c++: Fix class NTTP with template arguments [PR92948]

This PR points out an ICE with an alias template and class NTTP, but I
found that there are more issues.  Trouble arise when we use a
(non-type) template parameter as an argument to the template arg list of
a template that accepts a class NTTP and a conversion to a class is
involved, e.g.

  struct A { A(int) { } };
  template<A a> struct B { };

  template<int X> void fn () {
      B<X> b;
  }

Normally for such a conversion we create a TARGET_EXPR with an
AGGR_INIT_EXPR that calls a __ct_comp with some arguments, but not in
this case: when converting X to type A in convert_nontype_argument we
are in a template and the template parameter 'X' is value-dependent, and
AGGR_INIT_EXPR don't work in templates.  So it just creates a TARGET_EXPR
that contains "A::A(*this, X)", but with that overload resolution fails:
  error: no matching function for call to 'A::A(A*, int)'
That happens because finish_call_expr for a BASELINK creates a dummy
object, so we have 'this' twice.  I thought for the value-dependent case
we could use IMPLICIT_CONV_EXPR, as in the patch below.  Note that I
only do this when we convert to a different type than the type of the
expr.  The point is to avoid the call to a converting ctor.

The second issue was an ICE in tsubst_copy: when there's a conversion
like the above involved then
 /* Wrapper to make a C++20 template parameter object const.  */
  op = tsubst_copy (op, args, complain, in_decl);
might not produce a _ZT... VAR_DECL with const type, so the assert
  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op)));
fails.  Allowing IMPLICIT_CONV_EXPR there probably makes sense.

And since convert_nontype_argument uses value_dependent_expression_p
a lot, I used a dedicated bool to speed things up.

2020-01-29  Marek Polacek  <polacek@redhat.com>

	PR c++/92948 - Fix class NTTP with template arguments.
	* pt.c (convert_nontype_argument): Use IMPLICIT_CONV_EXPR when
	converting a value-dependent expression to a class type.
	(tsubst_copy) <case VIEW_CONVERT_EXPR>: Allow IMPLICIT_CONV_EXPR
	as the result of the tsubst_copy call.

	* g++.dg/cpp2a/nontype-class28.C: New test.
	* g++.dg/cpp2a/nontype-class29.C: New test.

gcc/cp/ChangeLog

+2020-01-29  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/92948 - Fix class NTTP with template arguments.
+	* pt.c (convert_nontype_argument): Use IMPLICIT_CONV_EXPR when
+	converting a value-dependent expression to a class type.
+	(tsubst_copy) <case VIEW_CONVERT_EXPR>: Allow IMPLICIT_CONV_EXPR
+	as the result of the tsubst_copy call.
+
 2020-01-29  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/91118

gcc/cp/pt.c

@@ -7074,7 +7074,8 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
   if (non_dep)
     expr = instantiate_non_dependent_expr_internal (expr, complain);
 
-  if (value_dependent_expression_p (expr))
+  const bool val_dep_p = value_dependent_expression_p (expr);
+  if (val_dep_p)
     expr = canonicalize_expr_argument (expr, complain);
 
   /* 14.3.2/5: The null pointer{,-to-member} conversion is applied
@@ -7100,10 +7101,17 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
 	     a conversion function with a value-dependent argument, which
 	     could invoke taking the address of a temporary representing
 	     the result of the conversion.  */
-	  if (COMPOUND_LITERAL_P (expr)
-	      && CONSTRUCTOR_IS_DEPENDENT (expr)
-	      && MAYBE_CLASS_TYPE_P (expr_type)
-	      && TYPE_HAS_CONVERSION (expr_type))
+	  if (!same_type_ignoring_top_level_qualifiers_p (type, expr_type)
+	      && ((COMPOUND_LITERAL_P (expr)
+		   && CONSTRUCTOR_IS_DEPENDENT (expr)
+		   && MAYBE_CLASS_TYPE_P (expr_type)
+		   && TYPE_HAS_CONVERSION (expr_type))
+		  /* Similarly, converting e.g. an integer to a class
+		     involves a constructor call.  convert_like would
+		     create a TARGET_EXPR, but in a template we can't
+		     use AGGR_INIT_EXPR, and the TARGET_EXPR would lead
+		     to a bogus error.  */
+		  || (val_dep_p && MAYBE_CLASS_TYPE_P (type))))
 	    {
 	      expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
 	      IMPLICIT_CONV_EXPR_NONTYPE_ARG (expr) = true;
@@ -7189,8 +7197,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
 
