[clang][Interp] Fix ignoring MaterializeTemporaryExprs

[tbaeder]

This is split out from https://reviews.llvm.org/D149828, but I think that other one had problems (which made me write this version).

[cor3ntin]

Can you describe the problems?

Otherwise, this looks reasonable.

[tbaeder]

The changes in https://reviews.llvm.org/D149828 missed one case, the (E->getStorageDuration() == SD_Static) && SubExprT one.

[tbaeder]

Ping

[tbaeder]

Ping

[cor3ntin]

@aaron.ballman any opinion on that?

[tbaeder]

Ping

[tbaeder]

Ping

[aaron.ballman]

LGTM!

clang/lib/AST/Interp/ByteCodeExprGen.cpp

 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitMaterializeTemporaryExpr(
     const MaterializeTemporaryExpr *E) {
   const Expr *SubExpr = E->getSubExpr();
   std::optional<PrimType> SubExprT = classify(SubExpr);
 
+  // If we don't end up using the materialized temporary anyway, don't
+  // bother creating it.
+  if (DiscardResult)
+    return this->discard(SubExpr);
+
   if (E->getStorageDuration() == SD_Static) {
     if (std::optional<unsigned> GlobalIndex = P.createGlobal(E)) {
       const LifetimeExtendedTemporaryDecl *TempDecl =
           E->getLifetimeExtendedTemporaryDecl();
 
       if (!this->visitInitializer(SubExpr))
         return false;
 

clang/test/AST/Interp/literals.cpp

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