Semiautovivification with a concat assignment produces an erroneous code

[pepkin88]

Example:

a[]b ++= [1 2]

Result:

a.b || (a.b = []) = (a.b = []).concat([1, 2]);

Expected result:

a.b = (a.b || (a.b = [])).concat([1, 2]);

The current behavior produces a code, which gives a runtime error ReferenceError: Invalid left-hand side in assignment.

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The "expected result" block is a result of compiling:
(after merging #1026)

a.b = a[]b ++ [1 2]

which is a simplification of (currently uncompilable):

a[]b = a[]b ++ [1 2]

because the effects of semiautovivification are canceled by an assign.

[pepkin88]

The issue also applies to other assignments, but this one has a practical use case.

[determin1st]

that's the craziest syntax i've ever seen 8)