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, 4 insertions, 0 deletions

diff --git a/clang/lib/Parse/ParseInit.cpp b/clang/lib/Parse/ParseInit.cpp
index 4a68942f6d2..fa6b75daed9 100644
--- a/clang/lib/Parse/ParseInit.cpp
+++ b/clang/lib/Parse/ParseInit.cpp
@@ -404,6 +404,10 @@ ExprResult Parser::ParseBraceInitializer() {
     return Actions.ActOnInitList(LBraceLoc, None, ConsumeBrace());
   }
 
+  // Enter an appropriate expression evaluation context for an initializer list.
+  EnterExpressionEvaluationContext EnterContext(
+      Actions, EnterExpressionEvaluationContext::InitList);
+
   bool InitExprsOk = true;
 
   while (1) {