[C++] Fix interaction between aka changes and DR1558 (PR92206)

[Richard Sandiford <richard dot sandiford at arm dot com>]

One of the changes in r277281 was to make the typedef variant
handling in strip_typedefs pass the raw DECL_ORIGINAL_TYPE to the
recursive call, instead of applying TYPE_MAIN_VARIANT first.
This PR shows that that interacts badly with the implementation
of DR1558, because we then refuse to strip aliases with dependent
template parameters and trip:

  gcc_assert (!typedef_variant_p (result)
	      || ((flags & STF_USER_VISIBLE)
		  && !user_facing_original_type_p (result)));

Keeping the current behaviour but suppressing the ICE leads to a
duplicate error (the dg-bogus in the first test), so that didn't
seem like a good fix.

I assume keeping the alias should never actually be necessary for
DECL_ORIGINAL_TYPEs, because it will already have been checked
somewhere, even for implicit TYPE_DECLs.  This patch therefore
passes a flag to say that we can assume the type is validated
elsewhere.

It seems a rather clunky fix, sorry, but restoring the
TYPE_MAIN_VARIANT (...) isn't compatible with the aka stuff.

Bootstrapped & regression-tested on aarch64-linux-gnu.  OK to install?

Richard


2019-10-25  Richard Sandiford  <richard.sandiford@arm.com>

gcc/cp/
	PR c++/92206
	* cp-tree.h (STF_ASSUME_VALID): New constant.
	* tree.c (strip_typedefs): Add STF_ASSUME_VALID to the flags
	when calling strip_typedefs recursively on a DECL_ORIGINAL_TYPE.
	Don't apply the fix for DR1558 in that case; allow aliases with
	dependent template parameters to be stripped instead.

gcc/testsuite/
	PR c++/92206
	* g++.dg/pr92206-1.C: New test.
	* g++.dg/pr92206-2.C: Likewise.
	* g++.dg/pr92206-3.C: Likewise.

Index: gcc/cp/cp-tree.h
===================================================================
--- gcc/cp/cp-tree.h	2019-10-25 09:21:28.098331304 +0100
+++ gcc/cp/cp-tree.h	2019-10-25 14:51:22.618009886 +0100
@@ -5728,8 +5728,12 @@ #define TFF_POINTER		                (1
 
    STF_USER_VISIBLE: use heuristics to try to avoid stripping user-facing
        aliases of internal details.  This is intended for diagnostics,
-       where it should (for example) give more useful "aka" types.  */
+       where it should (for example) give more useful "aka" types.
+
+   STF_ASSUME_VALID: assume where possible that the given type is valid,
+       relying on code elsewhere to report any appropriate diagnostics.  */
 const unsigned int STF_USER_VISIBLE = 1U;
+const unsigned int STF_ASSUME_VALID = 1U << 1;
 
 /* Returns the TEMPLATE_DECL associated to a TEMPLATE_TEMPLATE_PARM
    node.  */
Index: gcc/cp/tree.c
===================================================================
--- gcc/cp/tree.c	2019-10-22 08:47:01.255327861 +0100
+++ gcc/cp/tree.c	2019-10-25 14:51:22.618009886 +0100
@@ -1489,7 +1489,8 @@ strip_typedefs (tree t, bool *remove_att
   if (t == TYPE_CANONICAL (t))
     return t;
 
-  if (dependent_alias_template_spec_p (t))
+  if (!(flags & STF_ASSUME_VALID)
+      && dependent_alias_template_spec_p (t))
     /* DR 1558: However, if the template-id is dependent, subsequent
        template argument substitution still applies to the template-id.  */
     return t;
@@ -1674,7 +1675,8 @@ strip_typedefs (tree t, bool *remove_att
 	      && !user_facing_original_type_p (t))
 	    return t;
 	  result = strip_typedefs (DECL_ORIGINAL_TYPE (TYPE_NAME (t)),
-				   remove_attributes, flags);
+				   remove_attributes,
+				   flags | STF_ASSUME_VALID);
 	}
       else
 	result = TYPE_MAIN_VARIANT (t);
Index: gcc/testsuite/g++.dg/pr92206-1.C
===================================================================
--- /dev/null	2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/g++.dg/pr92206-1.C	2019-10-25 14:51:22.618009886 +0100
@@ -0,0 +1,9 @@
+// { dg-require-effective-target c++11 }
+
+template<typename> struct A {};
+template<typename T1, typename T2 = typename T1::value> using alias1 = A<T1>;
+template<typename T> class B {
+  using alias2 = alias1<A<T>>; // { dg-error {no type named 'value'} }
+  A<alias2> a; // { dg-bogus {no type named 'value'} }
+};
+B<int> b;
Index: gcc/testsuite/g++.dg/pr92206-2.C
===================================================================
--- /dev/null	2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/g++.dg/pr92206-2.C	2019-10-25 14:51:22.618009886 +0100
@@ -0,0 +1,14 @@
+// { dg-require-effective-target c++11 }
+
+template <bool> struct A;
+class Vector {
+  template <typename> struct TypeIsGCThing {
+    template <typename T, typename A<T ::value>::Type> using Vector = Vector;
+    struct B;
+    template <typename> class ContainerIter {
+      using Action = B;
+      using ActionVector = Vector<Action, 0>;
+      ContainerIter<ActionVector> a;
+    };
+  };
+};
Index: gcc/testsuite/g++.dg/pr92206-3.C
===================================================================
--- /dev/null	2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/g++.dg/pr92206-3.C	2019-10-25 14:51:22.618009886 +0100
@@ -0,0 +1,8 @@
+// { dg-require-effective-target c++11 }
+
+template <typename> void a();
+template <typename> struct b;
+template <bool> using c = int;
+template <typename d, typename e = decltype(a<d>)> using f = e;
+template <typename e> using g = f<e>;
+template <typename h> c<b<g<h>>::i> j;