c++: ICE with noexcept and canonical types [PR101715]

  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).

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


base-commit: 952b7dbb418198f86d7829aaf9d7f9fc7714a8b3
  

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;
  

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?

> 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);
>   	  }
>   	else
>   	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> new file mode 100644
> index 00000000000..f1455b3b46b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> @@ -0,0 +1,21 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +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
> +
> +template <typename T> struct S2 {
> +  S2<T> bar1() noexcept(T::value);
> +  S2<T> bar2() noexcept(T::value);
> +  S2<T> bar3() noexcept(T::value);
> +  S2<T> bar4() noexcept(T::value);
> +  S2<T> bar5() noexcept(T::value);
> +  S2<T> baz() noexcept(T::value2);
> +  S2<T> foo() noexcept(T::value);
> +};
> +
> +template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> new file mode 100644
> index 00000000000..24524f3592a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> @@ -0,0 +1,13 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S { };
> +
> +template<typename T>
> +struct A
> +{
> +    A& foo(A&&) noexcept((S<T>::value));
> +    A& assign(A&&) noexcept((S<T>::value));
> +};
> +template<typename T>
> +A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}
> 
> base-commit: 952b7dbb418198f86d7829aaf9d7f9fc7714a8b3
  

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)?

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);
> >   	  }
> >   	else
> >   	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> > new file mode 100644
> > index 00000000000..f1455b3b46b
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> > @@ -0,0 +1,21 @@
> > +// PR c++/101715
> > +// { dg-do compile { target c++11 } }
> > +
> > +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
> > +
> > +template <typename T> struct S2 {
> > +  S2<T> bar1() noexcept(T::value);
> > +  S2<T> bar2() noexcept(T::value);
> > +  S2<T> bar3() noexcept(T::value);
> > +  S2<T> bar4() noexcept(T::value);
> > +  S2<T> bar5() noexcept(T::value);
> > +  S2<T> baz() noexcept(T::value2);
> > +  S2<T> foo() noexcept(T::value);
> > +};
> > +
> > +template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> > new file mode 100644
> > index 00000000000..24524f3592a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> > @@ -0,0 +1,13 @@
> > +// PR c++/101715
> > +// { dg-do compile { target c++11 } }
> > +
> > +template <typename T> struct S { };
> > +
> > +template<typename T>
> > +struct A
> > +{
> > +    A& foo(A&&) noexcept((S<T>::value));
> > +    A& assign(A&&) noexcept((S<T>::value));
> > +};
> > +template<typename T>
> > +A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}
> > 
> > base-commit: 952b7dbb418198f86d7829aaf9d7f9fc7714a8b3
> 

Marek
  

On Sat, Jan 15, 2022 at 09:24:05AM -0500, Patrick Palka wrote:
> 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;

Thanks for looking.  I can dig that (and verified it works), but it strikes
me more as a workaround for the duplicity problem.  I also don't see
TYPE_CANONICAL (...) = TYPE_CANONICAL (build_cp_fntype_variant (...))
anywhere in the codebase, if that means anything.

Marek
  

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