RFR: 8264346: nullptr_t undefined in global namespace for clang+libstdc++

[Stefan Karlsson]

There's a mismatch in some toolchains about what part should provide the nullptr_t definition. This patch takes the easy way out and include cstddef and changes the two usages nullptr_t to std::nullptr_t.

See the bug report for more details.

We could have redefined nullptr_t to resolve this, but that would have required more extensive testing, so I left that as a potential future cleanup.

I've tested this by compiling with clang on linux. I'm going to let GHA testing testing run, and will also run our tier1 testing.

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Commit messages:
 - 8264346: nullptr_t undefined in global namespace for clang+libstdc++

Changes: https://git.openjdk.java.net/jdk/pull/3269/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3269&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264346
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3269.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3269/head:pull/3269

PR: https://git.openjdk.java.net/jdk/pull/3269