[Sema] Discarded statment should be an evaluatable context

[erik.pilkington]

Summary

The constexpr evaluator was erroring out because these templates weren't defined. Despite being used in a discarded statement, we still need to constexpr evaluate them, which means that we need to instantiate them.

Fixes https://llvm.org/PR37585

Thanks!

[erik.pilkington]

Ping!

[rsmith]

Hmm, so this will mean that we can have internal linkage declarations marked Used for which there is no definition, and we need to not warn on that.

I think it might be better to avoid marking things used in this case (even though they're still technically odr-used).

[erik.pilkington]

In D48322#1148424, @rsmith wrote:

Hmm, so this will mean that we can have internal linkage declarations marked Used for which there is no definition, and we need to not warn on that.

I think it might be better to avoid marking things used in this case (even though they're still technically odr-used).

We already return false for discarded statements in isOdrUseContext, so neither MarkVarDeclReferenced nor MarkFunctionReferenced will set Used. With or without this patch, clang doesn't emit -Wundefined-internal for the following code:

static int p();
int main() {
  if constexpr (false) p();
}

Is this what you were concerned about?

[rsmith]

In D48322#1150316, @erik.pilkington wrote:

Is this what you were concerned about?

Yes, exactly. Thank you for checking.

clang/lib/Sema/SemaExpr.cpp

 
 /// Are we within a context in which some evaluation could be performed (be it
 /// constant evaluation or runtime evaluation)? Sadly, this notion is not quite
 /// captured by C++'s idea of an "unevaluated context".
 static bool isEvaluatableContext(Sema &SemaRef) {
   switch (SemaRef.ExprEvalContexts.back().Context) {
     case Sema::ExpressionEvaluationContext::Unevaluated:
     case Sema::ExpressionEvaluationContext::UnevaluatedAbstract:
-    case Sema::ExpressionEvaluationContext::DiscardedStatement:
       // Expressions in this context are never evaluated.
       return false;
 
     case Sema::ExpressionEvaluationContext::UnevaluatedList:
     case Sema::ExpressionEvaluationContext::ConstantEvaluated:
     case Sema::ExpressionEvaluationContext::PotentiallyEvaluated:
+    case Sema::ExpressionEvaluationContext::DiscardedStatement:
       // Expressions in this context could be evaluated.
       return true;
 
     case Sema::ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed:
       // Referenced declarations will only be used if the construct in the
       // containing expression is used, at which point we'll be given another
       // turn to mark them.
       return false;

clang/test/SemaCXX/constant-expression-cxx1z.cpp

namespace NoexceptFunctionTypes {
   static_assert(A<int>().g());
   static_assert(A<int>()());
 }
 
 namespace Cxx17CD_NB_GB19 {
   const int &r = 0;
   constexpr int n = r;
 }
+
+namespace PR37585 {
+template <class T> struct S { static constexpr bool value = true; };
+template <class T> constexpr bool f() { return true; }
+template <class T> constexpr bool v = true;
+
+void test() {
+  if constexpr (true) {}
+  else if constexpr (f<int>()) {}
+  else if constexpr (S<int>::value) {}
+  else if constexpr (v<int>) {}
+}
+}