c++: explicit spec of constrained member tmpl [PR107522]

When defining a explicit specialization of a constrained member template
(of a class template) such as f and g in the below testcase, the
DECL_TEMPLATE_PARMS of the corresponding TEMPLATE_DECL are partially
instantiated, whereas its associated constraints are carried over
from the original template and thus are in terms of the original
DECL_TEMPLATE_PARMS.  So during normalization for such an explicit
specialization we need to consider the (parameters of) the most general
template, since that's what the constraints are in terms of and since we
always use the full set of template arguments during satisfaction.

	PR c++/107522

gcc/cp/ChangeLog:

	* constraint.cc (get_normalized_constraints_from_decl): Use the
	most general template for an explicit specialization of a
	member template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-explicit-spec7.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 62daa81308c6c187059fcad98377146e30725fa5)

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index cbf6357ad6816fc3027942746d25a65b4841f27f..215a6c1fa938acd02c000f62c3ba6e9154da052e 100644 (file)
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -970,11 +970,19 @@ get_normalized_constraints_from_decl (tree d, bool diag = false)
      accepting the latter causes the template parameter level of U
      to be reduced in a way that makes it overly difficult substitute
      concrete arguments (i.e., eventually {int, int} during satisfaction.  */
-  if (tmpl)
-  {
-    if (DECL_LANG_SPECIFIC(tmpl) && !DECL_TEMPLATE_SPECIALIZATION (tmpl))
-      tmpl = most_general_template (tmpl);
-  }
+  if (tmpl && DECL_LANG_SPECIFIC (tmpl)
+      && (!DECL_TEMPLATE_SPECIALIZATION (tmpl)
+         /* DECL_TEMPLATE_SPECIALIZATION means TMPL is either a partial
+            specialization, or an explicit specialization of a member
+            template.  In the former case all is well: TMPL's constraints
+            are in terms of its parameters.  But in the latter case TMPL's
+            parameters are partially instantiated whereas its constraints
+            aren't, so we need to instead use (the parameters of) the most
+            general template.  The following test distinguishes between a
+            partial specialization and such an explicit specialization.  */
+         || (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (tmpl))
+             < TMPL_ARGS_DEPTH (DECL_TI_ARGS (tmpl)))))
+    tmpl = most_general_template (tmpl);
 
   d = tmpl ? tmpl : decl;
 

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C b/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C
new file mode 100644 (file)
index 0000000..9452159
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C
@@ -0,0 +1,30 @@
+// PR c++/107522
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct A {
+  template<int N>
+  static void f() requires (N == 42);
+
+  template<class U>
+  struct B {
+    template<int N>
+    static void g() requires (T(N) == 42);
+  };
+};
+
+template<>
+template<int N>
+void A<int>::f() requires (N == 42) { }
+
+template<>
+template<>
+template<int N>
+void A<int>::B<int>::g() requires (int(N) == 42) { }
+
+int main() {
+  A<int>::f<42>();
+  A<int>::f<43>(); // { dg-error "no match" }
+  A<int>::B<int>::g<42>();
+  A<int>::B<int>::g<43>(); // { dg-error "no match" }
+}