       /* Notice that there are constant expressions like '4 % 0' which
 	 do not fold into integer constants.  */
-      if (TREE_CODE (expr) != INTEGER_CST
-	  && !value_dependent_expression_p (expr))
+      if (TREE_CODE (expr) != INTEGER_CST && !val_dep_p)
 	{
 	  if (complain & tf_error)
 	    {
@@ -7265,7 +7272,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
 	Here, we do not care about functions, as they are invalid anyway
 	for a parameter of type pointer-to-object.  */
 
-      if (value_dependent_expression_p (expr))
+      if (val_dep_p)
 	/* Non-type template parameters are OK.  */
 	;
       else if (cxx_dialect >= cxx11 && integer_zerop (expr))
@@ -7333,8 +7340,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
 	    }
 	}
 
-      if (TYPE_REF_OBJ_P (TREE_TYPE (expr))
-	  && value_dependent_expression_p (expr))
+      if (TYPE_REF_OBJ_P (TREE_TYPE (expr)) && val_dep_p)
 	/* OK, dependent reference.  We don't want to ask whether a DECL is
 	   itself value-dependent, since what we want here is its address.  */;
       else
@@ -7412,7 +7418,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
 
       /* [temp.arg.nontype] bullet 1 says the pointer to member
          expression must be a pointer-to-member constant.  */
-      if (!value_dependent_expression_p (expr)
+      if (!val_dep_p
 	  && !check_valid_ptrmem_cst_expr (type, expr, complain))
 	return NULL_TREE;
 
@@ -7429,7 +7435,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
     {
       /* [temp.arg.nontype] bullet 1 says the pointer to member
          expression must be a pointer-to-member constant.  */
-      if (!value_dependent_expression_p (expr)
+      if (!val_dep_p
 	  && !check_valid_ptrmem_cst_expr (type, expr, complain))
 	return NULL_TREE;
 
@@ -7452,7 +7458,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
     {
       /* Replace the argument with a reference to the corresponding template
 	 parameter object.  */
-      if (!value_dependent_expression_p (expr))
+      if (!val_dep_p)
 	expr = get_template_parm_object (expr, complain);
       if (expr == error_mark_node)
 	return NULL_TREE;
@@ -16406,7 +16412,9 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 		}
 	      else
 		{
-		  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op)));
+		  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op))
+			      || (TREE_CODE (op) == IMPLICIT_CONV_EXPR
+				  && IMPLICIT_CONV_EXPR_NONTYPE_ARG (op)));
 		  return op;
 		}
 	    }

gcc/testsuite/ChangeLog

+2020-01-29  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/92948 - Fix class NTTP with template arguments.
+	* g++.dg/cpp2a/nontype-class28.C: New test.
+	* g++.dg/cpp2a/nontype-class29.C: New test.
+
 2020-01-29  Joel Hutton  <Joel.Hutton@arm.com>
 
 	PR target/93221

gcc/testsuite/g++.dg/cpp2a/nontype-class28.C

+// PR c++/92948 - Fix class NTTP with template arguments.
+// { dg-do compile { target c++2a } }
+
+struct A {
+  constexpr A(int) { }
+};
+
+template<A>
+struct B {
+  using U = unsigned;
+};
+
+template<A a>
+using U = B<a>;
+
+template<int X, typename Y = typename B<X>::U>
+void foo()
+{
+}
+
+template<int X, typename Y = typename U<X>::U>
+void foo2()
+{
+}
+
+template<typename Y = typename B<1>::U>
+void foo3()
+{
+}
+
+void
+g ()
+{
+  foo<1>();
+  foo2<1>();
+  foo3<>();
+}

gcc/testsuite/g++.dg/cpp2a/nontype-class29.C

+// PR c++/92948 - Fix class NTTP with template arguments.
+// { dg-do run { target c++2a } }
+
+struct A {
+  int i;
+  constexpr A(int n) : i(n) { }
+};
+
+template<A a>
+struct B {
+  static constexpr int i = a.i;
+};
+
+template<int X>
+void foo()
+{
+  B<X> b;
+  if (b.i != 42)
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  foo<42>();
+}