Incorrectly indented block scalar is successfully parsed

[martin-sucha]

The following yaml is invalid according to the spec:

args:
  - |
      aaa
    key:
      nest1: something
      nest2:
        nest2a: b

This is because when the Block Indentation Indicator is absent, content indentation level is determined by the leading spaces in first non-empty line (or largest number of spaces if there are only empty lines).

Section 8.1.1.1. of yaml v1.2.2 spec:

If no indentation indicator is given, then the content indentation level is equal to the number of leading spaces on the first non-empty line of the contents. If there is no non-empty line then the content indentation level is equal to the number of spaces on the longest line.

It is an error if any non-empty line does not begin with a number of spaces greater than or equal to the content indentation level.

go-yaml v1.10.0 does not return an error in this case.

Here is a test that reproduces the issue:

func TestParseBlockIndent(t *testing.T) {
	in := `args:
  - |
      aaa
    key:
      nest1: something
      nest2:
        nest2a: b
`
	_, err := parser.ParseBytes([]byte(in), 0)
	if err == nil {
		t.Fatalf("expected error, but no error was produced")
	}
}

For the record, correct version of the block scalar above should have the indentation indicator present, like this:

args:
  - |2
      aaa
    key:
      nest1: something
      nest2:
        nest2a: b