[C++ PATCH] PR c++/87366 - wrong error with alias template.

[Jason Merrill]

With this testcase the code in template_args_equal to treat aliases as
distinct wasn't sufficient, because it only looked at the top level, whereas
here we have a reference to the alias.  So let's also handle treating them
as distinct in structural_comptypes.  For GCC 10 I have a more comprehensive
patch, but for GCC 9 let's go with this smaller change.

Tested x86_64-pc-linux-gnu, applying to trunk.

	* typeck.c (structural_comptypes): When comparing_specializations,
	aliases are unequal.
	(comptypes): When comparing_specializations, do structural
	comparison.
---
 gcc/cp/typeck.c                            | 15 +++++++++++++++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-66.C | 21 +++++++++++++++++++++
 gcc/cp/ChangeLog                           |  8 ++++++++
 3 files changed, 44 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-66.C

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 7224d9bf9ed..fff88ab8df4 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -1443,6 +1443,17 @@ structural_comptypes (tree t1, tree t2, int strict)
       return false;
     }
 
+  /* Don't treat an alias template specialization with dependent
+     arguments as equivalent to its underlying type when used as a
+     template argument; we need them to be distinct so that we
+     substitute into the specialization arguments at instantiation
+     time.  And aliases can't be equivalent without being ==, so
+     we don't need to look any deeper.  */
+  if (comparing_specializations
+      && (dependent_alias_template_spec_p (t1)
+	  || dependent_alias_template_spec_p (t2)))
+    return false;
+
   /* If we get here, we know that from a target independent POV the
      types are the same.  Make sure the target attributes are also
      the same.  */
@@ -1455,6 +1466,10 @@ structural_comptypes (tree t1, tree t2, int strict)
 bool
 comptypes (tree t1, tree t2, int strict)
 {
+  if (strict == COMPARE_STRICT && comparing_specializations
+      && (t1 != TYPE_CANONICAL (t1) || t2 != TYPE_CANONICAL (t2)))
+    /* If comparing_specializations, treat dependent aliases as distinct.  */
+    strict = COMPARE_STRUCTURAL;
   if (strict == COMPARE_STRICT)
     {
       if (t1 == t2)
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-66.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-66.C
new file mode 100644
index 00000000000..acdea77a326
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-66.C
@@ -0,0 +1,21 @@
+// PR c++/87366
+// { dg-do compile { target c++11 } }
+
+struct A {};
+struct B {};
+
+template <typename T> struct wrapper {};
+
+template <typename> struct enable_if_A { };
+template<> struct enable_if_A<A> { using type = void; };
+
+template <typename T, typename = typename enable_if_A<T>::type> using ok_t = T;
+
+template <typename T> void not_even_called(wrapper<const ok_t<T>&> a);
+
+template <typename T> int called(wrapper<const T&> a);
+
+void test(wrapper<const B&>& val)
+{
+    called(val);
+}
diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 11fc9de29b0..d207d95256e 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,11 @@
+2019-04-22  Jason Merrill  <jason@redhat.com>
+
+	PR c++/87366 - wrong error with alias template.
+	* typeck.c (structural_comptypes): When comparing_specializations,
+	aliases are unequal.
+	(comptypes): When comparing_specializations, do structural
+	comparison.
+
 2019-04-19  Jason Merrill  <jason@redhat.com>
 
 	PR c++/90190 - CTAD with list-constructor.

base-commit: a796a13541419edf99653581cf945253fb417064
-- 
2.20.1