c++: print stub object as std::declval

If the result of build_stub_object gets printed by %E it looks something
like '(A&&)1', which seems confusing.  Let's instead print it as
'std::declval<A>()' since that's how the library writes the same idea.

gcc/cp/ChangeLog:

	* method.cc (is_stub_object): New.
	* cp-tree.h (is_stub_object): Declare.
	* error.cc (dump_expr): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/declare-variant-3.C: Update diagnostic.
	* g++.dg/gomp/declare-variant-5.C: Likewise.

5 files changed, 26 insertions, 4 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 1d741ec..6184d99 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7456,6 +7456,7 @@ extern tree get_copy_assign			(tree);
 extern tree get_default_ctor			(tree);
 extern tree get_dtor				(tree, tsubst_flags_t);
 extern tree build_stub_object			(tree);
+extern bool is_stub_object			(tree);
 extern tree build_invoke			(tree, const_tree,
 						 tsubst_flags_t);
 extern tree strip_inheriting_ctors		(tree);
diff --git a/gcc/cp/error.cc b/gcc/cp/error.cc
index feb6853..cc4cc4a 100644
--- a/gcc/cp/error.cc
+++ b/gcc/cp/error.cc
@@ -2604,6 +2604,15 @@ dump_expr (cxx_pretty_printer *pp, tree t, int flags)
 	  dump_expr (pp, CALL_EXPR_FN (t), flags | TFF_EXPR_IN_PARENS);
 	  dump_call_expr_args (pp, t, flags, true);
 	}
+      else if (is_stub_object (t))
+	{
+	  pp_string (pp, "std::declval<");
+	  if (lvalue_p (t)) /* T& */
+	    dump_type (pp, TREE_TYPE (STRIP_REFERENCE_REF (t)), flags);
+	  else /* T */
+	    dump_type (pp, TREE_TYPE (t), flags);
+	  pp_string (pp, ">()");
+	}
       else
 	{
 	  if (TREE_OPERAND (t,0) != NULL_TREE
diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 0f9f23a..64535f5 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -1914,6 +1914,18 @@ build_stub_object (tree reftype)
   return convert_from_reference (stub);
 }
 
+/* True iff EXPR is the result of build_stub_object.  */
+
+bool
+is_stub_object (tree expr)
+{
+  if (!REFERENCE_REF_P (expr))
+    return false;
+  expr = TREE_OPERAND (expr, 0);
+  return (TREE_CODE (expr) == CONVERT_EXPR
+	  && TREE_OPERAND (expr, 0) == integer_one_node);
+}
+
 /* Build a std::declval<TYPE>() expression and return it.  */
 
 tree
diff --git a/gcc/testsuite/g++.dg/gomp/declare-variant-3.C b/gcc/testsuite/g++.dg/gomp/declare-variant-3.C
index 376eefc..8c0cfd2 100644
--- a/gcc/testsuite/g++.dg/gomp/declare-variant-3.C
+++ b/gcc/testsuite/g++.dg/gomp/declare-variant-3.C
@@ -86,7 +86,7 @@ struct E { int e; };
 
 void fn19 (E, int);
 
-#pragma omp declare variant (fn19)match(user={condition(0)})	// { dg-error {could not convert '[^']*' from 'int' to 'E'} }
+#pragma omp declare variant (fn19)match(user={condition(0)})	// { dg-error {could not convert 'std::declval<int>\(\)' from 'int' to 'E'} }
 void fn20 (int, E);
 
 struct F { operator int () const { return 42; } int f; };
@@ -95,7 +95,7 @@ void fn21 (int, F);
 #pragma omp declare variant ( fn21 ) match (user = { condition ( 1 - 1 ) } )	// { dg-error "variant 'void fn21\\\(int, F\\\)' and base 'void fn22\\\(F, F\\\)' have incompatible types" }
 void fn22 (F, F);
 
-#pragma omp declare variant (fn19) match (user={condition(0)})		// { dg-error {could not convert '[^']*' from 'F' to 'E'} }
+#pragma omp declare variant (fn19) match (user={condition(0)})		// { dg-error {could not convert 'std::declval<F>\(\)' from 'F' to 'E'} }
 void fn23 (F, int);
 
 void fn24 (int);
diff --git a/gcc/testsuite/g++.dg/gomp/declare-variant-5.C b/gcc/testsuite/g++.dg/gomp/declare-variant-5.C
index a52fa52..a4747ac 100644
--- a/gcc/testsuite/g++.dg/gomp/declare-variant-5.C
+++ b/gcc/testsuite/g++.dg/gomp/declare-variant-5.C
@@ -74,7 +74,7 @@ struct E { int e; };
 
 void fn19 (E, int) {}
 
-#pragma omp declare variant (fn19)match(user={condition(0)})	// { dg-error {could not convert '[^']*' from 'int' to 'E'} }
+#pragma omp declare variant (fn19)match(user={condition(0)})	// { dg-error {could not convert 'std::declval<int>\(\)' from 'int' to 'E'} }
 void fn20 (int, E) {}
 
 struct F { operator int () const { return 42; } int f; };
@@ -83,7 +83,7 @@ void fn21 (int, F) {}
 #pragma omp declare variant ( fn21 ) match (user = { condition ( 1 - 1 ) } )	// { dg-error "variant 'void fn21\\\(int, F\\\)' and base 'void fn22\\\(F, F\\\)' have incompatible types" }
 void fn22 (F, F) {}
 
-#pragma omp declare variant (fn19) match (user={condition(0)})		// { dg-error {could not convert '[^']*' from 'F' to 'E'} }
+#pragma omp declare variant (fn19) match (user={condition(0)})		// { dg-error {could not convert 'std::declval<F>\(\)' from 'F' to 'E'} }
 void fn23 (F, int) {}
 
 void fn24 (int);