Add support for variadic functions in container flow

ql/lib/semmle/go/Scopes.qll

@@ -620,6 +620,9 @@ class Callable extends TCallable {
     result = this.asFuncLit().getType()
   }
 
+  /** Holds if this callable is variadic. */
+  predicate isVariadic() { this.getType().isVariadic() }
+
   /** Gets the name of this callable. */
   string getName() {
     result = this.asFunction().getName() or

ql/lib/semmle/go/dataflow/internal/ContainerFlow.qll

@@ -16,6 +16,14 @@ private import semmle.go.dataflow.ExternalFlow
 predicate containerStoreStep(Node node1, Node node2, Content c) {
   c instanceof ArrayContent and
   (
+    exists(ArgumentNode arg, ParameterNode parm, Callable fn |
+      arg = node1 and
+      parm = node2 and
+      arg.getCall().getACalleeIncludingExternals() = fn and
+      arg.isVariadic() and
+      parm.isParameterOf(fn, fn.getType().getNumParameter() - 1) = arg.getCall()
+    )
+    or
     (
       node2.getType() instanceof ArrayType or
       node2.getType() instanceof SliceType

ql/lib/semmle/go/dataflow/internal/DataFlowNodes.qll

@@ -542,6 +542,9 @@ module Public {
     /** Gets the data flow node corresponding to the receiver of this call, if any. */
     Node getReceiver() { result = this.getACalleeSource().(MethodReadNode).getReceiver() }
 
+    /** Holds if this callee calls a callable that is variadic. */
+    predicate isVariadic() { this.getACalleeIncludingExternals().isVariadic() }
+
     /** Holds if this call has an ellipsis after its last argument. */
     predicate hasEllipsis() { expr.hasEllipsis() }
   }
@@ -714,6 +717,12 @@ module Public {
       )
     }
 
+    /** Holds if this argument is to a call that has a variadic callee. */
+    predicate isVariadic() {
+      this.getCall().isVariadic() and
+      i >= this.getCall().getACalleeIncludingExternals().getType().getNumParameter() - 1
+    }
+
     /**
      * Gets the `CallNode` this is an argument to.
      */