diff --git a/clang-tools-extra/clangd/index/CanonicalIncludes.cpp b/clang-tools-extra/clangd/index/CanonicalIncludes.cpp
--- a/clang-tools-extra/clangd/index/CanonicalIncludes.cpp
+++ b/clang-tools-extra/clangd/index/CanonicalIncludes.cpp
@@ -716,8 +716,6 @@
   // There are two std::move()s, this is by far the most common.
   if (Scope == "std::" && Name == "move")
     return "<utility>";
-  if (Scope == "std::" && Name == "size_t")
-    return "<cstddef>";
   if (auto StdSym = tooling::stdlib::Symbol::named(Scope, Name, Lang))
     return StdSym->header().name();
   return "";
diff --git a/clang-tools-extra/clangd/unittests/CanonicalIncludesTests.cpp b/clang-tools-extra/clangd/unittests/CanonicalIncludesTests.cpp
--- a/clang-tools-extra/clangd/unittests/CanonicalIncludesTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CanonicalIncludesTests.cpp
@@ -51,6 +51,7 @@
   EXPECT_EQ("<cstdio>", CI.mapSymbol("std::", "printf", Language));
   // std::move is ambiguous, currently always mapped to <utility>
   EXPECT_EQ("<utility>", CI.mapSymbol("std::", "move", Language));
+  EXPECT_EQ("<cstddef>", CI.mapSymbol("std::", "size_t", Language));
   // Unknown std symbols aren't mapped.
   EXPECT_EQ("", CI.mapSymbol("std::", "notathing", Language));
   // iosfwd declares some symbols it doesn't own.
diff --git a/clang/lib/Tooling/Inclusions/Stdlib/StandardLibrary.cpp b/clang/lib/Tooling/Inclusions/Stdlib/StandardLibrary.cpp
--- a/clang/lib/Tooling/Inclusions/Stdlib/StandardLibrary.cpp
+++ b/clang/lib/Tooling/Inclusions/Stdlib/StandardLibrary.cpp
@@ -62,6 +62,7 @@
     break;
   case Lang::CXX:
 #include "StdSymbolMap.inc"
+#include "StdSpecialSymbolMap.inc"
     break;
   }
 #undef SYMBOL
@@ -130,6 +131,7 @@
     break;
   case Lang::CXX:
 #include "StdSymbolMap.inc"
+#include "StdSpecialSymbolMap.inc"
     break;
   }
 #undef SYMBOL
diff --git a/clang/unittests/Tooling/StandardLibraryTest.cpp b/clang/unittests/Tooling/StandardLibraryTest.cpp
--- a/clang/unittests/Tooling/StandardLibraryTest.cpp
+++ b/clang/unittests/Tooling/StandardLibraryTest.cpp
@@ -62,6 +62,16 @@
               ElementsAre(stdlib::Header::named("<istream>"),
                           stdlib::Header::named("<iostream>"),
                           stdlib::Header::named("<iosfwd>")));
+  EXPECT_THAT(stdlib::Symbol::named("std::", "size_t")->headers(),
+              ElementsAre(stdlib::Header::named("<cstddef>"),
+                          stdlib::Header::named("<cstdlib>"),
+                          stdlib::Header::named("<cstring>"),
+                          stdlib::Header::named("<cwchar>"),
+                          stdlib::Header::named("<cuchar>"),
+                          stdlib::Header::named("<ctime>"),
+                          stdlib::Header::named("<cstdio>")));
+  EXPECT_EQ(stdlib::Symbol::named("std::", "size_t")->header(),
+            stdlib::Header::named("<cstddef>"));
 
   EXPECT_THAT(stdlib::Header::all(), Contains(*VectorH));
   EXPECT_THAT(stdlib::Symbol::all(), Contains(*Vector));