Issue 36332: compile() error on AST object with assignment expression

Seems like compile() can't properly handle assignment expressions parsed in AST object.

Python 3.8.0a2+ (heads/master:06e1e68, Mar 17 2019, 14:27:19)
[Clang 10.0.0 (clang-1000.10.44.4)] on darwin
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>>> import ast
>>> compile("if a == 1:\n  True", '<string>', 'exec')
<code object <module> at 0x10a59ef60, file "<string>", line 1>
>>> compile(ast.parse("if a == 1:\n  True"), '<string>', 'exec')
<code object <module> at 0x10a5f6780, file "<string>", line 1>
>>> compile("if a := 1:\n  True", '<string>', 'exec')
<code object <module> at 0x10a59ef60, file "<string>", line 1>
>>> compile(ast.parse("if a := 1:\n  True"), '<string>', 'exec')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: unexpected expression
>>>

This issue seems to break IPython when trying to use any assignment expressions.
https://github.com/ipython/ipython/issues/11618

Confirmed. Emily, do you need help tracking this down?

validate_expr() needs a case for NamedExpr_kind.

I think also that it is better to remove the "default" clause (and move its code after the switch statement) and allow the compiler to warn about missed cases.

New changeset 0c9258a6d299e0484538ef8d4b23f30515283db2 by Pablo Galindo in branch 'master':
bpo-36332: Allow compile() to handle AST objects with assignment expressions (GH-12398)
https://github.com/python/cpython/commit/0c9258a6d299e0484538ef8d4b23f30515283db2