On 1/18/22 11:05, Marek Polacek wrote:
On Mon, Jan 17, 2022 at 01:48:48PM -0500, Jason Merrill wrote:
On 1/14/22 19:22, Marek Polacek wrote:
This is a "canonical types differ for identical types" ICE, which started
with r11-4682. It's a bit tricky to explain. Consider:
template <typename T> struct S {
S<T> bar() noexcept(T::value); // #1
S<T> foo() noexcept(T::value); // #2
};
template <typename T> S<T> S<T>::foo() noexcept(T::value) {} // #3
We ICE because #3 and #2 have the same type, but their canonical types
differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
The member functions #1 and #2 have the same type. However, since their
noexcept-specifier is deferred, when parsing them, we create a variant for
both of them, because DEFERRED_PARSE cannot be compared. In other words,
build_cp_fntype_variant's
tree v = TYPE_MAIN_VARIANT (type);
for (; v; v = TYPE_NEXT_VARIANT (v))
if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
return v;
will *not* find an existing variant when creating a method_type for #2, so we
have to create a new one.
But then we perform delayed parsing and call fixup_deferred_exception_variants
for #1 and #2. f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
parsed noexcept-specifier. It also sets TYPE_CANONICAL (#2) to #1. Both
noexcepts turned out to be the same, so now we have two equivalent variants in
the list! I.e.,
+-----------------+ +-----------------+ +-----------------+
| main | | #2 | | #1 |
| S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
| - | | noex(T::value) | | noex(T::value) |
+-----------------+ +-----------------+ +-----------------+
Then we get to #3. As for #1 and #2, grokdeclarator calls build_memfn_type,
which ends up calling build_cp_fntype_variant, which will use the loop
above to look for an existing variant. The first one that matches
cp_check_qualified_type will be used, so we use #2 rather than #1, and the
TYPE_CANONICAL mismatch follows. Hopefully that makes sense.
Why doesn't the TYPE_CANONICAL (v) == v check prevent this?
In other words, I think you're asking: why did fixup_deferred_exception_variants
set TYPE_CANONICAL (#2) to #1 (which then differs from TYPE_CANONICAL (#3),
which is #2)?
I meant to ask why TYPE_CANONICAL (#3) got set to #2 instead of #1?
And to answer my own question, it's because the check I mention is in
fixup_deferred_exception_variants, and #3 doesn't go through there at
all; the loop in build_cp_fntype_variant assumes no duplicate variants,
which your patch fixes.
The method_type for #1 (I'll mark is as #1 here) is built with it being its own
canonical type.
The first call to fixup_deferred_exception_variants does not change it: in
there, VARIANT is #1, the loop with 'TYPE_CANONICAL (v) == v' cannot find
an existing variant that would match, so when we do
v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
rqual, cr, false);
we get #1 so
TYPE_CANONICAL (variant) = v;
is just
TYPE_CANONICAL (#1) = #1;
so no change.
The second call to fixup_deferred_exception_variants: here we're working with
VARIANT #2. Now we again scan the list of variants {main, #2, #1} where we
find a match for #2: #1. #1's TYPE_CANONICAL is #1 as per above, so we set
TYPE_CANONICAL (#2) = #1;
which I think is correct.
I think TYPE_CANONICAL (#3) should also be #1, not #2, which my patch attempts
to do.
Hope this explanation makes some sense, please ask away if it doesn't!
As for the fix, I didn't think I could rewrite the method_type #2 with #1
because the type may have escaped via decltype. So my approach is to
elide #2 from the list, so when looking for a matching variant, we always
find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
PR c++/101715
gcc/cp/ChangeLog:
* tree.c (fixup_deferred_exception_variants): Remove duplicate
variants after parsing the exception specifications.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/noexcept72.C: New test.
* g++.dg/cpp0x/noexcept73.C: New test.
---
gcc/cp/tree.c | 16 +++++++++++++++-
gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
3 files changed, 49 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 7f7de86b4e8..2efad49e7c1 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
/* Though sucky, this walk will process the canonical variants
first. */
+ tree prev = NULL_TREE;
for (tree variant = TYPE_MAIN_VARIANT (type);
- variant; variant = TYPE_NEXT_VARIANT (variant))
+ variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
if (TYPE_RAISES_EXCEPTIONS (variant) == original)
{
gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
@@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree
raises)
v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
rqual, cr, false);
TYPE_CANONICAL (variant) = v;
+
+ /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
+ of an existing variant in the variant list of TYPE after we
+ have parsed its exception specification, elide it. Otherwise,
+ build_cp_fntype_variant would use it, leading to "canonical
+ types differ for identical types." */
+ for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
+ if (v != variant
+ /* The main variant will not have TYPE_RAISES_EXCEPTIONS
+ so PREV should never be null. */
+ && cp_check_qualified_type (v, variant, var_quals,
+ rqual, cr, false))
+ TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
I think we don't two loops through the variants. It ought to work to
replace the existing loop with yours; if we find v, we prune and use its
TYPE_CANONICAL.
Jason
