C++ PATCH for c++/59791 (ICE with return type mentioning local variable)

[Jason Merrill <jason at redhat dot com>]

We already deal with PARM_DECLs that aren't available for lookup in a 
late-specified return type, but this case needs the same treatment for a 
local variable.  During normal instantiation of the lambda we find the 
local variable fine, but later when we're doing dump_template_bindings 
from cxx_print_decl, the variable isn't in scope anymore, and we can 
deal with that in the same way we handle PARM_DECLs.

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

commit 6930b77188740b3e1979260ee805b38733b0f698
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Jan 28 16:56:42 2014 -0500

    	PR c++/59791
    	* pt.c (tsubst_decl) [VAR_DECL]: Allow in unevaluated context.
    	(tsubst_copy): Use it if lookup fails.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 011db2c..7f1b6d5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10990,9 +10990,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 	    DECL_TEMPLATE_INFO (r) = build_template_info (tmpl, argvec);
 	    SET_DECL_IMPLICIT_INSTANTIATION (r);
 	  }
-	else if (cp_unevaluated_operand)
-	  gcc_unreachable ();
-	else
+	else if (!cp_unevaluated_operand)
 	  register_local_specialization (r, t);
 
 	DECL_CHAIN (r) = NULL_TREE;
@@ -12481,6 +12479,11 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 		}
 	      else
 		{
+		  /* This can happen for a variable used in a late-specified
+		     return type of a local lambda.  Just make a dummy decl
+		     since it's only used for its type.  */
+		  if (cp_unevaluated_operand)
+		    return tsubst_decl (t, args, complain);
 		  gcc_assert (errorcount || sorrycount);
 		  return error_mark_node;
 		}
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype1.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype1.C
new file mode 100644
index 0000000..0ab0cdd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype1.C
@@ -0,0 +1,21 @@
+// PR c++/59791
+// We force the gimple dump to trigger use of lang_decl_name.
+// { dg-options "-std=c++11 -fdump-tree-gimple" }
+// { dg-final { cleanup-tree-dump "gimple" } }
+
+template < class T > void
+f (T t)
+{
+  int i = t;
+  [](int)->decltype (i + t)
+  {
+    return 0;
+  }
+  (0);
+}
+
+void
+foo ()
+{
+  f (0);
+}