Add isVariadic() on FuncDecl and Function

ql/lib/semmle/go/Decls.qll

@@ -137,6 +137,8 @@ class FuncDef extends @funcdef, StmtParent, ExprParent {
    */
   DataFlow::CallNode getACall() { result.getACallee() = this }
 
+  predicate isVariadic() { getType().isVariadic() }
+
   override string getAPrimaryQlClass() { result = "FuncDef" }
 }
 

ql/lib/semmle/go/Scopes.qll

@@ -377,6 +377,12 @@ class Function extends ValueEntity, @functionobject {
   /** Gets the declaration of this function, if any. */
   FuncDecl getFuncDecl() { none() }
 
+  predicate isVariadic() {
+    this.(BuiltinFunction).getName() = ["append", "make", "print", "println"]
+    or
+    this.(DeclaredFunction).getFuncDecl().isVariadic()
+  }
+
   /** Holds if this function has no observable side effects. */
   predicate mayHaveSideEffects() { none() }