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 --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 01476fe92bec45d544df6e06811a47446cce515a..6c5f9815c981cd8938ff80348092709c523a4ab2 100644 (file)
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2011,12 +2011,9 @@ 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 23c49f477a8855a4cb2e111a1705f673654b834f..a6e21914eed1b946e25019d6930466efac444a43 100644 (file)
--- 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 (file)
index 0000000..2250df5
--- /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; });
+}