It took me a bit to see what's wrong with the behavior. In case it helps anyone else looking at this, the specific issue is that spec says

The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.

(link).

Below that it also gives this example for an []int, which works today (unlike strings, which this issue is about):

i = 2
x = []int{3, 5, 7}
for i, x[i] = range x {  // set i, x[2] = 0, x[0]
	break
}
// after this loop, i == 0 and x == []int{3, 5, 3}

/cc @alandonovan @ianlancetaylor @griesemer

I agree that this looks like a bug in the gc compiler.

gccgo gets it right.

I also agree with setting the milestone for this to 1.9.

Since the fix will be for 1.9 i will take a little more time for tuning the performance of the fix before submitting a CL.

We should add this test case to $GOROOT/test/range.go, analogous to what testslice already does for slices:

diff --git a/test/range.go b/test/range.go
index bae7a1c..b4b181b 100644
--- a/test/range.go
+++ b/test/range.go
@@ -311,6 +311,19 @@ func teststring2() {
        }
 }
 
+// Regression test for Go issue #18376.
+func teststring3() {
+       x := []rune{'a', 'b'}
+       i := 1
+       for i, x[i] = range "c" {
+               break
+       }
+       if i != 0 || x[0] != 'a' || x[1] != 'c' {
+               println("wrong parallel assignment", i, string(x[0]), string(x[1]))
+               panic("fail")
+       }
+}
+
 // test that range over map only evaluates
 // the expression after "range" once.
 
@@ -420,6 +433,7 @@ func main() {
        teststring()
        teststring1()
        teststring2()
+       teststring3()
        testmap()
        testmap1()
        testmap2()

FWIW, golang.org/x/tools/go/ssa also does the right thing.

@alandonovan sure, that what i did to verify the bug too (and then copied to the playground link) and will be in the CL. However i put the test into teststring function similar to testslice.

CL https://golang.org/cl/35955 mentions this issue.