[clang][Interp] Fix calling variadic call operators

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Differential Revision: https://phabricator.intern.facebook.com/D60250544

Author: tbaederr

clang/lib/AST/Interp/Interp.cpp

@@ -233,7 +233,8 @@ void cleanupAfterFunctionCall(InterpState &S, CodePtr OpPC) {
       assert(false && "Can't get arguments from that expression type");
 
     assert(NumArgs >= CurFunc->getNumWrittenParams());
-    NumVarArgs = NumArgs - CurFunc->getNumWrittenParams();
+    NumVarArgs = NumArgs - (CurFunc->getNumWrittenParams() +
+                            isa<CXXOperatorCallExpr>(CallSite));
     for (unsigned I = 0; I != NumVarArgs; ++I) {
       const Expr *A = Args[NumArgs - 1 - I];
       popArg(S, A);

clang/test/AST/Interp/cxx20.cpp

@@ -827,3 +827,17 @@ namespace CheckingNullPtrForInitialization {
     return x;
   }
 }
+
+namespace VariadicCallOperator {
+  class F {
+  public:
+    constexpr void operator()(int a, int b, ...) {}
+  };
+  constexpr int foo() {
+    F f;
+
+    f(1,2, 3);
+    return 1;
+  }
+  constexpr int A = foo();
+}