[all-commits] [llvm/llvm-project] f50b8e: [YAML I/O] Fix bug in emission of empty sequence

[Jonas Devlieghere via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: f50b8ee71faeb5056df7a950e7427f3047ff9987
      https://github.com/llvm/llvm-project/commit/f50b8ee71faeb5056df7a950e7427f3047ff9987
  Author: Jonas Devlieghere <jonas at devlieghere.com>
  Date:   2021-01-25 (Mon, 25 Jan 2021)

  Changed paths:
    M llvm/include/llvm/Support/YAMLTraits.h
    M llvm/lib/Support/YAMLTraits.cpp
    M llvm/unittests/Support/YAMLIOTest.cpp

  Log Message:
  -----------
  [YAML I/O] Fix bug in emission of empty sequence

Don't emit an output dash for an empty sequence. Take emitting a vector
of strings for example:

  std::vector<std::string> Strings = {"foo", "bar"};
  LLVM_YAML_IS_SEQUENCE_VECTOR(std::string)
  yout << Strings;

This emits the following YAML document.

  ---
  - foo
  - bar
  ...

When the vector is empty, this generates the following result:

  ---
  - []
  ...

Although this is valid YAML, it does not match what we meant to emit.
The result is a one-element sequence consisting of an empty list.
Indeed, if we were to try to read this again we get an error:

  YAML:2:4: error: not a mapping
  - []

The problem is the output dash before the empty list. The correct output
would be:

  ---
  []
  ...

This patch fixes that by not emitting the output dash for an empty
sequence.

Differential revision: https://reviews.llvm.org/D95280