c++: NTTP object wrapper substitution fixes [PR103346, ...]

This patch fixes some issues with substitution into a C++20 template
parameter object wrapper:

* The first testcase demonstrates a situation where the same_type_p
  assert in relevant case of tsubst_copy doesn't hold, because (partial)
  substitution of {int,} into the wrapper's TREE_TYPE yields A<int> but
  substitution into the underlying TEMPLATE_PARM_INDEX is a nop due to
  tsubst's level==1/tf_partial early exit tests, hence TREE_TYPE in the
  latter case remains A<T>.  So this patch just gets rid of the assert;
  the type mismatch doesn't seem to be a problem in practice.

* In the second testcase, dependent substitution into the underlying
  TEMPLATE_PARM_INDEX yields a CALL_EXPR with empty TREE_TYPE, which
  tsubst_copy doesn't expect.  This patch fixes this by handling empty
  TREE_TYPE the same way as a non-const TREE_TYPE.  Moreover, after
  this substitution we're left with a VIEW_CONVERT_EXPR wrapping a
  CALL_EXPR instead of a TEMPLATE_PARM_INDEX, which during the subsequent
  non-dependent substitution tsubst_copy doesn't expect either.  So
  this patch also relaxes tsubst_copy to accept such VIEW_CONVERT_EXPR
  too.

* In the third testcase, we end up never resolving the call to
  f.modify() because tsubst_copy doesn't do overload resolution.
  This patch fixes this by moving the handling of these
  VIEW_CONVERT_EXPR wrappers from tsubst_copy to tsubst_copy_and_build.
  For good measure tsubst_copy_and_build should also handle
  REF_PARENTHESIZED_P wrappers instead of delegating to tsubst_copy.

	PR c++/103346
	PR c++/104278
	PR c++/102553

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy) <case VIEW_CONVERT_EXPR>: Move the
	handling of C++20 template parameter object wrappers to ...
	(tsubst_copy_and_build) <case VIEW_CONVERT_EXPR>: ... here.
	Accept non-TEMPLATE_PARM_INDEX inner operand.  Handle empty
	TREE_TYPE on substituted inner operand.  Remove same_type_p
	assert.  Also handle REF_PARENTHESIZED_P VIEW_CONVERT_EXPRs.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class52a.C: New test.
	* g++.dg/cpp2a/nontype-class53.C: New test.
	* g++.dg/cpp2a/nontype-class54.C: New test.
	* g++.dg/cpp2a/nontype-class55.C: New test.

1 files changed, 39 insertions, 35 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 2b7b375..83da1ba 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -17338,42 +17338,16 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	      return maybe_wrap_with_location (op0, EXPR_LOCATION (t));
 	    }
 	  tree op = TREE_OPERAND (t, 0);
-	  if (code == VIEW_CONVERT_EXPR
-	      && TREE_CODE (op) == TEMPLATE_PARM_INDEX)
-	    {
-	      /* Wrapper to make a C++20 template parameter object const.  */
-	      op = tsubst_copy (op, args, complain, in_decl);
-	      if (!CP_TYPE_CONST_P (TREE_TYPE (op)))
-		{
-		  /* The template argument is not const, presumably because
-		     it is still dependent, and so not the const template parm
-		     object.  */
-		  tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
-		  gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
-				       (type, TREE_TYPE (op)));
-		  if (TREE_CODE (op) == CONSTRUCTOR
-		      || TREE_CODE (op) == IMPLICIT_CONV_EXPR)
-		    {
-		      /* Don't add a wrapper to these.  */
-		      op = copy_node (op);
-		      TREE_TYPE (op) = type;
-		    }
-		  else
-		    /* Do add a wrapper otherwise (in particular, if op is
-		       another TEMPLATE_PARM_INDEX).  */
-		    op = build1 (code, type, op);
-		}
-	      return op;
-	    }
 	  /* force_paren_expr can also create a VIEW_CONVERT_EXPR.  */
-	  else if (code == VIEW_CONVERT_EXPR && REF_PARENTHESIZED_P (t))
+	  if (code == VIEW_CONVERT_EXPR && REF_PARENTHESIZED_P (t))
 	    {
 	      op = tsubst_copy (op, args, complain, in_decl);
 	      op = build1 (code, TREE_TYPE (op), op);
 	      REF_PARENTHESIZED_P (op) = true;
 	      return op;
 	    }
-	  /* We shouldn't see any other uses of these in templates.  */
+	  /* We shouldn't see any other uses of these in templates
+	     (tsubst_copy_and_build handles C++20 tparm object wrappers).  */
 	  gcc_unreachable ();
 	}
 
@@ -21636,12 +21610,42 @@ tsubst_copy_and_build (tree t,
 
     case NON_LVALUE_EXPR:
     case VIEW_CONVERT_EXPR:
-      if (location_wrapper_p (t))
-	/* We need to do this here as well as in tsubst_copy so we get the
-	   other tsubst_copy_and_build semantics for a PARM_DECL operand.  */
-	RETURN (maybe_wrap_with_location (RECUR (TREE_OPERAND (t, 0)),
-					  EXPR_LOCATION (t)));
-      /* fallthrough.  */
+      {
+	tree op = RECUR (TREE_OPERAND (t, 0));
+
+	if (location_wrapper_p (t))
+	  /* We need to do this here as well as in tsubst_copy so we get the
+	     other tsubst_copy_and_build semantics for a PARM_DECL operand.  */
+	  RETURN (maybe_wrap_with_location (op, EXPR_LOCATION (t)));
+
+	gcc_checking_assert (TREE_CODE (t) == VIEW_CONVERT_EXPR);
+	if (REF_PARENTHESIZED_P (t))
+	  /* force_paren_expr can also create a VIEW_CONVERT_EXPR.  */
+	  RETURN (finish_parenthesized_expr (op));
+
+	/* Otherwise, we're dealing with a wrapper to make a C++20 template
+	   parameter object const.  */
+	if (TREE_TYPE (op) == NULL_TREE
+	    || !CP_TYPE_CONST_P (TREE_TYPE (op)))
+	  {
+	    /* The template argument is not const, presumably because
+	       it is still dependent, and so not the const template parm
+	       object.  */
+	    tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
+	    if (TREE_CODE (op) == CONSTRUCTOR
+		|| TREE_CODE (op) == IMPLICIT_CONV_EXPR)
+	      {
+		/* Don't add a wrapper to these.  */
+		op = copy_node (op);
+		TREE_TYPE (op) = type;
+	      }
+	    else
+	      /* Do add a wrapper otherwise (in particular, if op is
+		 another TEMPLATE_PARM_INDEX).  */
+	      op = build1 (VIEW_CONVERT_EXPR, type, op);
+	  }
+	RETURN (op);
+      }
 
     default:
       /* Handle Objective-C++ constructs, if appropriate.  */