Index: clang-tools-extra/trunk/clangd/Protocol.h
===================================================================
--- clang-tools-extra/trunk/clangd/Protocol.h
+++ clang-tools-extra/trunk/clangd/Protocol.h
@@ -616,7 +616,6 @@
 };
 bool fromJSON(const llvm::json::Value &, DocumentSymbolParams &);
 
-
 /// Represents a related message and source code location for a diagnostic.
 /// This should be used to point to code locations that cause or related to a
 /// diagnostics, e.g when duplicating a symbol in a scope.
@@ -666,11 +665,17 @@
 
 /// A LSP-specific comparator used to find diagnostic in a container like
 /// std:map.
-/// We only use the required fields of Diagnostic to do the comparsion to avoid
-/// any regression issues from LSP clients (e.g. VScode), see
-/// https://git.io/vbr29
+/// We only use as many fields of Diagnostic as is needed to make distinct
+/// diagnostics unique in practice, to avoid  regression issues from LSP clients
+/// (e.g. VScode), see https://git.io/vbr29
 struct LSPDiagnosticCompare {
   bool operator()(const Diagnostic &LHS, const Diagnostic &RHS) const {
+    if (!LHS.code.empty() && !RHS.code.empty()) {
+      // If the code is known for both, use the code to diambiguate, as e.g.
+      // two checkers could produce diagnostics with the same range and message.
+      return std::tie(LHS.range, LHS.message, LHS.code) <
+             std::tie(RHS.range, RHS.message, RHS.code);
+    }
     return std::tie(LHS.range, LHS.message) < std::tie(RHS.range, RHS.message);
   }
 };
Index: clang-tools-extra/trunk/clangd/test/fixits-duplication.test
===================================================================
--- clang-tools-extra/trunk/clangd/test/fixits-duplication.test
+++ clang-tools-extra/trunk/clangd/test/fixits-duplication.test
@@ -0,0 +1,221 @@
+# RUN: clangd -lit-test -clang-tidy-checks=modernize-use-nullptr,hicpp-use-nullptr < %s | FileCheck -strict-whitespace %s
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{"textDocument":{"codeAction":{"codeActionLiteralSupport":{}}}},"trace":"off"}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///foo.cpp","languageId":"cpp","version":1,"text":"void foo() { char* p = 0; }"}}}
+#      CHECK:    "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:  "params": {
+# CHECK-NEXT:    "diagnostics": [
+# CHECK-NEXT:      {
+# CHECK-NEXT:        "code": "hicpp-use-nullptr",
+# CHECK-NEXT:        "message": "Use nullptr (fix available)",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 24,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 23,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          }
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "severity": 2,
+# CHECK-NEXT:        "source": "clang-tidy"
+# CHECK-NEXT:      },
+# CHECK-NEXT:      {
+# CHECK-NEXT:        "code": "modernize-use-nullptr",
+# CHECK-NEXT:        "message": "Use nullptr (fix available)",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 24,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 23,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          }
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "severity": 2,
+# CHECK-NEXT:        "source": "clang-tidy"
+# CHECK-NEXT:      }
+# CHECK-NEXT:    ],
+# CHECK-NEXT:    "uri": "file:///{{.*}}/foo.cpp"
+# CHECK-NEXT:  }
+---
+{"jsonrpc":"2.0","id":2,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///foo.cpp"},"range":{"start":{"line":0,"character":23},"end":{"line":0,"character":24}},"context":{"diagnostics":[{"range":{"start": {"line": 0, "character": 23}, "end": {"line": 0, "character": 24}},"severity":2,"message":"Use nullptr (fix available)", "code": "hicpp-use-nullptr", "source": "clang-tidy"},{"range":{"start": {"line": 0, "character": 23}, "end": {"line": 0, "character": 24}},"severity":2,"message":"Use nullptr (fix available)", "code": "modernize-use-nullptr", "source": "clang-tidy"}]}}}
+#      CHECK:  "id": 2,
+# CHECK-NEXT:  "jsonrpc": "2.0",
+# CHECK-NEXT:  "result": [
+# CHECK-NEXT:    {
+# CHECK-NEXT:      "diagnostics": [
+# CHECK-NEXT:        {
+# CHECK-NEXT:          "code": "hicpp-use-nullptr",
+# CHECK-NEXT:          "message": "Use nullptr (fix available)",
+# CHECK-NEXT:          "range": {
+# CHECK-NEXT:            "end": {
+# CHECK-NEXT:              "character": 24,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            },
+# CHECK-NEXT:            "start": {
+# CHECK-NEXT:              "character": 23,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            }
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "severity": 2,
+# CHECK-NEXT:          "source": "clang-tidy"
+# CHECK-NEXT:        }
+# CHECK-NEXT:      ],
+# CHECK-NEXT:      "edit": {
+# CHECK-NEXT:        "changes": {
+# CHECK-NEXT:          "file://{{.*}}/foo.cpp": [
+# CHECK-NEXT:            {
+# CHECK-NEXT:              "newText": "nullptr",
+# CHECK-NEXT:              "range": {
+# CHECK-NEXT:                "end": {
+# CHECK-NEXT:                  "character": 24,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                },
+# CHECK-NEXT:                "start": {
+# CHECK-NEXT:                  "character": 23,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                }
+# CHECK-NEXT:              }
+# CHECK-NEXT:            }
