math/big: incorrect handling of aliased inputs in GCD

[bmkessler]

What version of Go are you using (go version)?

$ go version
go1.11.2 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

Calculate the cofactors x, y using the input variables (a, b) to store the result

d.GCD(x, y, a, b) --> x = 1, y = -1 (correct)
d.GCD(a, b, a, b) --> a = 1, b = 1 (incorrect)
d.GCD(b, a, a, b) --> b = 1, a = -6 (incorrect)

https://play.golang.org/p/KwX7hGGIHu_k

What did you expect to see?

Correct results that match when independent x, y are used.

What did you see instead?

Incorrect results when x and y reference the input variables a, b.

[gopherbot]

Change https://golang.org/cl/162517 mentions this issue: math/big: handle alias of cofactor inputs in GCD

[katiehockman]

/cc @griesemer

[gopherbot]

Change https://golang.org/cl/168257 mentions this issue: math/big: add comprehensive aliasing tests (and minor fixes to Exp, Rand)