[PATCH 2/2] c++: redeclaring member of constrained class template [PR96830]

[Patrick Palka <ppalka@redhat.com>]

When redeclaring a member of a constrained class template, we need to
repeat the class's constraints, but it turns out we don't verify
anywhere that the repeated constraints match the class's constraints.
A safe place to do so seems to be in push_template_decl, nearby a
similar consistency check for the template parameter list length of a
redeclaration.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/12?

	PR c++/96830

gcc/cp/ChangeLog:

	* pt.cc (push_inline_template_parms_recursive): Set
	TEMPLATE_PARMS_CONSTRAINTS.
	(push_template_decl): For a redeclaration, verify constraints
	for each enclosing template scope match those of the original
	template declaratation.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-class5.C: New test.
	* g++.dg/cpp2a/concepts-class5a.C: New test.
---
 gcc/cp/pt.cc                                  | 26 +++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-class5.C  | 37 ++++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C | 38 +++++++++++++++++++
 3 files changed, 101 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class5.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 053cc52b285..370a4f70f08 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -471,6 +471,8 @@ push_inline_template_parms_recursive (tree parmlist, int levels)
   current_template_parms
     = tree_cons (size_int (current_template_depth + 1),
 		 parms, current_template_parms);
+  TEMPLATE_PARMS_CONSTRAINTS (current_template_parms)
+    = TEMPLATE_PARMS_CONSTRAINTS (parmlist);
   TEMPLATE_PARMS_FOR_INLINE (current_template_parms) = 1;
 
   begin_scope (TREE_VEC_LENGTH (parms) ? sk_template_parms : sk_template_spec,
@@ -6134,6 +6136,30 @@ push_template_decl (tree decl, bool is_friend)
 	  DECL_INTERFACE_KNOWN (decl) = 1;
 	  return error_mark_node;
 	}
+
+      /* Check that the constraints for each enclosing template scope are
+	 consistent with the original template declaration.  */
+      if (flag_concepts)
+	{
+	  tree decl_parms = DECL_TEMPLATE_PARMS (tmpl);
+	  tree scope_parms = current_template_parms;
+	  if (PRIMARY_TEMPLATE_P (tmpl))
+	    {
+	      decl_parms = TREE_CHAIN (decl_parms);
+	      scope_parms = TREE_CHAIN (scope_parms);
+	    }
+	  while (decl_parms)
+	    {
+	      if (!template_requirements_equivalent_p (decl_parms, scope_parms))
+		{
+		  error ("redeclaration of %qD with different constraints",
+			 TPARMS_PRIMARY_TEMPLATE (TREE_VALUE (decl_parms)));
+		  break;
+		}
+	      decl_parms = TREE_CHAIN (decl_parms);
+	      scope_parms = TREE_CHAIN (scope_parms);
+	    }
+	}
     }
 
   gcc_checking_assert (!tmpl || DECL_TEMPLATE_RESULT (tmpl) == decl);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class5.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class5.C
new file mode 100644
index 00000000000..5d893d9f6a7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class5.C
@@ -0,0 +1,37 @@
+// PR c++/96830
+// { dg-do compile { target c++20 } }
+
+template<class T> concept C = true;
+
+template<C T> requires true
+struct A {
+  void f();
+  template<class U> void g();
+
+  struct B;
+  template<class U> struct C;
+
+  static int D;
+  template<class U> static int E;
+};
+
+template<C T>
+void A<T>::f() { }  // { dg-error "different constraints" }
+
+template<class T> requires true
+template<class U>
+void A<T>::g() { }  // { dg-error "different constraints" }
+
+template<C T> requires (!!true)
+struct A<T>::B { }; // { dg-error "different constraints" }
+
+template<class T> requires true
+template<class U>
+struct A<T>::C { }; // { dg-error "different constraints" }
+
+template<C T>
+int A<T>::D = 0;    // { dg-error "different constraints" }
+
+template<class T> requires true
+template<class U>
+int A<T>::E = 0;    // { dg-error "different constraints" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C
new file mode 100644
index 00000000000..a6a3684c258
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C
@@ -0,0 +1,38 @@
+// PR c++/96830
+// A valid version of concepts-class5.C.
+// { dg-do compile { target c++20 } }
+
+template<class T> concept C = true;
+
+template<C T> requires true
+struct A {
+  void f();
+  template<class U> void g();
+
+  struct B;
+  template<class U> struct C;
+
+  static int D;
+  template<class U> static int E;
+};
+
+template<C T> requires true
+void A<T>::f() { }
+
+template<C T> requires true
+template<class U>
+void A<T>::g() { }
+
+template<C T> requires true
+struct A<T>::B { };
+
+template<C T> requires true
+template<class U>
+struct A<T>::C { };
+
+template<C T> requires true
+int A<T>::D = 0;
+
+template<C T> requires true
+template<class U>
+int A<T>::E = 0;
-- 
2.40.0