c++: spec_hasher::equal and PARM_DECLs [PR94632]

In the testcase below, during specialization of c<int>::d, we build two
identical specializations of the parameter type b<decltype(e)::k> -- one when
substituting into c<int>::d's TYPE_ARG_TYPES and another when substituting into
c<int>::d's DECL_ARGUMENTS.

We don't reuse the first specialization the second time around as a consequence
of the fix for PR c++/56247 which made PARM_DECLs always compare different from
one another during spec_hasher::equal.  As a result, when looking up existing
specializations of 'b', spec_hasher::equal considers the template argument
decltype(e')::k to be different from decltype(e'')::k, where e' and e'' are the
result of two calls to tsubst_copy on the PARM_DECL e.

Since the two specializations are considered different due to the mentioned fix,
their TYPE_CANONICAL points to themselves even though they are otherwise
identical types, and this triggers an ICE in maybe_rebuild_function_decl_type
when comparing the TYPE_ARG_TYPES of c<int>::d to its DECL_ARGUMENTS.

This patch fixes this issue at the spec_hasher::equal level by ignoring the
'comparing_specializations' flag in cp_tree_equal whenever the DECL_CONTEXTs of
the two parameters are identical.  This seems to be a sufficient condition to be
able to correctly compare PARM_DECLs structurally.  (This also subsumes the
CONSTRAINT_VAR_P check since constraint variables all have empty, and therefore
identical, DECL_CONTEXTs.)

gcc/cp/ChangeLog:

	PR c++/94632
	* tree.c (cp_tree_equal) <case PARM_DECL>: Ignore
	comparing_specializations if the parameters' contexts are identical.

gcc/testsuite/ChangeLog:

	PR c++/94632
	* g++.dg/template/canon-type-14.C: New test.

gcc/cp/ChangeLog

 2020-04-18  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94632
+	* tree.c (cp_tree_equal) <case PARM_DECL>: Ignore
+	comparing_specializations if the parameters' contexts are identical.
+
 	PR c++/92187
 	* pt.c (splice_late_return_type): Propagate cv-qualifiers and
 	PLACEHOLDER_TYPE_CONSTRAINTS from the original auto node to the new one.

gcc/cp/tree.c

@@ -3723,11 +3723,12 @@ cp_tree_equal (tree t1, tree t2)
 	 up for expressions that involve 'this' in a member function
 	 template.  */
 
-      if (comparing_specializations && !CONSTRAINT_VAR_P (t1))
+      if (comparing_specializations
+	  && DECL_CONTEXT (t1) != DECL_CONTEXT (t2))
 	/* When comparing hash table entries, only an exact match is
 	   good enough; we don't want to replace 'this' with the
 	   version from another function.  But be more flexible
-	   with local parameters in a requires-expression.  */
+	   with parameters with identical contexts.  */
 	return false;
 
       if (same_type_p (TREE_TYPE (t1), TREE_TYPE (t2)))

gcc/testsuite/ChangeLog

 2020-04-18  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94632
+	* g++.dg/template/canon-type-14.C: New test.
+
 	PR c++/92187
 	* g++.dg/concepts/abbrev5.C: New test.
 	* g++.dg/concepts/abbrev6.C: New test.

gcc/testsuite/g++.dg/template/canon-type-14.C

+// PR c++/94632
+// { dg-do compile { target c++11 } }
+
+template <bool> struct b;
+template <typename> class c {
+  template <typename f> static void d(f e, b<decltype(e)::k>);
+};
+template class c<int>;