A libfuzzer run has discovered some inputs for which the demangler does not terminate.

When minimized, it looks like this: _Zcv1BIRT_EIS1_E

Deciphered:
_Z
cv    - conversion operator

      * result type
 1B   - "B"
 I    - template args begin
  R   - reference type              <.
   T_ - forward template reference   |  *
 E    - template args end            |  |
                                     |  |
      * parameter type               |  |
 I    - template args begin          |  |
  S1_ - substitution #1              * <'
 E    - template args end

The reason is: template-parameter refs in conversion operator result type create forward-references, while substitutions are instantly resolved via back-references. Together these can create a reference loop. It causes an infinite loop in ReferenceType::collapse().

I see three possible ways to avoid these loops:
1. check if resolving a forward reference creates a loop and reject the invalid input (hard to traverse AST at this point)
2. check if a substitution contains a malicious forward reference and reject the invalid input (hard to traverse AST at this point; substitutions are quite common: may affect performance; hard to clearly detect loops at this point)
3. detect loops in ReferenceType::collapse() (cannot reject the input)