replace_typedefs: handle templates in namespaces

GDB currently crashes with infinite recursion, if you set a breakpoint
on a function inside a namespace that includes a template on its fully
qualified name, and, the template's name is also used as typedef in
the global scope that expands to a name that includes the template
name in its qualified name.  For example, from the testcase added by
this commit:

 namespace NS1 { namespace NS2 {

 template<typename T> struct Templ1
 {
   T x;

   Templ1 (object_p) {}
 }} // namespace NS1::NS2

 using Templ1 = NS1::NS2::Templ1<unsigned>;

Setting a breakpoint like so:

(gdb) break NS1::NS2::Templ1<int>::Templ1(NS1::NS2::object*)

Results in infinite recursion, with this cycle (started by
cp_canonicalize_string_full) repeating until the stack is exhausted:

 ...
 #1709 0x000000000055533c in inspect_type (info=0x38ff720, ret_comp=0xd83be10, finder=0x0, data=0x0) at /home/pedro/gdb/mygit/src/gdb/cp-support.c:267
 #1710 0x0000000000555a6f in replace_typedefs (info=0x38ff720, ret_comp=0xd83be10, finder=0x0, data=0x0) at /home/pedro/gdb/mygit/src/gdb/cp-support.c:475
 #1711 0x0000000000555a36 in replace_typedefs (info=0x38ff720, ret_comp=0xd83be70, finder=0x0, data=0x0) at /home/pedro/gdb/mygit/src/gdb/cp-support.c:470
 #1712 0x0000000000555800 in replace_typedefs_qualified_name (info=0x38ff720, ret_comp=0xd839470, finder=0x0, data=0x0) at /home/pedro/gdb/mygit/src/gdb/cp-support.c:389
 #1713 0x0000000000555a8c in replace_typedefs (info=0x38ff720, ret_comp=0xd839470, finder=0x0, data=0x0) at /home/pedro/gdb/mygit/src/gdb/cp-support.c:479
 ...

The demangle component tree for that symbol name looks like this:

d_dump tree for NS1::NS2::Templ1<int>::Templ1(NS1::NS2::object*):
typed name
  qualified name
    name 'NS1'
    qualified name
      name 'NS2'
      qualified name
        template                  <<<<<<<<<<
          name 'Templ1'
          template argument list
            builtin type int
        name 'Templ1'
  function type
    argument list
      pointer
        qualified name
          name 'NS1'
          qualified name
            name 'NS2'
            name 'object'

The recursion starts at replace_typedefs_qualified_name, which doesn't
handle the "template" node, and thus doesn't realize that the template
name actually has the fully qualified name NS1::NS2::Templ1.
replace_typedefs_qualified_name calls into replace_typedefs on the
template node, and that ends up in inspect_type looking up for a
symbol named "Templ1", which finds the global namespace typedef, which
itself expands to NS1::NS2::Templ1.  GDB then tries replacing typedefs
in that newly expanded name, which ends up again in
replace_typedefs_qualified_name, trying to expand a fully qualified
name with "NS::NS2::Templ1<unsigned>" in its name, which results in
recursion whenever the template node is reached.

Fix this by teaching replace_typedefs_qualified_name how to handle
template nodes.  It needs handling in two places: the first spot
handles the symbol above; the second spot handles a symbol like this,
from the new test:

(gdb) b NS1::NS2::grab_it(NS1::NS2::Templ1<int>*)
d_dump tree for NS1::NS2::grab_it(NS1::NS2::Templ1<int>*):
typed name
  qualified name
    name 'NS1'
    qualified name
      name 'NS2'
      name 'grab_it'
  function type
    argument list
      pointer
        qualified name
          name 'NS1'
          qualified name
            name 'NS2'
            template             <<<<<<<<
              name 'Templ1'
              template argument list
                builtin type int

What's different in this case is that the template node appears on the
right child node of a qualified name, instead of on the left child.

The testcase includes a test that checks whether template aliases are
correctly replaced by GDB too.  That fails with GCC due to GCC PR
95437, which makes GDB not know about a typedef for
"NS1::NS2::AliasTempl<int>".  GCC emits a typedef named
"NS1::NS2::AliasTempl" instead, with no template parameter info.  The
test passes with Clang (5.0.2 at least).  See more details here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95437

gdb/ChangeLog:
2020-05-30  Pedro Alves  <palves@redhat.com>

	* cp-support.c (replace_typedefs_template): New.
	(replace_typedefs_qualified_name): Handle
	DEMANGLE_COMPONENT_TEMPLATE.

gdb/testsuite/ChangeLog:
2020-05-30  Pedro Alves  <palves@redhat.com>

	* gdb.linespec/cp-replace-typedefs-ns-template.cc: New.
	* gdb.linespec/cp-replace-typedefs-ns-template.exp: New.

