c++: Fix error-recovery with concepts.

Here, push_tinst_level refused to push into the scope of Foo::Foo
because it was triggered from the ill-formed function fun.  But we didn't
check the return value and tried to pop the un-pushed level.

	PR c++/93551
	* constraint.cc (satisfy_declaration_constraints): Check return
	value of push_tinst_level.

gcc/cp/ChangeLog

 2020-02-04  Jason Merrill  <jason@redhat.com>
 
+	PR c++/93551
+	* constraint.cc (satisfy_declaration_constraints): Check return
+	value of push_tinst_level.
+
 	PR c++/90951
 	* constexpr.c (cxx_eval_array_reference): {}-initialize missing
 	elements instead of value-initializing them.

gcc/cp/constraint.cc

@@ -2692,7 +2692,8 @@ satisfy_declaration_constraints (tree t, subst_info info)
   tree result = boolean_true_node;
   if (norm)
     {
-      push_tinst_level (t);
+      if (!push_tinst_level (t))
+	return result;
       push_access_scope (t);
       result = satisfy_associated_constraints (norm, args, info);
       pop_access_scope (t);

gcc/testsuite/g++.dg/cpp2a/concepts-err1.C

+// PR c++/93551
+// { dg-do compile { target concepts } }
+
+namespace std {
+  template<typename _Tp, _Tp __v>
+  struct integral_constant
+  {
+    static constexpr _Tp                  value = __v;
+    typedef _Tp                           value_type;
+    typedef integral_constant<_Tp, __v>   type;
+    constexpr operator value_type() const noexcept { return value; }
+  };
+  template<typename _Base, typename _Derived>
+  struct is_base_of
+    : public integral_constant<bool, __is_base_of(_Base, _Derived)>
+  { };
+  template <typename _Base, typename _Derived>
+  inline constexpr bool is_base_of_v = is_base_of<_Base, _Derived>::value;
+}
+class Bar { };
+struct Foo {
+  template <typename P> requires std::is_base_of_v<Bar, P>
+  Foo(P const&);
+};
+template <typename P>
+Foo fun(P const& arg) {
+  (bool)arg;			// { dg-error "" }
+  return Foo {arg};
+}
+int main() {
+  fun(Bar{});
+  return 0;
+}