CompilerPanic for concat on empty(Bytes)

[cyberthirst]

Version Information

• vyper Version (output of vyper --version OR linkable commit hash vyperlang/vyper@commitish): 0.4.0rc3+commit.80708e6d

Issue description

Certain contracts which contain empty and concat fail to compile with CompilerPanic.

POC

These contracts

#@version 0.4.0.rc3

@external
def foo():
    s: Bytes[63] = concat(empty(Bytes[31]), empty(bytes32))

#@version 0.4.0.rc3

@external
def foo():
    s: Bytes[63] = concat(empty(Bytes[31]), b"a")

fail to compile with:

vyper.exceptions.CompilerPanic: tried to modify non-pointer type

credits: @trocher