Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
-- >8 --
The original PR114393 testcase is unfortunately still not accepted after
r14-9938-g081c1e93d56d35 due to return type deduction confusion when a
lambda-expr is used as a default template argument.
The below reduced testcase demonstrates the bug. Here, when forming the
dependent specialization b_v<U> we substitute the default argument of F,
a lambda-expr, with _Descriptor=U. (In this case in_template_context is
true since we're in the context of the template c_v so we don't defer.)
This substitution in turn lowers the level of its auto return type from
2 to 1. So later, when instantiating c_v<int, char> we incorrectly
replace the auto with the template argument at level=0, index=0, i.e.
int, instead of going through do_auto_deduction which would yield char.
One way to fix this would be to use a level-less auto to represent a
deduced return type of a lambda, but that might be too invasive of a
change at this stage.
Another way would be to pass tf_partial during dependent substitution
into a default template argument from coerce_template_parms so that the
levels of deeper parameters don't get lowered, but that wouldn't do
the right thing currently due to the tf_partial early exit in the
LAMBDA_EXPR case of tsubst_expr.
Another way, and the approach that this patch takes, is to just defer
all dependent substitution into a lambda-expr, building upon the logic
added in r14-9938-g081c1e93d56d35. This seems like the right thing to
do in light of the way we build up LAMBDA_EXPR_REGEN_INFO which should
consist only of the concrete template arguments used to regenerate the
lambda.
PR c++/114393
gcc/cp/ChangeLog:
* pt.cc (tsubst_lambda_expr): Also defer all dependent
substitution into a lambda-expr.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/lambda-targ2a.C: New test.
---
gcc/cp/pt.cc | 9 +++++++--
gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C | 14 ++++++++++++++
2 files changed, 21 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index ec259ee0fbf..bbd35417f8d 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -19622,11 +19622,16 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t
complain, tree in_decl)
in_decl = oldfn;
args = add_extra_args (LAMBDA_EXPR_EXTRA_ARGS (t), args, complain, in_decl);
- if (processing_template_decl && !in_template_context)
+ if (processing_template_decl
+ && (!in_template_context || any_dependent_template_arguments_p (args)))
{
/* Defer templated substitution into a lambda-expr if we lost the
necessary template context. This may happen for a lambda-expr
- used as a default template argument. */
+ used as a default template argument.
+
+ Defer dependent substitution as well so that we don't lower the
+ level of a deduced return type or any other auto or template
+ parameter. */
t = copy_node (t);
LAMBDA_EXPR_EXTRA_ARGS (t) = NULL_TREE;
LAMBDA_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args, complain);
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C
b/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C
new file mode 100644
index 00000000000..7136ce79872
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C
@@ -0,0 +1,14 @@
+// PR c++/114393
+// { dg-do compile { target c++20 } }
+
+template <auto _DescriptorFn> struct c1 {};
+
+template <class _Descriptor, auto F = [] { return _Descriptor(); }>
+inline constexpr auto b_v = F;
+
+template <class T, class U>
+inline constexpr auto c_v = b_v<U>;
+
+auto f = c_v<int, char>;
+using type = decltype(f());
+using type = char;
--
2.44.0.568.g436d4e5b14
