On 9/7/22 15:41, Patrick Palka wrote:
Here the use of the constexpr member/variable specialization 'value'
from within an unevaluated context causes us to overeagerly instantiate
it, via maybe_instantiate_decl called from mark_used, despite only its
declaration not its definition being needed.
If the issue is with unevaluated context, maybe maybe_instantiate_decl
should guard the call to decl_maybe_constant_var_p with
!cp_unevaluated_operand?
We used to have the same issue for constexpr function specializations
until r6-1309-g81371eff9bc7ef made us delay their instantiation until
necessary during constexpr evaluation.
So this patch makes us avoid unnecessarily instantiating constexpr
variable template specializations from mark_used as well. To that end
this patch pulls out the test in maybe_instantiate_decl
(decl_maybe_constant_var_p (decl)
|| (TREE_CODE (decl) == FUNCTION_DECL
&& DECL_OMP_DECLARE_REDUCTION_P (decl))
|| undeduced_auto_decl (decl))
into each of its three callers (including mark_used) and refines the
test appropriately. The net result is that only mark_used is changed,
because the other two callers, resolve_address_of_overloaded_function
and decl_constant_var_p, already guard the call appropriately. And
presumably decl_constant_var_p will take care of instantiation when
needed for e.g. constexpr evaluation.
Bootstrapped and regteste on x86_64-pc-linux-gnu, does this look OK for
trunk?
PR c++/99130
gcc/cp/ChangeLog:
* decl2.cc (maybe_instantiate_decl): Adjust function comment.
Check VAR_OR_FUNCTION_DECL_P. Pull out the disjunction into ...
(mark_used): ... here, removing the decl_maybe_constant_var_p
part of it.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/var-templ70.C: New test.
---
gcc/cp/decl2.cc | 33 ++++++++----------------
gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 19 ++++++++++++++
2 files changed, 30 insertions(+), 22 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 89ab2545d64..cd188813bee 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -5381,24 +5381,15 @@ possibly_inlined_p (tree decl)
return true;
}
-/* Normally, we can wait until instantiation-time to synthesize DECL.
- However, if DECL is a static data member initialized with a constant
- or a constexpr function, we need it right now because a reference to
- such a data member or a call to such function is not value-dependent.
- For a function that uses auto in the return type, we need to instantiate
- it to find out its type. For OpenMP user defined reductions, we need
- them instantiated for reduction clauses which inline them by hand
- directly. */
+/* If DECL is a function or variable template specialization, instantiate
+ its definition now. */
void
maybe_instantiate_decl (tree decl)
{
- if (DECL_LANG_SPECIFIC (decl)
+ if (VAR_OR_FUNCTION_DECL_P (decl)
+ && DECL_LANG_SPECIFIC (decl)
&& DECL_TEMPLATE_INFO (decl)
- && (decl_maybe_constant_var_p (decl)
- || (TREE_CODE (decl) == FUNCTION_DECL
- && DECL_OMP_DECLARE_REDUCTION_P (decl))
- || undeduced_auto_decl (decl))
&& !DECL_DECLARED_CONCEPT_P (decl)
&& !uses_template_parms (DECL_TI_ARGS (decl)))
{
@@ -5700,15 +5691,13 @@ mark_used (tree decl, tsubst_flags_t complain)
return false;
}
- /* Normally, we can wait until instantiation-time to synthesize DECL.
- However, if DECL is a static data member initialized with a constant
- or a constexpr function, we need it right now because a reference to
- such a data member or a call to such function is not value-dependent.
- For a function that uses auto in the return type, we need to instantiate
- it to find out its type. For OpenMP user defined reductions, we need
- them instantiated for reduction clauses which inline them by hand
- directly. */
- maybe_instantiate_decl (decl);
+ /* If DECL has a deduced return type, we need to instantiate it now to
+ find out its type. For OpenMP user defined reductions, we need them
+ instantiated for reduction clauses which inline them by hand directly. */
+ if (undeduced_auto_decl (decl)
+ || (TREE_CODE (decl) == FUNCTION_DECL
+ && DECL_OMP_DECLARE_REDUCTION_P (decl)))
+ maybe_instantiate_decl (decl);
if (processing_template_decl || in_template_function ())
return true;
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
new file mode 100644
index 00000000000..80965657c32
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
@@ -0,0 +1,19 @@
+// PR c++/99130
+// { dg-do compile { target c++14 } }
+
+template<class T>
+struct A {
+ static constexpr int value = T::value;
+};
+
+struct B {
+ template<class T>
+ static constexpr int value = T::value;
+};
+
+template<class T>
+constexpr int value = T::value;
+
+using ty1 = decltype(A<int>::value);
+using ty2 = decltype(B::value<int>);
+using ty3 = decltype(value<int>);
