[pushed] c++: -Wunused, constant, and generic lambda [PR96311]

[Jason Merrill via Gcc-patches]

We never called mark_use for a return value in a function with dependent
return type.  In that situation we don't know if the use is as an rvalue or
lvalue, but we can use mark_exp_read instead.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog:

        PR c++/96311
        * typeck.c (check_return_expr): Call mark_exp_read in dependent
        case.

gcc/testsuite/ChangeLog:

        PR c++/96311
        * g++.dg/cpp1y/lambda-generic-Wunused.C: New test.
---
 gcc/cp/typeck.c                                |  3 +++
 .../g++.dg/cpp1y/lambda-generic-Wunused.C      | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/lambda-generic-Wunused.C

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 8535ecc2d93..11dee7d8753 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -10215,6 +10215,9 @@ check_return_expr (tree retval, bool *no_warning)
     dependent:
       /* We should not have changed the return value.  */
       gcc_assert (retval == saved_retval);
+      /* We don't know if this is an lvalue or rvalue use, but
+        either way we can mark it as read.  */
+      mark_exp_read (retval);
       return retval;
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-Wunused.C 
b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-Wunused.C
new file mode 100644
index 00000000000..b43cbe6b675
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-Wunused.C
@@ -0,0 +1,18 @@
+// PR c++/96311
+// { dg-do compile { target c++14 } }
+// { dg-additional-options -Wunused }
+
+auto foo()
+{
+  constexpr int used = 0;
+  return
+    [](auto unused)
+    {
+      return used;
+    };
+}
+
+int main()
+{
+  foo()(42);
+}

base-commit: 9f4c41147a41d08a74990eafe69a1064a3c13435
prerequisite-patch-id: 3be338a0d789cd159b27785251afb4e692b30d15
-- 
2.27.0