c++: Diagnose invalid type of bitfield widths in templates [PR125674]

As the first testcase shows, outside of templates or when
the bitfield width is not type dependent, we diagnose it
in grokbitfield:
  if (width != error_mark_node)
    {
      /* The width must be an integer type.  */
      if (!type_dependent_expression_p (width)
          && !INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (TREE_TYPE (width)))
        error ("width of bit-field %qD has non-integral type %qT", value,
               TREE_TYPE (width));
      else if (!check_for_bare_parameter_packs (width))
        {
          /* Temporarily stash the width in DECL_BIT_FIELD_REPRESENTATIVE.
             check_bitfield_decl picks it from there later and sets DECL_SIZE
             accordingly.  */
          DECL_BIT_FIELD_REPRESENTATIVE (value) = width;
          SET_DECL_C_BIT_FIELD (value);
        }
    }
Later on in check_bitfield_decl we verify it is a constant expression,
folded into INTEGER_CST, non-negative etc.
But during instantiation, we don't repeat that check, so only call
check_bitfield_decl later on which can sometimes emit different diagnostics
(so e.g.
bit-field ‘D<1.0e+0>::d’ width not an integer constant
instead of
width of bit-field ‘D<N>::d’ has non-integral type ‘double’
) but what the second testcase shows, we can ICE during cxx_constant_value
even before that if the type is even more problematic.

The following patch fixes that by repeating the test from grokbitfield
during tsubst_decl.

2026-06-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/125674
	* pt.cc (tsubst_decl): Diagnose bit-field widths
	with invalid type.

	* g++.dg/template/bitfield5.C: New test.
	* g++.dg/template/bitfield6.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

Author: jakubjelinek

gcc/cp/pt.cc

@@ -16058,11 +16058,26 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain,
 	    cp_apply_type_quals_to_decl (cp_type_quals (type), r);
 
 	    if (DECL_C_BIT_FIELD (r))
-	      /* For bit-fields, DECL_BIT_FIELD_REPRESENTATIVE gives the
-		 number of bits.  */
-	      DECL_BIT_FIELD_REPRESENTATIVE (r)
-		= tsubst_expr (DECL_BIT_FIELD_REPRESENTATIVE (t), args,
-			       complain, in_decl);
+	      {
+		/* For bit-fields, DECL_BIT_FIELD_REPRESENTATIVE gives the
+		   number of bits.  */
+		tree width
+		  = tsubst_expr (DECL_BIT_FIELD_REPRESENTATIVE (t), args,
+				 complain, in_decl);
+		if (width
+		    && width != error_mark_node
+		    && !type_dependent_expression_p (width)
+		    && !INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P
+			 (TREE_TYPE (width)))
+		  {
+		    if (complain & tf_error)
+		      error_at (DECL_SOURCE_LOCATION (t),
+		      		"width of bit-field %qD has non-integral "
+				"type %qT", r, TREE_TYPE (width));
+		    RETURN (error_mark_node);
+		  }
+		DECL_BIT_FIELD_REPRESENTATIVE (r) = width;
+	      }
 	    if (DECL_INITIAL (t))
 	      {
 		/* Set up DECL_TEMPLATE_INFO so that we can get at the

gcc/testsuite/g++.dg/template/bitfield5.C

@@ -0,0 +1,16 @@
+// PR c++/125674
+// { dg-do compile { target c++20 } }
+
+struct A { int a : 1.; };	// { dg-error "width of bit-field 'a' has non-integral type 'double'" }
+template <typename T>
+struct B { int b : 1.; };	// { dg-error "width of bit-field 'b' has non-integral type 'double'" }
+template <typename T>
+struct C { T c : 1.; };		// { dg-error "width of bit-field 'c' has non-integral type 'double'" }
+template <auto N>
+struct D { int d : N; };	// { dg-error "width of bit-field 'D<N>::d' has non-integral type 'double'" }
+template <typename T, auto N>
+struct E { T e : N; };		// { dg-error "width of bit-field 'E<T, N>::e' has non-integral type 'double'" }
+B <int> b;
+C <int> c;
+D <1.> d;
+E <int, 1.> e;

gcc/testsuite/g++.dg/template/bitfield6.C

@@ -0,0 +1,9 @@
+// PR c++/125674
+// { dg-do compile }
+
+template <typename T>
+struct A {
+  int f ();
+  int i : f;	// { dg-error "width of bit-field 'A<T>::i' has non-integral type '<unresolved overloaded function type>'" }
+};
+A<int> a;