+  AddCXXSummary(
+      cpp_category_sp, lldb_private::formatters::LibcxxContainerSummaryProvider,
+      "libc++ std::list summary provider",
+      // A POSIX variant of: "^std::__(?!cxx11:)[[:alnum:]]+::list<.+>(( )?&)?$"
+      // so that it does not clash with: "^std::(__cxx11::)?list<.+>(( )?&)?$"
+      ConstString("^std::__([A-Zabd-z0-9]|cx?[A-Za-wyz0-9]|cxx1?[A-Za-z02-9]|"
+                  "cxx11[[:alnum:]])[[:alnum:]]*::list<.+>(( )?&)?$"),
+      stl_summary_flags, true);
   AddCXXSummary(cpp_category_sp,
                 lldb_private::formatters::LibcxxContainerSummaryProvider,
                 "libc++ std::map summary provider",