[clang][Interp] Fix ignoring TypeTraitExprs

[aaron.ballman]

clang/test/AST/Interp/literals.cpp

898

Let's make sure we still reject this:

constexpr int oh_my() {
  int x = 0;
  sizeof(int[x++]); // This would usually be evaluated because VLAs are terrible
  return x;
}

https://godbolt.org/z/E3jx6co46

[tbaeder]

clang/test/AST/Interp/literals.cpp

898

Hm, no, doesn't get rejected:

array.cpp:1240:3: warning: expression result unused [-Wunused-value]
 1240 |   sizeof(int[x++]); // This would usually be evaluated because VLAs are terrible
      |   ^~~~~~~~~~~~~~~~
array.cpp:1243:15: error: static assertion failed due to requirement 'oh_my() == 1'
 1243 | static_assert(oh_my() == 1);
      |               ^~~~~~~~~~~~
array.cpp:1243:23: note: expression evaluates to '0 == 1'
 1243 | static_assert(oh_my() == 1);
      |               ~~~~~~~~^~~~

[aaron.ballman]

clang/test/AST/Interp/literals.cpp

898

It looks to me like we're not modelling the side effects properly (we would return 1 if we were, so the static_assert would have passed), which might explain why we're failing to reject the code.

[tbaeder]

clang/test/AST/Interp/literals.cpp

898

Right, I understand, I was just showing the current state. From looking at it though, this is not something to be fixed here, the x++ is passed stand-alone to evaluateAsRValue(). Not sure yet what the exact problem is, I guess this is called before we even registered a local variable for x. I'll look into it.

[tbaeder]

clang/test/AST/Interp/literals.cpp

898

Fun, the ArgumentExpr is just x++:

|-UnaryExprOrTypeTraitExpr 0x621000072430 <line:1280:3, col:18> 'unsigned long' sizeof 'int[x++]'
| `-UnaryOperator 0x621000072378 <col:14, col:15> 'int' postfix '++'
|   `-DeclRefExpr 0x621000072350 <col:14> 'int' lvalue Var 0x6210000721a0 'x' 'int'

it's simple to implement just rejecting it by just doing discard(ArgumentExpr), but that doesn't emit any diagnostics.

[tbaeder]

clang/test/AST/Interp/literals.cpp

898

... and now I just realized that the tests for this patch don't even fit with the changes themselves. TypeTraitExpr is not for sizeof and alignof...

[aaron.ballman]

LGTM!

clang/lib/AST/Interp/ByteCodeExprGen.cpp

if (T) {
     }
   }
 
   return false;
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitTypeTraitExpr(const TypeTraitExpr *E) {
+  if (DiscardResult)
+    return true;
   return this->emitConstBool(E->getValue(), E);
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitLambdaExpr(const LambdaExpr *E) {
   // XXX We assume here that a pointer-to-initialize is on the stack.
 
   const Record *R = P.getOrCreateRecord(E->getLambdaClass());

clang/test/AST/Interp/literals.cpp

constexpr int ignoredExprs() {
 
     /// Ignored MaterializeTemporaryExpr.
     struct B{ const int &a; };
     (void)B{12};
 
     (void)5, (void)6;
 
     1 ? 0 : 1;
+    __is_trivial(int);
+
     return 0;
   }
 
   /// Ignored comma expressions still have their
   /// expressions evaluated.
   constexpr int Comma(int start) {
       int i = start;