Re: [PATCH] c++: decltype of by-ref capture proxy of ref [PR115504]

[Jason Merrill]

On 6/25/24 11:45, Patrick Palka wrote:

On Tue, 25 Jun 2024, Jason Merrill wrote:

On 6/25/24 11:03, Patrick Palka wrote:

On Mon, 24 Jun 2024, Jason Merrill wrote:

On 6/24/24 21:00, Patrick Palka wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk/14?

-- >8 --

The capture proxy handling in finish_decltype_type added in r14-5330
was stripping the reference type of a capture proxy's captured variable,
which is desirable for a by-value capture, but not for a by-ref capture
(of a reference).

I'm not sure why we would want it for by-value, either; regardless of the
capture kind, decltype(x) is int&.

Ah, makes sense.  But I guess that means

    void f(int& x) {
      [x]() {
        decltype(auto) a = x;
      }
    }

is ill-formed since decltype(x) is int& but the corresponding closure
member is const?  It works if we make the lambda mutable.

Yes, and clang agrees.  Let's also test that case.

Like so?

OK.

-- >8 --

Subject: [PATCH] c++: decltype of capture proxy of ref [PR115504]

The capture proxy handling in finish_decltype_type added in r14-5330
was stripping the reference type of a capture proxy's captured variable.

        PR c++/115504

gcc/cp/ChangeLog:

        * semantics.cc (finish_decltype_type): Don't strip the reference
        type (if any) of a capture proxy's captured variable.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1y/decltype-auto8.C: New test.
---
  gcc/cp/semantics.cc                         |  1 -
  gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C | 22 +++++++++++++++++++++
  2 files changed, 22 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 6c1813d37c6..6a383c0f7f9 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12071,7 +12071,6 @@ finish_decltype_type (tree expr, bool 
id_expression_or_member_access_p,
                {
                  expr = DECL_CAPTURED_VARIABLE (expr);
                  type = TREE_TYPE (expr);
-                 type = non_reference (type);
                }
              else
                {
diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C 
b/gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C
new file mode 100644
index 00000000000..55135cecf72
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C
@@ -0,0 +1,22 @@
+// PR c++/115504
+// { dg-do compile { target c++14 } }
+
+void f(int& x, const int& y) {
+  [&x]() {
+    decltype(auto) a = x;
+    using type = decltype(x);
+    using type = decltype(a);
+    using type = int&; // not 'int'
+  };
+
+  [x]() {
+    decltype(auto) a = x; // { dg-error "discards qualifiers" }
+  };
+
+  [x]() mutable {
+    decltype(auto) a = x;
+    using type = decltype(x);
+    using type = decltype(a);
+    using type = int&;
+  };
+}