[gcc r14-10697] c++: decltype(auto) deduction of statement-expression [PR116418]

[Patrick Palka]

https://gcc.gnu.org/g:abdea396e12e66185d810435bbc363845cf4664a

commit r14-10697-gabdea396e12e66185d810435bbc363845cf4664a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Sep 12 12:45:03 2024 -0400

    c++: decltype(auto) deduction of statement-expression [PR116418]
    
    r8-7538 for PR84968 made strip_typedefs_expr diagnose STATEMENT_LIST
    so that we reject statement-expressions in noexcept-specifiers to
    match our behavior in template arguments (which the parser diagnoses
    directly).
    
    Later r11-7452 made decltype(auto) deduction canonicalize the expression
    (as an implementation detail) which in turn calls strip_typedefs_expr,
    and so ever since we inadvertently reject decltype(auto) deduction of a
    statement-expression.
    
    This patch just removes the diagnostic in strip_typedefs_expr and instead
    treats statement-expressions similar to lambda-expressions.  The function
    doesn't seem like the right place for such a diagnostic and so it seems
    easier to just accept rather than try to reject them in a suitable place.
    
            PR c++/116418
    
    gcc/cp/ChangeLog:
    
            * tree.cc (strip_typedefs_expr) <case STATEMENT_LIST>: Replace
            this error path with ...
            <case STMT_EXPR>: ... this, returning the original tree.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/eh/pr84968.C: No longer expect an ahead of time diagnostic
            for the statement-expresssion.  Instantiate the template and expect
            an incomplete type error instead.
            * g++.dg/ext/stmtexpr26.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit 12bdcc3d7970860b9d66ed4dea203bde8fd68d4d)

Diff:
---
 gcc/cp/tree.cc                        |  5 +----
 gcc/testsuite/g++.dg/eh/pr84968.C     |  4 +++-
 gcc/testsuite/g++.dg/ext/stmtexpr26.C | 10 ++++++++++
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index d90244609c8a..a056237e8669 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2008,12 +2008,9 @@ strip_typedefs_expr (tree t, bool *remove_attributes, unsigned int flags)
       }
 
     case LAMBDA_EXPR:
+    case STMT_EXPR:
       return t;
 
-    case STATEMENT_LIST:
-      error ("statement-expression in a constant expression");
-      return error_mark_node;
-
     default:
       break;
     }
diff --git a/gcc/testsuite/g++.dg/eh/pr84968.C b/gcc/testsuite/g++.dg/eh/pr84968.C
index 23c49f477a88..a6e21914eed1 100644
--- a/gcc/testsuite/g++.dg/eh/pr84968.C
+++ b/gcc/testsuite/g++.dg/eh/pr84968.C
@@ -9,7 +9,9 @@ struct S {
   void a()
     try {
     } catch (int ()
-	     noexcept (({ union b a; true; }))) // { dg-error "constant" }
+	     noexcept (({ union b a; true; }))) // { dg-error "'b a' has incomplete type" }
   {
   }
 };
+
+template void S::a<int>(); // { dg-message "required from here" }
diff --git a/gcc/testsuite/g++.dg/ext/stmtexpr26.C b/gcc/testsuite/g++.dg/ext/stmtexpr26.C
new file mode 100644
index 000000000000..2250df550d48
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/stmtexpr26.C
@@ -0,0 +1,10 @@
+// PR c++/116418
+// { dg-do compile { target c++14 } }
+// { dg-options "" }
+
+void foo ();
+template <typename>
+void bar ()
+{
+  decltype(auto) v = ({ foo (); 3; });
+}