[clang][Interp] Remove args from called functions in more cases

When calling functions in the checkingPotentialConstantExpression mode,
we cannot have arguments (including This + RVO pointers) for the
toplevel callee, but the functions called from within can work just
fine, or at least we succeed in pushing their arguments on the stack, so
we must also succeed in removing them again.

Differential Revision: https://reviews.llvm.org/D150358

clang/lib/AST/Interp/Interp.h

@@ -177,13 +177,12 @@ enum class ArithOp { Add, Sub };
 // Returning values
 //===----------------------------------------------------------------------===//
 
-template <PrimType Name, bool Builtin = false,
-          class T = typename PrimConv<Name>::T>
+template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
   const T &Ret = S.Stk.pop<T>();
 
   assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
-  if (Builtin || !S.checkingPotentialConstantExpression())
+  if (!S.checkingPotentialConstantExpression() || S.Current->Caller)
     S.Current->popArgs();
 
   if (InterpFrame *Caller = S.Current->Caller) {
@@ -200,10 +199,9 @@ bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
   return true;
 }
 
-template <bool Builtin = false>
 inline bool RetVoid(InterpState &S, CodePtr &PC, APValue &Result) {
   assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
-  if (Builtin || !S.checkingPotentialConstantExpression())
+  if (!S.checkingPotentialConstantExpression() || S.Current->Caller)
     S.Current->popArgs();
 
   if (InterpFrame *Caller = S.Current->Caller) {

clang/lib/AST/Interp/InterpBuiltin.cpp

@@ -64,12 +64,12 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F) {
   switch (F->getBuiltinID()) {
   case Builtin::BI__builtin_is_constant_evaluated:
     S.Stk.push<Boolean>(Boolean::from(S.inConstantContext()));
-    return Ret<PT_Bool, true>(S, OpPC, Dummy);
+    return Ret<PT_Bool>(S, OpPC, Dummy);
   case Builtin::BI__builtin_assume:
-    return RetVoid<true>(S, OpPC, Dummy);
+    return RetVoid(S, OpPC, Dummy);
   case Builtin::BI__builtin_strcmp:
     if (interp__builtin_strcmp(S, OpPC, Frame))
-      return Ret<PT_Sint32, true>(S, OpPC, Dummy);
+      return Ret<PT_Sint32>(S, OpPC, Dummy);
     return false;
   default:
     return false;

clang/test/AST/Interp/functions.cpp

@@ -257,3 +257,11 @@ namespace InvalidCall {
                    // ref-note {{in call to 'SS()'}}
 
 }
+
+namespace CallWithArgs {
+  /// This used to call problems during checkPotentialConstantExpression() runs.
+  constexpr void g(int a) {}
+  constexpr void f() {
+    g(0);
+  }
+}