It seems that clang-format erroneously treats {{int a; as a malformed initializer list. (The first brace in the repro is on a separate line only because it's followed by a single line comment.)

The problem appears to be caused by UnwrappedLineParser.

• UnwrappedLineParser::tryToParseBracedList calls calculateBraceTypes to see whether a left brace starts an initializer list or a block. If the left brace starts an initializer list, parseBracedList is called to consume it.
• UnwrappedLineParser::calculateBraceTypes thinks {// asdf starts an initializer list, because its matching right brace is followed by a left brace (which makes sense in a constructor, e.g. in class Foo { int mem; Foo() : mem{} {};, the {} after mem is an initializer list, but does not make sense otherwise).
• UnwrappedLineParser::parseBracedList consumes the initializer list and stops at the semicolon after int a (because it was called with ContinueOnSemicolons being the default, i.e. false). It is unaware that the left brace has a matching right brace.

mentioned in issue llvm/llvm-bugzilla-archive#48438

This issue still exists in version 14:

bash$ echo $'void foo(){{//asdf\n{int a;}}{{int b;}}}' | clang-format -style LLVM
void foo() {
  {// asdf
   {int a;
}
}
{
  { int b; }
}
}

Duplicate of #33891