Compiler freezes if type parameter is omitted

[ansvonwa]

Compiler version

3.0.0

Minimized code

scala> ???.asInstanceOf[scala.collection.Seq] += 1

Note:

• The type parameter of s.c.Seq is omitted (Compiler does not freeze if it is given)
• += is not a member of collection.Seq
• May be related to REPL crashes at completion if type parameter is omitted #12465

Output (click arrow to expand)

Absolutely nothing: No output, no error, no new prompt.
Same with -Xprint:typer:

$ time scala -Xprint:typer
scala> ???.asInstanceOf[scala.collection.Seq] += 1                                              
^C
real	1m5,798s
user	0m8,007s
sys	0m0,721s

[griggt]

Already fixed on master by #12223

Now:

scala> ???.asInstanceOf[scala.collection.Seq] += 1
1 |???.asInstanceOf[scala.collection.Seq] += 1
  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |collection.Seq takes type parameters