The suggested resolution for CWG1286, which we implemented, ignores default
template arguments, but this PR is an example of why that doesn't make
sense: the templates aren't functionally equivalent.
Tested x86_64-pc-linux-gnu, applying to trunk.
PR c++/103852
DR 1286
gcc/cp/ChangeLog:
* pt.cc (get_underlying_template): Compare default template args.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/alias-decl-dr1286a.C: Default args now matter.
* g++.dg/cpp1z/class-deduction-alias1.C: New test.
---
gcc/cp/pt.cc | 13 +++++++++++++
gcc/testsuite/g++.dg/cpp0x/alias-decl-dr1286a.C | 16 ++++++++--------
.../g++.dg/cpp1z/class-deduction-alias1.C | 17 +++++++++++++++++
3 files changed, 38 insertions(+), 8 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction-alias1.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 75ed9a34018..1f0231f70e6 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -6637,6 +6637,18 @@ get_underlying_template (tree tmpl)
if (!comp_template_args (TI_ARGS (tinfo), alias_args))
break;
+ /* Are any default template arguments equivalent? */
+ tree aparms = INNERMOST_TEMPLATE_PARMS (DECL_TEMPLATE_PARMS (tmpl));
+ tree uparms = INNERMOST_TEMPLATE_PARMS (DECL_TEMPLATE_PARMS
(underlying));
+ const int nparms = TREE_VEC_LENGTH (aparms);
+ for (int i = 0; i < nparms; ++i)
+ {
+ tree adefarg = TREE_PURPOSE (TREE_VEC_ELT (aparms, i));
+ tree udefarg = TREE_PURPOSE (TREE_VEC_ELT (uparms, i));
+ if (!template_args_equal (adefarg, udefarg))
+ goto top_break;
+ }
+
/* If TMPL adds or changes any constraints, it isn't equivalent. I think
it's appropriate to treat a less-constrained alias as equivalent. */
if (!at_least_as_constrained (underlying, tmpl))
@@ -6645,6 +6657,7 @@ get_underlying_template (tree tmpl)
/* Alias is equivalent. Strip it and repeat. */
tmpl = underlying;
}
+ top_break:;
return tmpl;
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-dr1286a.C
b/gcc/testsuite/g++.dg/cpp0x/alias-decl-dr1286a.C
index 1780c9a47b7..fbd63d891d0 100644
--- a/gcc/testsuite/g++.dg/cpp0x/alias-decl-dr1286a.C
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-dr1286a.C
@@ -11,13 +11,13 @@ template<typename T, typename U = T> struct A;
template<template <class...> class> struct X;
// equivalent to A
-template<typename V, typename W>
+template<typename V, typename W = V>
using B = A<V, W>;
same<X<A>,X<B>> s1;
// not equivalent to A: not all parameters used
-template<typename V, typename W>
+template<typename V, typename W = V>
using C = A<V>;
different<X<A>,X<C>> d1;
@@ -29,32 +29,32 @@ using D = A<V>;
different<X<A>,X<D>> d2;
// not equivalent to A: template-arguments in wrong order
-template<typename V, typename W>
+template<typename V, typename W = V>
using E = A<W, V>;
different<X<A>,X<E>> d3;
-// equivalent to A: default arguments not considered
+// NOT equivalent to A: default arguments now considered
template<typename V, typename W = int>
using F = A<V, W>;
-same<X<A>,X<F>> s2;
+different<X<A>,X<F>> s2;
// equivalent to A and B
-template<typename V, typename W>
+template<typename V, typename W = V>
using G = A<V, W>;
same<X<A>,X<G>> s3;
same<X<B>,X<G>> s3b;
// equivalent to E
-template<typename V, typename W>
+template<typename V, typename W = V>
using H = E<V, W>;
same<X<E>,X<H>> s4;
// not equivalent to A: argument not identifier
-template<typename V, typename W>
+template<typename V, typename W = V>
using I = A<V, typename W::type>;
different<X<A>,X<I>> d4;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction-alias1.C
b/gcc/testsuite/g++.dg/cpp1z/class-deduction-alias1.C
new file mode 100644
index 00000000000..1ec90b58e3a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction-alias1.C
@@ -0,0 +1,17 @@
+// PR c++/103852
+// { dg-do compile { target c++17 } }
+
+template <class T> struct b{};
+template <class T, class T1 = b<T>>
+struct s
+{
+ s(T);
+};
+s c(100);
+template <class T, class T1 = b<T>>
+using ss = s<T, T1>; // equivalent under proposed resolution of DR 1286
+ss tt(1); // OK
+
+template <class T, class T1 = T>
+using ss2 = s<T, T1>; // different default arg makes it non-equivalent
+ss2 tt2(1); // { dg-error "alias template deduction" "" { target c++17_only } }
base-commit: 2f0610acbc056052a108e4a46911fc21d0dca2ab
--
2.27.0
