With test-case gdb.dwarf2/struct-with-sig.exp and target board cc-with-debug-names I run into:
...
(gdb) ptype struct s^M
/home/vries/gdb_versions/devel/src/gdb/dwarf2/read.c:3963: internal-error: dw2_expand_symtabs_matching_one: Assertion `symtab != nullptr' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
----- Backtrace -----^M
FAIL: gdb.dwarf2/struct-with-sig.exp: struct s with int i (GDB internal error)
...

This is easily enough fixed by:
...
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 017cde0dcd9..97be0aaa0db 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -3960,9 +3960,8 @@ dw2_expand_symtabs_matching_one
 
       compunit_symtab *symtab
        = dw2_instantiate_symtab (per_cu, per_objfile, false);
-      gdb_assert (symtab != nullptr);
 
-      if (expansion_notify != NULL && symtab_was_null)
+      if (symtab != nullptr && expansion_notify != NULL && symtab_was_null)

        return expansion_notify (symtab);
     }
   return true;
...

Still, we're trying to print struct s, in the context of main, which has a struct s decl with sig reference to a struct s in a TU.

So why are we're trying to instantiate the dummy CU at offset 0x9a:
...
  Compilation Unit @ offset 0x8f:
   Length:        0x7 (32-bit)
   Version:       4
   Abbrev Offset: 0x59
   Pointer Size:  8
  Compilation Unit @ offset 0x9a:
...
which contains no symbols and is indeed not listed as containing any symbols in the .debug_names index?

Is this because the index is missing information?  That is, it has:
...
[ 15] #0002b618 s:
        <5> DW_TAG_structure_type DW_IDX_type_unit=0 DW_IDX_GNU_internal=1
        <5> DW_TAG_structure_type DW_IDX_type_unit=1 DW_IDX_GNU_internal=1
...
but there's no indication that the foo and main compilation units also contain those types (using aforementioned decl-with-sig-ref).