[clang][NFC] refactors value type traits so we can have more than bools

[cjdb]

Summary

Since all the type traits up until now have had Boolean vaules, we've
always been able to assume that the expressions are bool. This is
about to change (D151952 introduces a trait that returns size_t), so
we need to restructure the code so it doesn't become unwieldy.

This is achieved by giving traits a designated "return" type.

[nickdesaulniers]

clang/lib/Sema/SemaExprCXX.cpp

5591–5594

llvm-project/clang/lib/Sema/SemaExprCXX.cpp:5591:3: warning: default label in switch which covers all enumeration values [-Wcovered-switch-default]
  default:
  ^

[nickdesaulniers]

clang/lib/Sema/SemaExprCXX.cpp

5591–5594

commit caf00725923c ("[Sema] Fix a warning")
via @kazu

clang/lib/Sema/SemaExprCXX.cpp

static bool EvaluateUnaryTypeTrait(Sema &Self, TypeTrait UTT,
   case UTT_IsTriviallyEqualityComparable:
     return T.isTriviallyEqualityComparableType(C);
   }
 }
 
 static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, QualType LhsT,
                                     QualType RhsT, SourceLocation KeyLoc);
 
-static bool evaluateTypeTrait(Sema &S, TypeTrait Kind, SourceLocation KWLoc,
+static bool EvaluateBooleanTypeTrait(Sema &S, TypeTrait Kind,
+                                     SourceLocation KWLoc,
                                      ArrayRef<TypeSourceInfo *> Args,
-                              SourceLocation RParenLoc) {
+                                     SourceLocation RParenLoc,
+                                     bool IsDependent) {
+  if (IsDependent)
+    return false;
+
   if (Kind <= UTT_Last)
     return EvaluateUnaryTypeTrait(S, Kind, KWLoc, Args[0]->getType());
 
   // Evaluate BTT_ReferenceBindsToTemporary alongside the IsConstructible
   // traits to avoid duplication.
   if (Kind <= BTT_Last && Kind != BTT_ReferenceBindsToTemporary)
     return EvaluateBinaryTypeTrait(S, Kind, Args[0]->getType(),
                                    Args[1]->getType(), RParenLoc);
bool Sema::CheckTypeTraitArity(unsigned Arity, SourceLocation Loc, size_t N) {
   if (!Arity && N == 0) {
     Diag(Loc, diag::err_type_trait_arity)
         << 1 << 1 << 1 << (int)N << SourceRange(Loc);
     return false;
   }
   return true;
 }
 
+enum class TypeTraitReturnType {
+  Bool,
+};
+
+static TypeTraitReturnType GetReturnType(TypeTrait Kind) {
+  return TypeTraitReturnType::Bool;
+}
+
 ExprResult Sema::BuildTypeTrait(TypeTrait Kind, SourceLocation KWLoc,
                                 ArrayRef<TypeSourceInfo *> Args,
                                 SourceLocation RParenLoc) {
   if (!CheckTypeTraitArity(getTypeTraitArity(Kind), KWLoc, Args.size()))
     return ExprError();
-  QualType ResultType = Context.getLogicalOperationType();
 
   if (Kind <= UTT_Last && !CheckUnaryTypeTraitTypeCompleteness(
                                *this, Kind, KWLoc, Args[0]->getType()))
     return ExprError();
 
   DiagnoseBuiltinDeprecation(*this, Kind, KWLoc);
 
   bool Dependent = false;
   for (unsigned I = 0, N = Args.size(); I != N; ++I) {
     if (Args[I]->getType()->isDependentType()) {
       Dependent = true;
       break;
     }
   }
 
-  bool Result = false;
-  if (!Dependent)
-    Result = evaluateTypeTrait(*this, Kind, KWLoc, Args, RParenLoc);
-
-  return TypeTraitExpr::Create(Context, ResultType, KWLoc, Kind, Args,
-                               RParenLoc, Result);
+  switch (GetReturnType(Kind)) {
+  case TypeTraitReturnType::Bool: {
+    bool Result = EvaluateBooleanTypeTrait(*this, Kind, KWLoc, Args, RParenLoc,
+                                           Dependent);
+    return TypeTraitExpr::Create(Context, Context.getLogicalOperationType(),
+                                 KWLoc, Kind, Args, RParenLoc, Result);
+  }
+  default:
+    llvm_unreachable("reached the end of BuildTypeTrait because the type "
+                     "trait's type is unaccounted for");
+  }
 }
 
 ExprResult Sema::ActOnTypeTrait(TypeTrait Kind, SourceLocation KWLoc,
                                 ArrayRef<ParsedType> Args,
                                 SourceLocation RParenLoc) {
   SmallVector<TypeSourceInfo *, 4> ConvertedArgs;
   ConvertedArgs.reserve(Args.size());