On Fri, 14 Jan 2022, Marek Polacek via Gcc-patches 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.
>
> 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).
I wonder about instead making build_cp_fntype_variant set the TYPE_CANONICAL for
#3 to TYPE_CANONICAL(#2) (i.e. #1) instead of to #2? Something like:
-- >8 --
gcc/cp/tree.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 7f7de86b4e8..b89135fa121 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -2779,8 +2779,9 @@ build_cp_fntype_variant (tree type, cp_ref_qualifier
rqual,
else if (TYPE_CANONICAL (type) != type || cr != raises || late)
/* Build the underlying canonical type, since it is different
from TYPE. */
- TYPE_CANONICAL (v) = build_cp_fntype_variant (TYPE_CANONICAL (type),
- rqual, cr, false);
+ TYPE_CANONICAL (v)
+ = TYPE_CANONICAL (build_cp_fntype_variant (TYPE_CANONICAL (type),
+ rqual, cr, false));
else
/* T is its own canonical type. */
TYPE_CANONICAL (v) = v;
