c++: Improve diagnostic for class tmpl/class redecl [PR103749]

For code like

  template<typename>
  struct bar;

  struct bar {
    int baz;
  };

  bar var;

we emit a fairly misleading and unwieldy diagnostic:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$ g++ -c u.cc
u.cc:6:8: error: template argument required for 'struct bar'
    6 | struct bar {
      |        ^~~
u.cc:10:5: error: class template argument deduction failed:
   10 | bar var;
      |     ^~~
u.cc:10:5: error: no matching function for call to 'bar()'
u.cc:3:17: note: candidate: 'template<class> bar()-> bar< <template-parameter-1-1> >'
    3 |   friend struct bar;
      |                 ^~~
u.cc:3:17: note:   template argument deduction/substitution failed:
u.cc:10:5: note:   couldn't deduce template parameter '<template-parameter-1-1>'
   10 | bar var;
      |     ^~~
u.cc:3:17: note: candidate: 'template<class> bar(bar< <template-parameter-1-1> >)-> bar< <template-parameter-1-1> >'
    3 |   friend struct bar;
      |                 ^~~
u.cc:3:17: note:   template argument deduction/substitution failed:
u.cc:10:5: note:   candidate expects 1 argument, 0 provided
   10 | bar var;
      |     ^~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

but with this patch we get:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
z.C:4:10: error: class template 'bar' redeclared as non-template
    4 |   struct bar {
      |          ^~~
z.C:2:10: note: previous declaration here
    2 |   struct bar;
      |          ^~~
z.C:8:7: error: 'bar<...auto...> var' has incomplete type
    8 |   bar var;
      |       ^~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

which is clearer about what the problem is.

I thought it'd be nice to avoid printing the messages about failed CTAD,
too.  To that end, I'm using CLASSTYPE_ERRONEOUS to suppress CTAD.  Not
sure if that's entirely kosher.

The other direction (first a non-template class declaration followed by
a class template definition) we handle quite well:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
z.C:11:8: error: 'bar' is not a template
   11 | struct bar {};
      |        ^~~
z.C:8:8: note: previous declaration here
    8 | struct bar;
      |        ^~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

	PR c++/103749

gcc/cp/ChangeLog:

	* decl.c (lookup_and_check_tag): Give an error when a class was
	declared as template but no template header has been provided.
	* pt.c (do_class_deduction): Don't deduce CLASSTYPE_ERRONEOUS
	types.

gcc/testsuite/ChangeLog:

	* g++.dg/template/redecl4.C: Adjust dg-error.
	* g++.dg/diagnostic/redeclaration-2.C: New test.

gcc/cp/decl.c

@@ -15438,6 +15438,16 @@ lookup_and_check_tag (enum tag_types tag_code, tree name,
       return error_mark_node;
     }
 
+  if (DECL_CLASS_TEMPLATE_P (decl)
+      && !template_header_p
+      && how == TAG_how::CURRENT_ONLY)
+    {
+      error ("class template %qD redeclared as non-template", name);
+      inform (location_of (decl), "previous declaration here");
+      CLASSTYPE_ERRONEOUS (TREE_TYPE (decl)) = true;
+      return error_mark_node;
+    }
+
   if (DECL_CLASS_TEMPLATE_P (decl)
       /* If scope is TAG_how::CURRENT_ONLY we're defining a class,
 	 so ignore a template template parameter.  */

gcc/cp/pt.c

@@ -29601,6 +29601,13 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
   if (DECL_TEMPLATE_TEMPLATE_PARM_P (tmpl))
     return ptype;
 
+  /* If the class was erroneous, don't try to deduce, because that
+     can generate a lot of diagnostic.  */
+  if (TREE_TYPE (tmpl)
+      && TYPE_LANG_SPECIFIC (TREE_TYPE (tmpl))
+      && CLASSTYPE_ERRONEOUS (TREE_TYPE (tmpl)))
+    return ptype;
+
   /* Wait until the enclosing scope is non-dependent.  */
   if (DECL_CLASS_SCOPE_P (tmpl)
       && dependent_type_p (DECL_CONTEXT (tmpl)))

gcc/testsuite/g++.dg/diagnostic/redeclaration-2.C

@@ -0,0 +1,19 @@
+// PR c++/103749
+
+struct foo {
+  template<typename>
+  friend struct bar;
+};
+
+struct bar { // { dg-error "redeclared as non-template" }
+  int baz;
+};
+
+template<typename>
+struct T; // { dg-message "previous" }
+
+struct T { // { dg-error "redeclared as non-template" }
+};
+
+bar var; // { dg-error "" }
+T t; // { dg-error "" }

gcc/testsuite/g++.dg/template/redecl4.C

@@ -2,4 +2,4 @@
 // { dg-do compile }
 
 template<int> union A;  // { dg-message "previous" }
-struct A;               // { dg-error "non-template" }
+struct A;               // { dg-error "redeclared as non-template" }