PR23135: Don't instantiate constexpr functions referenced in unevaluated operands where possible.

This implements something like the current direction of DR1581: we use a narrow
syntactic check to determine the set of places where a constant expression
could be evaluated, and only instantiate a constexpr function or variable if
it's referenced in one of those contexts, or is odr-used.

It's not yet clear whether this is the right set of syntactic locations; we
currently consider all contexts within templates that would result in odr-uses
after instantiation, and contexts within list-initialization (narrowing
conversions take another victim...), as requiring instantiation. We could in
principle restrict the former cases more (only const integral / reference
variable initializers, and contexts in which a constant expression is required,
perhaps). However, this is sufficient to allow us to accept libstdc++ code,
which relies on GCC's behavior (which appears to be somewhat similar to this
approach).

llvm-svn: 291318

1 files changed, 7 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index befee05713e..45eff5ee6b6 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -6561,6 +6561,13 @@ InitializationSequence::Perform(Sema &S,
     break;
   }
 
+  // Promote from an unevaluated context to an unevaluated list context in
+  // C++11 list-initialization; we need to instantiate entities usable in
+  // constant expressions here in order to perform narrowing checks =(
+  EnterExpressionEvaluationContext Evaluated(
+      S, EnterExpressionEvaluationContext::InitList,
+      CurInit.get() && isa<InitListExpr>(CurInit.get()));
+
   // C++ [class.abstract]p2:
   //   no objects of an abstract class can be created except as subobjects
   //   of a class derived from it