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diff --git a/clangd/unittests/CanonicalIncludesTests.cpp b/clangd/unittests/CanonicalIncludesTests.cpp
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+++ b/clangd/unittests/CanonicalIncludesTests.cpp
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+//===-- CanonicalIncludesTests.cpp - --------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "index/CanonicalIncludes.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+
+TEST(CanonicalIncludesTest, CXXStandardLibrary) {
+  CanonicalIncludes CI;
+  addSystemHeadersMapping(&CI);
+
+  // Usual standard library symbols are mapped correctly.
+  EXPECT_EQ("<vector>", CI.mapHeader("path/vector.h", "std::vector"));
+  // std::move is ambiguous, currently mapped only based on path
+  EXPECT_EQ("<utility>", CI.mapHeader("libstdc++/bits/move.h", "std::move"));
+  EXPECT_EQ("path/utility.h", CI.mapHeader("path/utility.h", "std::move"));
+  // Unknown std symbols aren't mapped.
+  EXPECT_EQ("foo/bar.h", CI.mapHeader("foo/bar.h", "std::notathing"));
+  // iosfwd declares some symbols it doesn't own.
+  EXPECT_EQ("<ostream>", CI.mapHeader("iosfwd", "std::ostream"));
+  // And (for now) we assume it owns the others.
+  EXPECT_EQ("<iosfwd>", CI.mapHeader("iosfwd", "std::notwathing"));
+}
+
+TEST(CanonicalIncludesTest, PathMapping) {
+  // As used for IWYU pragmas.
+  CanonicalIncludes CI;
+  CI.addMapping("foo/bar", "<baz>");
+
+  EXPECT_EQ("<baz>", CI.mapHeader("foo/bar", "some::symbol"));
+  EXPECT_EQ("bar/bar", CI.mapHeader("bar/bar", "some::symbol"));
+}
+
+TEST(CanonicalIncludesTest, SymbolMapping) {
+  // As used for standard library.
+  CanonicalIncludes CI;
+  CI.addSymbolMapping("some::symbol", "<baz>");
+
+  EXPECT_EQ("<baz>", CI.mapHeader("foo/bar", "some::symbol"));
+  EXPECT_EQ("foo/bar", CI.mapHeader("foo/bar", "other::symbol"));
+}
+
+TEST(CanonicalIncludesTest, Precedence) {
+  CanonicalIncludes CI;
+  CI.addMapping("some/path", "<path>");
+  CI.addSymbolMapping("some::symbol", "<symbol>");
+
+  // Symbol mapping beats path mapping.
+  EXPECT_EQ("<symbol>", CI.mapHeader("some/path", "some::symbol"));
+}
+
+} // namespace
+} // namespace clangd
+} // namespace clang