interp: fix mismatch assign statement panic (traefik#1606)

_test/assign19.go

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	a, b, c := 1, 2
+	_, _, _ = a, b, c
+}
+
+// Error:
+// _test/assign19.go:4:2: cannot assign 2 values to 3 variables

interp/cfg.go

@@ -644,6 +644,14 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 				sbase = len(n.child) - n.nright
 			}
 
+			// If len(RHS) > 1, each node must be single-valued, and the nth expression
+			// on the right is assigned to the nth operand on the left, so the number of
+			// nodes on the left and right sides must be equal
+			if n.nright > 1 && n.nright != n.nleft {
+				err = n.cfgErrorf("cannot assign %d values to %d variables", n.nright, n.nleft)
+				return
+			}
+
 			wireChild(n)
 			for i := 0; i < n.nleft; i++ {
 				dest, src := n.child[i], n.child[sbase+i]

interp/interp_consistent_test.go

@@ -37,6 +37,7 @@ func TestInterpConsistencyBuild(t *testing.T) {
 			file.Name() == "assign11.go" || // expect error
 			file.Name() == "assign12.go" || // expect error
 			file.Name() == "assign15.go" || // expect error
+			file.Name() == "assign19.go" || // expect error
 			file.Name() == "bad0.go" || // expect error
 			file.Name() == "break0.go" || // expect error
 			file.Name() == "cont3.go" || // expect error