On 4/16/20 1:53 PM, Patrick Palka wrote:
In this PR (which I think is misclassified as ice-on-invalid instead of
ice-on-valid), we're ICEing on a use of an 'int... a' template parameter pack as
part of the variadic lambda init-capture [...z=a].
The unexpected thing about this variadic init-capture is that it is not
type-dependent, and so when we call do_auto_deduction from
lambda_capture_field_type it actually resolves its type to 'int' instead of
exiting early like it would do for a type-dependent variadic initializer. This
later confuses add_capture which, according to one of its comments, assumes that
'type' is always 'auto' for a variadic init-capture.
The simplest fix, and the approach that this patch takes, seems to be to avoid
doing auto deduction in lambda_capture_field_type when the initializer uses
parameter packs, so that we always return 'auto' even in the non-type-dependent
case.
Hmm, that will make 'z' more dependent than necessary, but I suppose its
packness will tend to make uses dependent anyway, so OK.
Passes 'make check-c++', does this look OK to commit after full
bootstrap/regtesting?
gcc/cp/ChangeLog:
PR c++/94483
* lambda.c (lambda_capture_field_type): Avoid doing auto deduction if
the explicit initializer has parameter packs.
gcc/testsuite/ChangeLog:
PR c++/94483
* g++.dg/cpp2a/lambda-pack-init5.C: New test.
---
gcc/cp/lambda.c | 5 ++++-
gcc/testsuite/g++.dg/cpp2a/lambda-pack-init5.C | 18 ++++++++++++++++++
2 files changed, 22 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-pack-init5.C
diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c
index 4f39f99756b..b55c2f85d27 100644
--- a/gcc/cp/lambda.c
+++ b/gcc/cp/lambda.c
@@ -223,7 +223,10 @@ lambda_capture_field_type (tree expr, bool explicit_init_p,
/* Add the reference now, so deduction doesn't lose
outermost CV qualifiers of EXPR. */
type = build_reference_type (type);
- type = do_auto_deduction (type, expr, auto_node);
+ if (uses_parameter_packs (expr))
+ /* Stick with 'auto' even if the type could be deduced. */;
+ else
+ type = do_auto_deduction (type, expr, auto_node);
}
else if (!is_this && type_dependent_expression_p (expr))
{
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init5.C
b/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init5.C
new file mode 100644
index 00000000000..492fc479e94
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init5.C
@@ -0,0 +1,18 @@
+// PR c++/94483
+// { dg-do compile { target c++2a } }
+
+template<int... a> constexpr auto x1
+ = [...z = -a] (auto F) { return F(z...); };
+
+template<const int&... a> constexpr auto x2
+ = [&...z = a] (auto F) { return F(z...); };
+
+template<int... a> constexpr auto x3
+ = [z = -a] (auto F) { return F(z); }; // { dg-error "packs not expanded" }
+
+
+constexpr auto sum = [] (auto... xs) { return (xs + ... + 0); };
+const int y1 = 1, y2 = 2, y3 = 3;
+
+static_assert(x1<1,2,3>(sum) == -6);
+static_assert(x2<y1,y2,y3>(sum) == 6);
