On Wed, 28 Apr 2021, Jason Merrill wrote:
> On 4/28/21 2:24 PM, Patrick Palka wrote:
> > This makes tsubst_arg_types substitute into a function's parameter types
> > in left-to-right order instead of right-to-left order, in accordance with
> > DR 1227.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk? [ diff generated with -w to hide noisy whitespace changes ]
>
> OK. We'll still substitute lambda default args in reverse order, but that
> shouldn't happen in sfinae context, so that shouldn't be a problem. Maybe add
> an assert (complain & tf_error) in the lambda case?
Apparently cpp2a/lambda-uneval2.C trips over such an assert during
deduction for:
template <class T>
auto j(T t) -> decltype([](auto x) -> decltype(x.invalid) { } (t));
which makes sense, I suppose.
It looks like we can just move up the default argument processing to
before the recursive call to tsubst_arg_types, as in the below.
Bootstrapped and regtested on x86_64-pc-linux-gnu.
BTW, now that LAMBDA_EXPR has LAMBDA_EXPR_REGEN_INFO I think we have
enough information to defer substituting into lambda default arguments
until they're actually needed. Would that be something to look into as
a followup patch?
-- >8 --
Subject: [PATCH] c++: Substitute into function parms in lexical order
[PR96560]
This makes tsubst_arg_types substitute into a function's parameter types
in left-to-right order instead of right-to-left order, in accordance with
DR 1227.
gcc/cp/ChangeLog:
DR 1227
PR c++/96560
* pt.c (tsubst_arg_types): Rearrange so that we substitute into
TYPE_ARG_TYPES in forward order while short circuiting
appropriately. Adjust formatting.
gcc/testsuite/ChangeLog:
DR 1227
PR c++/96560
* g++.dg/template/sfinae-dr1227.C: New test.
---
gcc/cp/pt.c | 51 ++++++++++---------
gcc/testsuite/g++.dg/template/sfinae-dr1227.C | 23 +++++++++
2 files changed, 51 insertions(+), 23 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/template/sfinae-dr1227.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index eaf46659f85..e6d65595e2f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -15068,20 +15068,13 @@ tsubst_arg_types (tree arg_types,
tsubst_flags_t complain,
tree in_decl)
{
- tree remaining_arg_types;
tree type = NULL_TREE;
- int i = 1;
+ int len = 1;
tree expanded_args = NULL_TREE;
- tree default_arg;
if (!arg_types || arg_types == void_list_node || arg_types == end)
return arg_types;
- remaining_arg_types = tsubst_arg_types (TREE_CHAIN (arg_types),
- args, end, complain, in_decl);
- if (remaining_arg_types == error_mark_node)
- return error_mark_node;
-
if (PACK_EXPANSION_P (TREE_VALUE (arg_types)))
{
/* For a pack expansion, perform substitution on the
@@ -15092,7 +15085,7 @@ tsubst_arg_types (tree arg_types,
if (TREE_CODE (expanded_args) == TREE_VEC)
/* So that we'll spin through the parameters, one by one. */
- i = TREE_VEC_LENGTH (expanded_args);
+ len = TREE_VEC_LENGTH (expanded_args);
else
{
/* We only partially substituted into the parameter
@@ -15101,14 +15094,15 @@ tsubst_arg_types (tree arg_types,
expanded_args = NULL_TREE;
}
}
+ else
+ type = tsubst (TREE_VALUE (arg_types), args, complain, in_decl);
- while (i > 0) {
- --i;
-
+ /* Check if a substituted type is erroneous before substituting into
+ the rest of the chain. */
+ for (int i = 0; i < len; i++)
+ {
if (expanded_args)
type = TREE_VEC_ELT (expanded_args, i);
- else if (!type)
- type = tsubst (TREE_VALUE (arg_types), args, complain, in_decl);
if (type == error_mark_node)
return error_mark_node;
@@ -15122,15 +15116,12 @@ tsubst_arg_types (tree arg_types,
}
return error_mark_node;
}
-
- /* Do array-to-pointer, function-to-pointer conversion, and ignore
- top-level qualifiers as required. */
- type = cv_unqualified (type_decays_to (type));
+ }
/* We do not substitute into default arguments here. The standard
mandates that they be instantiated only when needed, which is
done in build_over_call. */
- default_arg = TREE_PURPOSE (arg_types);
+ tree default_arg = TREE_PURPOSE (arg_types);
/* Except that we do substitute default arguments under tsubst_lambda_expr,
since the new op() won't have any associated template arguments for us
@@ -15139,20 +15130,34 @@ tsubst_arg_types (tree arg_types,
default_arg = tsubst_copy_and_build (default_arg, args, complain, in_decl,
false/*fn*/, false/*constexpr*/);
+ tree remaining_arg_types = tsubst_arg_types (TREE_CHAIN (arg_types),
+ args, end, complain, in_decl);
+ if (remaining_arg_types == error_mark_node)
+ return error_mark_node;
+
+ for (int i = len-1; i >= 0; i--)
+ {
+ if (expanded_args)
+ type = TREE_VEC_ELT (expanded_args, i);
+
+ /* Do array-to-pointer, function-to-pointer conversion, and ignore
+ top-level qualifiers as required. */
+ type = cv_unqualified (type_decays_to (type));
+
if (default_arg && TREE_CODE (default_arg) == DEFERRED_PARSE)
{
/* We've instantiated a template before its default arguments
have been parsed. This can happen for a nested template
class, and is not an error unless we require the default
argument in a call of this function. */
- remaining_arg_types =
- tree_cons (default_arg, type, remaining_arg_types);
+ remaining_arg_types
+ = tree_cons (default_arg, type, remaining_arg_types);
vec_safe_push (DEFPARSE_INSTANTIATIONS (default_arg),
remaining_arg_types);
}
else
- remaining_arg_types =
- hash_tree_cons (default_arg, type, remaining_arg_types);
+ remaining_arg_types
+ = hash_tree_cons (default_arg, type, remaining_arg_types);
}
return remaining_arg_types;
diff --git a/gcc/testsuite/g++.dg/template/sfinae-dr1227.C
b/gcc/testsuite/g++.dg/template/sfinae-dr1227.C
new file mode 100644
index 00000000000..821ff0313b6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/sfinae-dr1227.C
@@ -0,0 +1,23 @@
+// PR c++/96560
+// DR 1227
+// Test that we substitute function parameter types in lexical order.
+
+template <class T>
+struct A { typedef typename T::type type; }; // { dg-error "void" }
+
+template <class T> void f(typename T::type, typename A<T>::type);
+template <class T> long f(...);
+
+long x = f<int>(0, 0); // { dg-bogus "" } OK
+
+
+template <class T> void g(T, typename A<T>::type);
+template <class T> long g(...);
+
+long y = g<void>(0, 0); // { dg-bogus "" } OK
+
+
+template <class T> void h(typename A<T>::type, T);
+template <class T> long h(...);
+
+long z = h<void>(0, 0); // { dg-message "required from here" } hard error
--
2.31.1.362.g311531c9de
