cmd/gc: prohibit short variable declarations containing duplicate symbols

Fixes #6764.
Fixes #8435.

LGTM=rsc
R=golang-codereviews, r, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/116440046

src/cmd/gc/dcl.c

@@ -488,6 +488,10 @@ colasdefn(NodeList *left, Node *defn)
 	NodeList *l;
 	Node *n;
 
+	for(l=left; l; l=l->next)
+		if(l->n->sym != S)
+			l->n->sym->flags |= SymUniq;
+
 	nnew = 0;
 	nerr = 0;
 	for(l=left; l; l=l->next) {
@@ -499,6 +503,13 @@ colasdefn(NodeList *left, Node *defn)
 			nerr++;
 			continue;
 		}
+		if((n->sym->flags & SymUniq) == 0) {
+			yyerrorl(defn->lineno, "%S repeated on left side of :=", n->sym);
+			n->diag++;
+			nerr++;
+			continue;
+		}
+		n->sym->flags &= ~SymUniq;
 		if(n->sym->block == block)
 			continue;
 

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
+	}
 }