c++: NRVO, constexpr, lambda [PR118053]

Here during constant evaluation we encounter a VAR_DECL with DECL_VALUE_EXPR
of the RESULT_DECL, where the latter has been adjusted for
pass-by-invisible-reference.  We already had the code to deal with this, we
just need to use it in the non-capture case of DECL_VALUE_EXPR as well.

	PR c++/118053

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Generalize
	DECL_VALUE_EXPR invisiref handling.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-lambda1.C: New test.

2 files changed, 30 insertions, 9 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 299b134..59dd066 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7683,18 +7683,19 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 		 definition, so don't try to look at the closure.  But if the
 		 captured variable is constant, try to evaluate it directly. */
 	      r = DECL_CAPTURED_VARIABLE (t);
-	      tree type = TREE_TYPE (t);
-	      if (TYPE_REF_P (type) != TYPE_REF_P (TREE_TYPE (r)))
-		{
-		  /* Adjust r to match the reference-ness of t.  */
-		  if (TYPE_REF_P (type))
-		    r = build_address (r);
-		  else
-		    r = convert_from_reference (r);
-		}
 	    }
 	  else
 	    r = DECL_VALUE_EXPR (t);
+
+	  tree type = TREE_TYPE (t);
+	  if (TYPE_REF_P (type) != TYPE_REF_P (TREE_TYPE (r)))
+	    {
+	      /* Adjust r to match the reference-ness of t.  */
+	      if (TYPE_REF_P (type))
+		r = build_address (r);
+	      else
+		r = convert_from_reference (r);
+	    }
 	  return cxx_eval_constant_expression (ctx, r, lval, non_constant_p,
 					       overflow_p);
 	}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda1.C
new file mode 100644
index 0000000..68152e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda1.C
@@ -0,0 +1,20 @@
+// PR c++/118053
+// { dg-do compile { target c++14 } }
+
+template <typename _Tp> struct vector {
+  _Tp * _M_finish;
+  vector(_Tp);
+  vector(const vector &);
+  constexpr auto back() { return *_M_finish; }
+};
+template <typename Funct> void
+run(Funct funct) { funct(1); }
+
+vector<int>
+runner() try {
+  vector<int> vec{1};
+  run([&](auto) { vec.back(); });
+  return vec;
+} catch (...) {
+  return 1;
+}