cmd/gc: prohibit short variable declarations containing duplicate symbols

Fixes issue 6764.
Fixes issue 8435.

LGTM=rsc
R=golang-codereviews, r, gobot, rsc
CC=golang-codereviews
https://codereview.appspot.com/116440046

Committer: Russ Cox <rsc@golang.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/test/assign.go b/test/assign.go
index da0192f83..6611f8ce3 100644
--- a/test/assign.go
+++ b/test/assign.go
@@ -53,4 +53,16 @@ func main() {
 		_ = x
 		_ = y
 	}
+	{
+		var x = 1
+		{
+			x, x := 2, 3 // ERROR "x repeated on left side of :="
+			_ = x
+		}
+		_ = x
+	}
+	{
+		a, a := 1, 2 // ERROR "a repeated on left side of :="
+		_ = a
+	}
 }