[clang][Interp] Allow evaluating standalone complex expressions

[tbaeder]

Summary

We reach visitExpr() when evaluating standalone expressions. However,
the RetValue() code path was unused before, because we never reach it,
even with structs or arrays.

RetValue expects a pointer on the stack it can take apart to return an
APValue, so provide it with one.

[tbaeder]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

2280

I added these changes to make the test work, but they are thematically different from the purpose of this patch. However, without them, the test doesn't work.

[aaron.ballman]

LGTM, but you should change the summary to Allow evaluating standalone complex expressions when landing.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

if (std::optional<PrimType> T = classify(E)) {
     if (!visit(E))
       return false;
     return this->emitRet(*T, E);
   }
 
   // Expressions with a composite return type.
   // For us, that means everything we don't
   // have a PrimType for.
-  if (!visit(E))
+  if (std::optional<unsigned> LocalOffset = this->allocateLocal(E)) {
+    if (!this->visitLocalInitializer(E, *LocalOffset))
+      return false;
+
+    if (!this->emitGetPtrLocal(*LocalOffset, E))
       return false;
     return this->emitRetValue(E);
   }
 
+  return false;
+}
+
 /// Toplevel visitDecl().
 /// We get here from evaluateAsInitializer().
 /// We need to evaluate the initializer and return its value.
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::visitDecl(const VarDecl *VD) {
   assert(!VD->isInvalidDecl() && "Trying to constant evaluate an invalid decl");
   std::optional<PrimType> VarT = classify(VD->getType());
 
case UO_Not:    // ~x
     return DiscardResult ? this->emitPop(*T, E) : this->emitComp(*T, E);
   case UO_Real: { // __real x
     assert(!T);
     if (!this->visit(SubExpr))
       return false;
     if (!this->emitConstUint8(0, E))
       return false;
     return this->emitArrayElemPtrPopUint8(E);
   }
   case UO_Imag: { // __imag x
     assert(!T);
     if (!this->visit(SubExpr))
       return false;
     if (!this->emitConstUint8(1, E))
       return false;
     return this->emitArrayElemPtrPopUint8(E);
   }

clang/test/AST/Interp/complex.cpp

 static_assert(__real(D1) == 0, "");
 static_assert(__imag(D1) == 0, "");
 
 constexpr _Complex int I2 = {};
 static_assert(__real(I2) == 0, "");
 static_assert(__imag(I2) == 0, "");
 
 
+// Standalone complex expressions.
+static_assert(__real((_Complex float){1.0, 3.0}) == 1.0, "");
+
+
 #if 0
 /// FIXME: This should work in the new interpreter.
 constexpr _Complex double D2 = {12};
 static_assert(__real(D2) == 12, "");
 static_assert(__imag(D2) == 12, "");
 
 constexpr _Complex int I3 = {15};
 static_assert(__real(I3) == 15, "");