Add failing test for flow out of varargs parameter

Author: owen-mc

go/ql/test/library-tests/semmle/go/dataflow/VarArgs/main.go

@@ -4,7 +4,7 @@ func source() string {
 	return "untrusted data"
 }
 
-func sink(string) {
+func sink(any) {
 }
 
 type A struct {
@@ -19,6 +19,10 @@ func functionWithVarArgsParameter(s ...string) string {
 	return s[1]
 }
 
+func functionWithVarArgsOutParameter(in string, out ...*string) {
+	*out[0] = in
+}
+
 func functionWithSliceOfStructsParameter(s []A) string {
 	return s[1].f
 }
@@ -38,6 +42,12 @@ func main() {
 	sink(functionWithVarArgsParameter(sSlice...)) // $ hasValueFlow="call to functionWithVarArgsParameter"
 	sink(functionWithVarArgsParameter(s0, s1))    // $ hasValueFlow="call to functionWithVarArgsParameter"
 
+	var out1 *string
+	var out2 *string
+	functionWithVarArgsOutParameter(source(), out1, out2)
+	sink(out1) // $ MISSING: hasValueFlow="out1"
+	sink(out2) // $ MISSING: hasValueFlow="out2"
+
 	sliceOfStructs := []A{{f: source()}}
 	sink(sliceOfStructs[0].f) // $ hasValueFlow="selection of f"