c++: Instantiate less when evaluating __is_convertible

Jon reported that evaluating __is_convertible in a test led to
instantiating something ill-formed and so we failed to compile the test.
__is_convertible doesn't and shouldn't need to instantiate so much, so
let's limit it with a cp_unevaluated guard.  Use a helper function to
implement both built-ins.

	PR c++/106784

gcc/cp/ChangeLog:

	* method.cc (is_convertible_helper): New.
	(is_convertible): Use it.
	(is_nothrow_convertible): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible3.C: New test.
	* g++.dg/ext/is_nothrow_convertible3.C: New test.

3 files changed, 33 insertions, 8 deletions

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index c35a59f..9f917f1 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2236,6 +2236,19 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
   return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
 }
 
+/* Worker for is_{,nothrow_}convertible.  Attempt to perform an implicit
+   conversion from FROM to TO and return the result.  */
+
+static tree
+is_convertible_helper (tree from, tree to)
+{
+  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
+    return integer_one_node;
+  cp_unevaluated u;
+  tree expr = build_stub_object (from);
+  return perform_implicit_conversion (to, expr, tf_none);
+}
+
 /* Return true if FROM can be converted to TO using implicit conversions,
    or both FROM and TO are possibly cv-qualified void.  NB: This doesn't
    implement the "Access checks are performed as if from a context unrelated
@@ -2244,10 +2257,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
 bool
 is_convertible (tree from, tree to)
 {
-  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
-    return true;
-  tree expr = build_stub_object (from);
-  expr = perform_implicit_conversion (to, expr, tf_none);
+  tree expr = is_convertible_helper (from, to);
   if (expr == error_mark_node)
     return false;
   return !!expr;
@@ -2258,10 +2268,7 @@ is_convertible (tree from, tree to)
 bool
 is_nothrow_convertible (tree from, tree to)
 {
-  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
-    return true;
-  tree expr = build_stub_object (from);
-  expr = perform_implicit_conversion (to, expr, tf_none);
+  tree expr = is_convertible_helper (from, to);
   if (expr == NULL_TREE || expr == error_mark_node)
     return false;
   return expr_noexcept_p (expr, tf_none);
diff --git a/gcc/testsuite/g++.dg/ext/is_convertible3.C b/gcc/testsuite/g++.dg/ext/is_convertible3.C
new file mode 100644
index 0000000..7a986d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_convertible3.C
@@ -0,0 +1,9 @@
+// PR c++/106784
+// { dg-do compile { target c++11 } }
+// Make sure we don't reject this at runtime by trying to instantiate
+// something that would be ill-formed.
+
+struct A;
+struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
+
+static_assert(__is_convertible(const A&, B), "");
diff --git a/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
new file mode 100644
index 0000000..05b1e1d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
@@ -0,0 +1,9 @@
+// PR c++/106784
+// { dg-do compile { target c++11 } }
+// Make sure we don't reject this at runtime by trying to instantiate
+// something that would be ill-formed.
+
+struct A;
+struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
+
+static_assert(__is_nothrow_convertible(const A&, B), "");