[Bug c++/94827] New: [ICE] [regression] crash on requires clause in tparam list

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94827

            Bug ID: 94827
           Summary: [ICE] [regression] crash on requires clause in tparam
                    list
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

Try the following code with -std=c++2a

template <typename T,
  bool = requires { requires (sizeof(T)==0); } >
void foo(T) {}

The result:

<source>:20:44: internal compiler error: tree check: accessed elt 1 of
'tree_vec' with 0 elts in map_arguments, at cp/constraint.cc:553
   20 |   bool = requires { requires (sizeof(T)==0); } >
      |                                            ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.
Compiler returned: 1

This compiles with gcc-9 with -std=c++2a -fconcepts, so this is a regression.