c++: decltype of capture proxy [PR79378, PR96917]

We typically don't see capture proxies in finish_decltype_type because
process_outer_var_ref is a no-op within an unevaluated context and so a
use of a captured variable within decltype resolves to the captured
variable, not the capture.  But we can see them during decltype(auto)
deduction and for decltype of an init-capture, which suggests we need to
handle capture proxies specially within finish_decltype_type after all.
This patch adds such handling.

	PR c++/79378
	PR c++/96917

gcc/cp/ChangeLog:

	* semantics.cc (finish_decltype_type): Handle an id-expression
	naming a capture proxy specially.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto7.C: New test.
	* g++.dg/cpp1y/lambda-init20.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

3 files changed, 96 insertions, 1 deletions

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 4059e74..f5ae8b5 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -11643,8 +11643,28 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
           /* Fall through for fields that aren't bitfields.  */
 	  gcc_fallthrough ();
 
-        case FUNCTION_DECL:
         case VAR_DECL:
+	  if (is_capture_proxy (expr))
+	    {
+	      if (is_normal_capture_proxy (expr))
+		{
+		  expr = DECL_CAPTURED_VARIABLE (expr);
+		  type = TREE_TYPE (expr);
+		  type = non_reference (type);
+		}
+	      else
+		{
+		  expr = DECL_VALUE_EXPR (expr);
+		  gcc_assert (TREE_CODE (expr) == COMPONENT_REF);
+		  expr = TREE_OPERAND (expr, 1);
+		  type = TREE_TYPE (expr);
+		}
+	      break;
+	    }
+	  /* Fall through for variables that aren't capture proxies.  */
+	  gcc_fallthrough ();
+
+	case FUNCTION_DECL:
         case CONST_DECL:
         case PARM_DECL:
         case RESULT_DECL:
diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto7.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto7.C
new file mode 100644
index 0000000..a37b9db
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto7.C
@@ -0,0 +1,53 @@
+// PR c++/96917
+// { dg-do compile { target c++14 } }
+
+int main() {
+  int x = 0;
+  int y = 0;
+  const int cx = 0;
+  const int cy = 0;
+
+  [x, &y, cx, &cy] {
+    decltype(auto) a = x;
+    using ty1 = int;
+    using ty1 = decltype(x);
+    using ty1 = decltype(a);
+
+    decltype(auto) b = y;
+    using ty2 = int;
+    using ty2 = decltype(y);
+    using ty2 = decltype(b);
+
+    decltype(auto) ca = cx;
+    using ty3 = const int;
+    using ty3 = decltype(cx);
+    using ty3 = decltype(ca);
+
+    decltype(auto) cb = cy;
+    using ty4 = const int;
+    using ty4 = decltype(cy);
+    using ty4 = decltype(cb);
+  };
+
+  [x=x, &y=y, cx=cx, &cy=cy] {
+    decltype(auto) a = x;
+    using ty1 = int;
+    using ty1 = decltype(x);
+    using ty1 = decltype(a);
+
+    decltype(auto) b = y;
+    using ty2 = int&;
+    using ty2 = decltype(y);
+    using ty2 = decltype(b);
+
+    decltype(auto) ca = cx;
+    using ty3 = int;
+    using ty3 = decltype(cx);
+    using ty3 = decltype(ca);
+
+    decltype(auto) cb = cy;
+    using ty4 = const int&;
+    using ty4 = decltype(cy);
+    using ty4 = decltype(cb);
+  };
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-init20.C b/gcc/testsuite/g++.dg/cpp1y/lambda-init20.C
new file mode 100644
index 0000000..a06b77a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-init20.C
@@ -0,0 +1,22 @@
+// PR c++/79378
+// { dg-do compile { target c++14 } }
+
+int main() {
+  int x = 0;
+  [x=x, &r=x] {
+    using ty1 = int;
+    using ty1 = decltype(x);
+
+    using ty2 = int&;
+    using ty2 = decltype(r);
+  };
+
+  const int cx = 0;
+  [x=cx, &r=cx] {
+    using ty1 = int;
+    using ty1 = decltype(x);
+
+    using ty2 = const int&;
+    using ty2 = decltype(r);
+  };
+}