+# CHECK-NEXT:          ]
+# CHECK-NEXT:        }
+# CHECK-NEXT:      },
+# CHECK-NEXT:      "kind": "quickfix",
+# CHECK-NEXT:      "title": "change '0' to 'nullptr'"
+# CHECK-NEXT:    },
+# CHECK-NEXT:    {
+# CHECK-NEXT:      "diagnostics": [
+# CHECK-NEXT:        {
+# CHECK-NEXT:          "code": "modernize-use-nullptr",
+# CHECK-NEXT:          "message": "Use nullptr (fix available)",
+# CHECK-NEXT:          "range": {
+# CHECK-NEXT:            "end": {
+# CHECK-NEXT:              "character": 24,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            },
+# CHECK-NEXT:            "start": {
+# CHECK-NEXT:              "character": 23,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            }
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "severity": 2,
+# CHECK-NEXT:          "source": "clang-tidy"
+# CHECK-NEXT:        }
+# CHECK-NEXT:      ],
+# CHECK-NEXT:      "edit": {
+# CHECK-NEXT:        "changes": {
+# CHECK-NEXT:          "file://{{.*}}/foo.cpp": [
+# CHECK-NEXT:            {
+# CHECK-NEXT:              "newText": "nullptr",
+# CHECK-NEXT:              "range": {
+# CHECK-NEXT:                "end": {
+# CHECK-NEXT:                  "character": 24,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                },
+# CHECK-NEXT:                "start": {
+# CHECK-NEXT:                  "character": 23,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                }
+# CHECK-NEXT:              }
+# CHECK-NEXT:            }
+# CHECK-NEXT:          ]
+# CHECK-NEXT:        }
+# CHECK-NEXT:      },
+# CHECK-NEXT:      "kind": "quickfix",
+# CHECK-NEXT:      "title": "change '0' to 'nullptr'"
+# CHECK-NEXT:    }
+# CHECK-NEXT:  ]
+---
+{"jsonrpc":"2.0","id":3,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///foo.cpp"},"range":{"start":{"line":0,"character":23},"end":{"line":0,"character":24}},"context":{"diagnostics":[{"range":{"start": {"line": 0, "character": 23}, "end": {"line": 0, "character": 24}},"severity":2,"message":"Use nullptr (fix available)", "source": "clang-tidy"},{"range":{"start": {"line": 0, "character": 23}, "end": {"line": 0, "character": 24}},"severity":2,"message":"Use nullptr (fix available)", "source": "clang-tidy"}]}}}
+#      CHECK:  "id": 3,
+# CHECK-NEXT:  "jsonrpc": "2.0",
+# CHECK-NEXT:  "result": [
+# CHECK-NEXT:    {
+# CHECK-NEXT:      "diagnostics": [
+# CHECK-NEXT:        {
+# CHECK-NEXT:          "message": "Use nullptr (fix available)",
+# CHECK-NEXT:          "range": {
+# CHECK-NEXT:            "end": {
+# CHECK-NEXT:              "character": 24,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            },
+# CHECK-NEXT:            "start": {
+# CHECK-NEXT:              "character": 23,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            }
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "severity": 2,
+# CHECK-NEXT:          "source": "clang-tidy"
+# CHECK-NEXT:        }
+# CHECK-NEXT:      ],
+# CHECK-NEXT:      "edit": {
+# CHECK-NEXT:        "changes": {
+# CHECK-NEXT:          "file://{{.*}}/foo.cpp": [
+# CHECK-NEXT:            {
+# CHECK-NEXT:              "newText": "nullptr",
+# CHECK-NEXT:              "range": {
+# CHECK-NEXT:                "end": {
+# CHECK-NEXT:                  "character": 24,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                },
+# CHECK-NEXT:                "start": {
+# CHECK-NEXT:                  "character": 23,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                }
+# CHECK-NEXT:              }
+# CHECK-NEXT:            }
+# CHECK-NEXT:          ]
+# CHECK-NEXT:        }
+# CHECK-NEXT:      },
+# CHECK-NEXT:      "kind": "quickfix",
+# CHECK-NEXT:      "title": "change '0' to 'nullptr'"
+# CHECK-NEXT:    },
+# CHECK-NEXT:    {
+# CHECK-NEXT:      "diagnostics": [
+# CHECK-NEXT:        {
+# CHECK-NEXT:          "message": "Use nullptr (fix available)",
+# CHECK-NEXT:          "range": {
+# CHECK-NEXT:            "end": {
+# CHECK-NEXT:              "character": 24,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            },
+# CHECK-NEXT:            "start": {
+# CHECK-NEXT:              "character": 23,
+# CHECK-NEXT:              "line": 0
+# CHECK-NEXT:            }
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "severity": 2,
+# CHECK-NEXT:          "source": "clang-tidy"
+# CHECK-NEXT:        }
+# CHECK-NEXT:      ],
+# CHECK-NEXT:      "edit": {
+# CHECK-NEXT:        "changes": {
+# CHECK-NEXT:          "file://{{.*}}/foo.cpp": [
+# CHECK-NEXT:            {
+# CHECK-NEXT:              "newText": "nullptr",
+# CHECK-NEXT:              "range": {
+# CHECK-NEXT:                "end": {
+# CHECK-NEXT:                  "character": 24,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                },
+# CHECK-NEXT:                "start": {
+# CHECK-NEXT:                  "character": 23,
+# CHECK-NEXT:                  "line": 0
+# CHECK-NEXT:                }
+# CHECK-NEXT:              }
+# CHECK-NEXT:            }
+# CHECK-NEXT:          ]
+# CHECK-NEXT:        }
+# CHECK-NEXT:      },
+# CHECK-NEXT:      "kind": "quickfix",
+# CHECK-NEXT:      "title": "change '0' to 'nullptr'"
+# CHECK-NEXT:    }
+# CHECK-NEXT:  ]
+---
+{"jsonrpc":"2.0","id":4,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}
+