1 files changed, 72 insertions, 4 deletions

diff --git a/gdb/cp-support.c b/gdb/cp-support.c
index 1e54aae..11e54c2 100644
--- a/gdb/cp-support.c
+++ b/gdb/cp-support.c
@@ -294,6 +294,42 @@ inspect_type (struct demangle_parse_info *info,
   return 0;
 }
 
+/* Helper for replace_typedefs_qualified_name to handle
+   DEMANGLE_COMPONENT_TEMPLATE.  TMPL is the template node.  BUF is
+   the buffer that holds the qualified name being built by
+   replace_typedefs_qualified_name.  REPL is the node that will be
+   rewritten as a DEMANGLE_COMPONENT_NAME node holding the 'template
+   plus template arguments' name with typedefs replaced.  */
+
+static bool
+replace_typedefs_template (struct demangle_parse_info *info,
+			   string_file &buf,
+			   struct demangle_component *tmpl,
+			   struct demangle_component *repl,
+			   canonicalization_ftype *finder,
+			   void *data)
+{
+  demangle_component *tmpl_arglist = d_right (tmpl);
+
+  /* Replace typedefs in the template argument list.  */
+  replace_typedefs (info, tmpl_arglist, finder, data);
+
+  /* Convert 'template + replaced template argument list' to a string
+     and replace the REPL node.  */
+  gdb::unique_xmalloc_ptr<char> tmpl_str = cp_comp_to_string (tmpl, 100);
+  if (tmpl_str == nullptr)
+    {
+      /* If something went astray, abort typedef substitutions.  */
+      return false;
+    }
+  buf.puts (tmpl_str.get ());
+
+  repl->type = DEMANGLE_COMPONENT_NAME;
+  repl->u.s_name.s = obstack_strdup (&info->obstack, buf.string ());
+  repl->u.s_name.len = buf.size ();
+  return true;
+}
+
 /* Replace any typedefs appearing in the qualified name
    (DEMANGLE_COMPONENT_QUAL_NAME) represented in RET_COMP for the name parse
    given in INFO.  */
@@ -314,6 +350,29 @@ replace_typedefs_qualified_name (struct demangle_parse_info *info,
      substituted name.  */
   while (comp->type == DEMANGLE_COMPONENT_QUAL_NAME)
     {
+      if (d_left (comp)->type == DEMANGLE_COMPONENT_TEMPLATE)
+	{
+	  /* Convert 'template + replaced template argument list' to a
+	     string and replace the top DEMANGLE_COMPONENT_QUAL_NAME
+	     node.  */
+	  if (!replace_typedefs_template (info, buf,
+					  d_left (comp), d_left (ret_comp),
+					  finder, data))
+	    return;
+
+	  buf.clear ();
+	  d_right (ret_comp) = d_right (comp);
+	  comp = ret_comp;
+
+	  /* Fallback to DEMANGLE_COMPONENT_NAME processing.  We want
+	     to call inspect_type for this template, in case we have a
+	     template alias, like:
+	       template<typename T> using alias = base<int, t>;
+	     in which case we want inspect_type to do a replacement like:
+	       alias<int> -> base<int, int>
+	  */
+	}
+
       if (d_left (comp)->type == DEMANGLE_COMPONENT_NAME)
 	{
 	  struct demangle_component newobj;
@@ -370,11 +429,20 @@ replace_typedefs_qualified_name (struct demangle_parse_info *info,
       comp = d_right (comp);
     }
 
-  /* If the next component is DEMANGLE_COMPONENT_NAME, save the qualified
-     name assembled above and append the name given by COMP.  Then use this
-     reassembled name to check for a typedef.  */
+  /* If the next component is DEMANGLE_COMPONENT_TEMPLATE or
+     DEMANGLE_COMPONENT_NAME, save the qualified name assembled above
+     and append the name given by COMP.  Then use this reassembled
+     name to check for a typedef.  */
 
-  if (comp->type == DEMANGLE_COMPONENT_NAME)
+  if (comp->type == DEMANGLE_COMPONENT_TEMPLATE)
+    {
+      /* Replace the top (DEMANGLE_COMPONENT_QUAL_NAME) node with a
+	 DEMANGLE_COMPONENT_NAME node containing the whole name.  */
+      if (!replace_typedefs_template (info, buf, comp, ret_comp, finder, data))
+	return;
+      inspect_type (info, ret_comp, finder, data);
+    }
+  else if (comp->type == DEMANGLE_COMPONENT_NAME)
     {
       buf.write (comp->u.s_name.s, comp->u.s_name.len);