c++: Fix mismatch in template argument deduction [PR90505]

My GCC 9 patch for C++20 P0846R0 (ADL and function templates) tweaked
cp_parser_template_name to only return an identifier if name lookup
didn't find anything.  In the deduce4.C case it means that we now
return an OVERLOAD.  That means that cp_parser_template_id will call
lookup_template_function whereby producing a TEMPLATE_ID_EXPR with
unknown_type_node.  Previously, we created a TEMPLATE_ID_EXPR with
no type, making it type-dependent.  What we have now is no longer
type-dependent.  And so, when we call finish_call_expr after we've
parsed "foo<int>(10)", even though we're in a template, we still do
the normal processing, thus perform overload resolution.  When adding
the template candidate foo we need to deduce the template arguments,
and that is where things go downhill.

When fn_type_unification sees that we have explicit template arguments,
but they aren't complete, it will use them to substitute the function
type.  So we substitute e.g. "void <T33d> (U)".  But the explicit
template argument was for a different parameter so we don't actually
substitute anything.  But the problem here was that we reduced the
template level of 'U' anyway.  So then when we're actually deducing
the template arguments via type_unification_real, we fail in unify:
22932       if (TEMPLATE_TYPE_LEVEL (parm)
22933           != template_decl_level (tparm))
22934         /* The PARM is not one we're trying to unify.  Just check
22935            to see if it matches ARG.  */
because 'parm' has been reduced but 'tparm' has not yet.

Therefore we shouldn't reduce the template level of template parameters
when tf_partial aka template argument deduction substitution.  But we
can only return after performing the cp_build_qualified_type etc.
business otherwise things break horribly.

2020-03-03  Jason Merrill  <jason@redhat.com>
	    Marek Polacek  <polacek@redhat.com>

	PR c++/90505 - mismatch in template argument deduction.
	* pt.c (tsubst): Don't reduce the template level of template
	parameters when tf_partial.

	* g++.dg/template/deduce4.C: New test.
	* g++.dg/template/deduce5.C: New test.
	* g++.dg/template/deduce6.C: New test.
	* g++.dg/template/deduce7.C: New test.

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 7c22aeaaedc75a3d9dcf63e0636fe6b566875589..590c898866632aea3614ea0e843b76c3ec1e03aa 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2020-03-03  Jason Merrill  <jason@redhat.com>
+           Marek Polacek  <polacek@redhat.com>
+
+       PR c++/90505 - mismatch in template argument deduction.
+       * pt.c (tsubst): Don't reduce the template level of template
+       parameters when tf_partial.
+
 2020-03-03  Jakub Jelinek  <jakub@redhat.com>
 
        PR c++/93998

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 230331f60cbfac09e8cf94d5cf7f696b843ffa5b..1c721b311764a312068d996315d738d6cfa3308f 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -15057,12 +15057,6 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
        int levels;
        tree arg = NULL_TREE;
 
-       /* Early in template argument deduction substitution, we don't
-          want to reduce the level of 'auto', or it will be confused
-          with a normal template parm in subsequent deduction.  */
-       if (is_auto (t) && (complain & tf_partial))
-         return t;
-
        r = NULL_TREE;
 
        gcc_assert (TREE_VEC_LENGTH (args) > 0);
@@ -15193,6 +15187,14 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
             about the template parameter in question.  */
          return t;
 
+       /* Early in template argument deduction substitution, we don't
+          want to reduce the level of 'auto', or it will be confused
+          with a normal template parm in subsequent deduction.
+          Similarly, don't reduce the level of template parameters to
+          avoid mismatches when deducing their types.  */
+       if (complain & tf_partial)
+         return t;
+
        /* If we get here, we must have been looking at a parm for a
           more deeply nested template.  Make a new version of this
           template parameter, but with a lower level.  */

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index fdc7768f93ecf1c61b38c8a459452453592e1aed..5e8a8356a4237f6e4495b492abccfafd6ddbf9fe 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,11 @@
+2020-03-03  Marek Polacek  <polacek@redhat.com>
+
+       PR c++/90505 - mismatch in template argument deduction.
+       * g++.dg/template/deduce4.C: New test.
+       * g++.dg/template/deduce5.C: New test.
+       * g++.dg/template/deduce6.C: New test.
+       * g++.dg/template/deduce7.C: New test.
+
 2020-03-03  Jakub Jelinek  <jakub@redhat.com>
 
        PR c++/93998

diff --git a/gcc/testsuite/g++.dg/template/deduce4.C b/gcc/testsuite/g++.dg/template/deduce4.C
new file mode 100644 (file)
index 0000000..e2c165d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce4.C
@@ -0,0 +1,17 @@
+// PR c++/90505 - mismatch in template argument deduction.
+// { dg-do compile }
+
+template <typename T>
+struct S {
+  template <typename U, typename V>
+  static void foo(V) { }
+
+  void bar () { foo<int>(10); }
+};
+
+void
+test ()
+{
+  S<int> s;
+  s.bar ();
+}

diff --git a/gcc/testsuite/g++.dg/template/deduce5.C b/gcc/testsuite/g++.dg/template/deduce5.C
new file mode 100644 (file)
index 0000000..9d382bf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce5.C
@@ -0,0 +1,17 @@
+// PR c++/90505 - mismatch in template argument deduction.
+// { dg-do compile { target c++11 } }
+
+template <typename T>
+struct S {
+  template <typename U, typename V = void>
+  static void foo(U) { }
+
+  void bar () { foo<int>(10); }
+};
+
+void
+test ()
+{
+  S<int> s;
+  s.bar ();
+}

diff --git a/gcc/testsuite/g++.dg/template/deduce6.C b/gcc/testsuite/g++.dg/template/deduce6.C
new file mode 100644 (file)
index 0000000..8fee612
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce6.C
@@ -0,0 +1,17 @@
+// PR c++/90505 - mismatch in template argument deduction.
+// { dg-do compile { target c++11 } }
+
+template <typename T>
+struct S {
+  template <typename U = int, typename V>
+  static void foo(V) { }
+
+  void bar () { foo<>(10); }
+};
+
+void
+test ()
+{
+  S<int> s;
+  s.bar ();
+}

diff --git a/gcc/testsuite/g++.dg/template/deduce7.C b/gcc/testsuite/g++.dg/template/deduce7.C
new file mode 100644 (file)
index 0000000..fbc28e5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce7.C
@@ -0,0 +1,10 @@
+// PR c++/90505 - mismatch in template argument deduction.
+// { dg-do compile { target c++11 } }
+
+template <typename> class a {
+  using b = int;
+  using c = int;
+  b d;
+  void e() { g<c>(d); }
+  template <typename... f> static void g(f...);
+};