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

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).

	PR c++/101715

gcc/cp/ChangeLog:

	* tree.cc (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.