c++/modules: Handle mismatching TYPE_CANONICAL when deduping partial specs [PR118101]

In r15-4862 we ensured that merging a partial specialisation would
properly update its TYPE_CANONICAL.  However, this confuses the deduping
mechanism, since the canonical type has updated out from under it,
causing is_matching_decl to crash when seeing the equivalent types with
different TYPE_CANONICAL.

This patch solves the issue by forcing structural equality checking for
this case; this way mismatching TYPE_CANONICAL doesn't cause issues, but
we still can handle the case that the types are legitimately different.

	PR c++/118101

gcc/cp/ChangeLog:

	* module.cc (trees_in::decl_value): Use structural equality when
	deduping partial specs with mismatching canonical types.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/partial-7.h: New test.
	* g++.dg/modules/partial-7_a.C: New test.
	* g++.dg/modules/partial-7_b.C: New test.
	* g++.dg/modules/partial-7_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

5 files changed, 39 insertions, 1 deletions

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 6fe64bb..813c143 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -8650,9 +8650,18 @@ trees_in::decl_value ()
 	  if (stub_decl)
 	    TREE_TYPE (stub_decl) = type;
 
+	  tree etype = TREE_TYPE (existing);
+
 	  /* Handle separate declarations with different attributes.  */
-	  tree &eattr = TYPE_ATTRIBUTES (TREE_TYPE (existing));
+	  tree &eattr = TYPE_ATTRIBUTES (etype);
 	  eattr = merge_attributes (eattr, TYPE_ATTRIBUTES (type));
+
+	  /* When merging a partial specialisation, the existing decl may have
+	     had its TYPE_CANONICAL adjusted.  If so we should use structural
+	     equality to ensure is_matching_decl doesn't get confused.  */
+	  if ((spec_flags & 2)
+	      && TYPE_CANONICAL (type) != TYPE_CANONICAL (etype))
+	    SET_TYPE_STRUCTURAL_EQUALITY (type);
 	}
 
       if (inner_tag)
diff --git a/gcc/testsuite/g++.dg/modules/partial-7.h b/gcc/testsuite/g++.dg/modules/partial-7.h
new file mode 100644
index 0000000..cf04118
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/partial-7.h
@@ -0,0 +1,5 @@
+// PR c++/118101
+template <typename> struct A;
+template <typename T> struct A<T*> {};
+template <typename T> struct B { A<T*> f(); };
+B<int> inst();
diff --git a/gcc/testsuite/g++.dg/modules/partial-7_a.C b/gcc/testsuite/g++.dg/modules/partial-7_a.C
new file mode 100644
index 0000000..6aa7aee
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/partial-7_a.C
@@ -0,0 +1,8 @@
+// PR c++/118101
+// { dg-additional-options "-fmodules" }
+// { dg-module-cmi A }
+
+module;
+#include "partial-7.h"
+export module A;
+B<int> a;
diff --git a/gcc/testsuite/g++.dg/modules/partial-7_b.C b/gcc/testsuite/g++.dg/modules/partial-7_b.C
new file mode 100644
index 0000000..1ca391a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/partial-7_b.C
@@ -0,0 +1,9 @@
+// PR c++/118101
+// { dg-additional-options "-fmodules" }
+// { dg-module-cmi B }
+
+module;
+#include "partial-7.h"
+export module B;
+import A;
+B<int> b;
diff --git a/gcc/testsuite/g++.dg/modules/partial-7_c.C b/gcc/testsuite/g++.dg/modules/partial-7_c.C
new file mode 100644
index 0000000..3d2a287
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/partial-7_c.C
@@ -0,0 +1,7 @@
+// PR c++/118101
+// { dg-additional-options "-fmodules" }
+
+template <typename> struct A {};
+template <typename T> struct B { A<T*> f(); };
+template struct B<int>;
+import B;