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

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 safely strip aliases with
dependent template parameters.

2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/cp/
	PR c++/92206
	* cp-tree.h (STF_STRIP_DEPENDENT): New constant.
	* tree.c (strip_typedefs): Add STF_STRIP_DEPENDENT 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/cpp0x/alias-decl-pr92206-1.C: New test.
	* g++.dg/cpp0x/alias-decl-pr92206-2.C: Likewise.
	* g++.dg/cpp0x/alias-decl-pr92206-3.C: Likewise.

From-SVN: r278119

3 files changed, 19 insertions, 3 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index bf50b1b..5dcf13b 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,12 @@
+2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>
+
+	PR c++/92206
+	* cp-tree.h (STF_STRIP_DEPENDENT): New constant.
+	* tree.c (strip_typedefs): Add STF_STRIP_DEPENDENT 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.
+
 2019-11-12  Nathan Sidwell  <nathan@acm.org>
 
 	* name-lookup.c (lookup_using_decl): New function, merged from ...
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index adc021b..42afe1b 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5759,8 +5759,13 @@ enum auto_deduction_context
 
    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_STRIP_DEPENDENT: allow the stripping of aliases with dependent
+       template parameters, relying on code elsewhere to report any
+       appropriate diagnostics.  */
 const unsigned int STF_USER_VISIBLE = 1U;
+const unsigned int STF_STRIP_DEPENDENT = 1U << 1;
 
 /* Returns the TEMPLATE_DECL associated to a TEMPLATE_TEMPLATE_PARM
    node.  */
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index ba635d4..6c39c00 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -1488,7 +1488,8 @@ strip_typedefs (tree t, bool *remove_attributes, unsigned int flags)
   if (t == TYPE_CANONICAL (t))
     return t;
 
-  if (dependent_alias_template_spec_p (t))
+  if (!(flags & STF_STRIP_DEPENDENT)
+      && 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;
@@ -1673,7 +1674,8 @@ strip_typedefs (tree t, bool *remove_attributes, unsigned int flags)
 	      && !user_facing_original_type_p (t))
 	    return t;
 	  result = strip_typedefs (DECL_ORIGINAL_TYPE (TYPE_NAME (t)),
-				   remove_attributes, flags);
+				   remove_attributes,
+				   flags | STF_STRIP_DEPENDENT);
 	}
       else
 	result = TYPE_MAIN_VARIANT (t);