PR c++/87748 - substitution failure error with decltype.

This issue is similar to PR 87480; in both cases we were doing non-dependent
substitution with processing_template_decl set, leading to member access
expressions seeming still instantiation-dependent, and therefore decltype
not being simplified to its actual type.  And as in that PR, the fix is to
clear processing_template_decl while substituting a default template
argument.

	* pt.c (most_specialized_partial_spec): Clear
	processing_template_decl.

From-SVN: r269921

gcc/cp/ChangeLog

+2019-03-25  Jason Merrill  <jason@redhat.com>
+
+	PR c++/87748 - substitution failure error with decltype.
+	* pt.c (most_specialized_partial_spec): Clear
+	processing_template_decl.
+
 2019-03-25  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/89214 - ICE when initializing aggregates with bases.

gcc/cp/pt.c

@@ -23550,6 +23550,11 @@ most_specialized_partial_spec (tree target, tsubst_flags_t complain)
       args = INNERMOST_TEMPLATE_ARGS (args);
     }
 
+  /* The caller hasn't called push_to_top_level yet, but we need
+     get_partial_spec_bindings to be done in non-template context so that we'll
+     fully resolve everything.  */
+  processing_template_decl_sentinel ptds;
+
   for (t = DECL_TEMPLATE_SPECIALIZATIONS (main_tmpl); t; t = TREE_CHAIN (t))
     {
       tree spec_args;

gcc/testsuite/g++.dg/cpp0x/sfinae64.C

+// PR c++/87748
+// { dg-do compile { target c++11 } }
+
+template <class T> T&& declval();
+template <bool B, class T> struct enable_if;
+template <class T> struct enable_if <true, T> { typedef T type; };
+struct true_type { static const bool value = true; };
+struct false_type { static const bool value = false; };
+
+struct string
+{
+  string (const char *p);
+};
+
+/// Template to map all type arguments to void, useful for SFINAE, see also WG21 N3911.
+template<class...> using void_t = void;
+
+/// REQUIRES<value> - Simplified version of enable_if<> to use SFINAE in function templates.
+template<bool value> using REQUIRES = typename ::enable_if<value, bool>::type;
+
+/// DerivesString<T> - Check if @a T is of type 'string'.
+template<class T> struct DerivesString
+{
+  static const int value = __is_same_as (T, string);
+};
+
+/// Has__aida_visit__<T,Visitor> - Check if @a T provides a member template __aida_visit__<>(Visitor).
+template<class, class, class = void> struct Has__aida_visit__ : false_type {};
+template<class T, class V>
+struct Has__aida_visit__<T, V, void_t< decltype (declval<T>().template __aida_visit__<V> (*(V*) 0)) >> : true_type {};
+
+struct Foo {
+  template<class V> void __accept__ (V*);
+};
+
+/// Base template for Visitor classes, dispatches operator() to visit_<type>() methods.
+template<class DerivedVisitor>
+class VisitorDispatcher {
+  DerivedVisitor* derived () { return static_cast<DerivedVisitor*> (this); }
+protected:
+  typedef const char* Name; ///< Member name argument type for visit() methods.
+public:
+
+  // dispatch for calls like: visitor (field, "field");
+
+  template<class A,
+           REQUIRES< (!Has__aida_visit__<A, DerivedVisitor>::value) > = true>
+  void operator() (A &a, Name n)
+  { return derived()->visit_string (a, n); }
+};
+
+class PspecVisitor : public VisitorDispatcher<PspecVisitor> {
+public:
+  void visit_string (::string &a, Name name);
+};
+
+int
+main (int argc, char *argv[])
+{
+#ifdef WITHASSERT // define this to fix g++-8.1.1
+  static_assert (Has__aida_visit__<string, PspecVisitor>::value == false, "");
+#endif
+  PspecVisitor v;
+  string str ("A string");
+  v (str, "some_string");
+  static_assert (Has__aida_visit__<string, PspecVisitor>::value == false, "");
+  return 0;
+}
+// g++ -std=gnu++14 -Wall -O2 aidavisit.cc && ./a.out