c++: extract_local_specs and unevaluated contexts [PR100295]

Here during partial instantiation of the constexpr if, extra_local_specs
walks the statement looking for local specializations within to capture.
However, we're thwarted by the fact that 'ts' first appears inside an
unevaluated context, and so the calls to process_outer_var_ref for its
local specializations are a no-op.  And since we walk each tree exactly
once, we end up not capturing the local specializations despite 'ts'
later occurring in an evaluated context.

This patch fixes this by making extract_local_specs walk evaluated
contexts first before walking unevaluated contexts.  We could probably
get away with not walking unevaluated contexts at all, but this approach
seems more clearly safe.

	PR c++/100295
	PR c++/107579

gcc/cp/ChangeLog:

	* pt.cc (el_data::skip_unevaluated_operands): New data member.
	(extract_locals_r): If skip_unevaluated_operands is true,
	don't walk into unevaluated contexts.
	(extract_local_specs): Walk the pattern twice, first with
	skip_unevaluated_operands true followed by it set to false.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-if-lambda5.C: New test.

1 files changed, 18 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 38e3a16..b9933ec 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -13016,17 +13016,26 @@ public:
   /* List of local_specializations used within the pattern.  */
   tree extra;
   tsubst_flags_t complain;
+  /* True iff we don't want to walk into unevaluated contexts.  */
+  bool skip_unevaluated_operands = false;
 
   el_data (tsubst_flags_t c)
     : extra (NULL_TREE), complain (c) {}
 };
 static tree
-extract_locals_r (tree *tp, int */*walk_subtrees*/, void *data_)
+extract_locals_r (tree *tp, int *walk_subtrees, void *data_)
 {
   el_data &data = *reinterpret_cast<el_data*>(data_);
   tree *extra = &data.extra;
   tsubst_flags_t complain = data.complain;
 
+  if (data.skip_unevaluated_operands
+      && unevaluated_p (TREE_CODE (*tp)))
+    {
+      *walk_subtrees = 0;
+      return NULL_TREE;
+    }
+
   if (TYPE_P (*tp) && typedef_variant_p (*tp))
     /* Remember local typedefs (85214).  */
     tp = &TYPE_NAME (*tp);
@@ -13118,6 +13127,14 @@ static tree
 extract_local_specs (tree pattern, tsubst_flags_t complain)
 {
   el_data data (complain);
+  /* Walk the pattern twice, ignoring unevaluated operands the first time
+     around, so that if a local specialization appears in both an evaluated
+     and unevaluated context we prefer to process it in the evaluated context
+     (since e.g. process_outer_var_ref is a no-op inside an unevaluated
+     context).  */
+  data.skip_unevaluated_operands = true;
+  cp_walk_tree (&pattern, extract_locals_r, &data, &data.visited);
+  data.skip_unevaluated_operands = false;
   cp_walk_tree (&pattern, extract_locals_r, &data, &data.visited);
   return data.extra;
 }