[clang][Interp] Fix variadic operator calls

Operator calls pass their instance member explicitly, so remove
it from NumParams when calling a variadic function

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -2842,7 +2842,8 @@ bool ByteCodeExprGen<Emitter>::VisitCallExpr(const CallExpr *E) {
         return false;
     } else if (Func->isVariadic()) {
       uint32_t VarArgSize = 0;
-      unsigned NumParams = Func->getNumWrittenParams();
+      unsigned NumParams =
+          Func->getNumWrittenParams() + isa<CXXOperatorCallExpr>(E);
       for (unsigned I = NumParams, N = E->getNumArgs(); I != N; ++I)
         VarArgSize += align(primSize(classify(E->getArg(I)).value_or(PT_Ptr)));
       if (!this->emitCallVar(Func, VarArgSize, E))

clang/test/AST/Interp/functions.cpp

@@ -555,3 +555,13 @@ namespace Local {
     return t;
   }
 }
+
+namespace VariadicOperator {
+  struct Callable {
+    float& operator()(...);
+  };
+
+  void test_callable(Callable c) {
+    float &fr = c(10);
+  }
+}