[1/2] C++ template type diff printing

diff --git a/gcc/c-family/c-format.c b/gcc/c-family/c-format.c
index 400eb66..fd40359 100644
--- a/gcc/c-family/c-format.c
+++ b/gcc/c-family/c-format.c
@@ -754,7 +754,7 @@  static const format_char_info gcc_cxxdiag_char_table[] =
   /* Custom conversion specifiers.  */
 
   /* These will require a "tree" at runtime.  */
-  { "ADEFKSTVX",0,STD_C89,{ T89_V,   BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN  }, "q+#",   "",   NULL },
+  { "ADEFHIKSTVX",0,STD_C89,{ T89_V,   BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN  }, "q+#",   "",   NULL },
 
   { "v", 0,STD_C89, { T89_I,   BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN,  BADLEN  }, "q#",  "",   NULL },
 
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 9ad2f6e..4648e60 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1342,6 +1342,10 @@  fdefault-inline
 C++ ObjC++ Ignore
 Does nothing.  Preserved for backward compatibility.
 
+fdiagnostics-show-template-tree
+C++ ObjC++ Var(flag_diagnostics_show_template_tree) Init(0)
+Print hierarchical comparisons when template types are mismatched.
+
 fdirectives-only
 C ObjC C++ ObjC++
 Preprocess directives only.
@@ -1361,6 +1365,10 @@  Write all declarations as Ada code for the given file only.
 felide-constructors
 C++ ObjC++ Var(flag_elide_constructors) Init(1)
 
+felide-type
+C++ ObjC++ Var(flag_elide_type) Init(1)
+-fno-elide-type Do not elide common elements in template comparisons.
+
 fenforce-eh-specs
 C++ ObjC++ Var(flag_enforce_eh_specs) Init(1)
 Generate code to check exception specifications.
diff --git a/gcc/c/c-objc-common.c b/gcc/c/c-objc-common.c
index 5e69488..ee0fa69 100644
--- a/gcc/c/c-objc-common.c
+++ b/gcc/c/c-objc-common.c
@@ -28,7 +28,7 @@  along with GCC; see the file COPYING3.  If not see
 #include "c-objc-common.h"
 
 static bool c_tree_printer (pretty_printer *, text_info *, const char *,
-			    int, bool, bool, bool);
+			    int, bool, bool, bool, const char **);
 
 bool
 c_missing_noreturn_ok_p (tree decl)
@@ -75,7 +75,8 @@  c_objc_common_init (void)
    diagnostic machinery.  */
 static bool
 c_tree_printer (pretty_printer *pp, text_info *text, const char *spec,
-		int precision, bool wide, bool set_locus, bool hash)
+		int precision, bool wide, bool set_locus, bool hash,
+		const char **)
 {
   tree t = NULL_TREE;
   tree name;
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index c15b8e4..737f312 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -10099,7 +10099,7 @@  perform_implicit_conversion_flags (tree type, tree expr,
 	  else if (invalid_nonstatic_memfn_p (loc, expr, complain))
 	    /* We gave an error.  */;
 	  else
-	    error_at (loc, "could not convert %qE from %qT to %qT", expr,
+	    error_at (loc, "could not convert %qE from %H to %I", expr,
 		      TREE_TYPE (expr), type);
 	}
       expr = error_mark_node;
diff --git a/gcc/cp/error.c b/gcc/cp/error.c
index a83ecb2..be9c78c 100644
--- a/gcc/cp/error.c
+++ b/gcc/cp/error.c
@@ -99,7 +99,47 @@  static void cp_diagnostic_starter (diagnostic_context *, diagnostic_info *);
 static void cp_print_error_function (diagnostic_context *, diagnostic_info *);
 
 static bool cp_printer (pretty_printer *, text_info *, const char *,
-			int, bool, bool, bool);
+			int, bool, bool, bool, const char **);
+
+/* Struct for handling %H or %I, which require delaying printing the
+   type until a postprocessing stage.  */
+
+struct deferred_printed_type
+{
+  deferred_printed_type ()
+  : m_tree (NULL_TREE), m_buffer_ptr (NULL), m_verbose (false)
+  {}
+
+  deferred_printed_type (tree type, const char **buffer_ptr, bool verbose)
+  : m_tree (type), m_buffer_ptr (buffer_ptr), m_verbose (verbose)
+  {
+    gcc_assert (type);
+    gcc_assert (buffer_ptr);
+  }
+
+  /* The tree is not GTY-marked: they are only non-NULL within a
+     call to pp_format.  */
+  tree m_tree;
+  const char **m_buffer_ptr;
+  bool m_verbose;
+};
+
+/* Subclass of format_postprocessor for the C++ frontend.
+   This handles the %H and %I formatting codes, printing them
+   in a postprocessing phase (since they affect each other).  */
+
+class cxx_format_postprocessor : public format_postprocessor
+{
+ public:
+  cxx_format_postprocessor ()
+  : m_type_a (), m_type_b ()
+  {}
+
+  void handle (pretty_printer *pp) FINAL OVERRIDE;
+
+  deferred_printed_type m_type_a;
+  deferred_printed_type m_type_b;
+};
 
 /* CONTEXT->printer is a basic pretty printer that was constructed
    presumably by diagnostic_initialize(), called early in the
@@ -123,6 +163,7 @@  cxx_initialize_diagnostics (diagnostic_context *context)
   diagnostic_starter (context) = cp_diagnostic_starter;
   /* diagnostic_finalizer is already c_diagnostic_finalizer.  */
   diagnostic_format_decoder (context) = cp_printer;
+  pp->m_format_postprocessor = new cxx_format_postprocessor ();
 }
 
 /* Dump a scope, if deemed necessary.  */
@@ -3565,6 +3606,370 @@  maybe_print_constexpr_context (diagnostic_context *context)
     }
 }
 
+
+/* Return true iff TYPE_A and TYPE_B are template types that are
+   meaningful to compare.  */
+
+static bool
+comparable_template_types_p (tree type_a, tree type_b)
+{
+  if (TREE_CODE (type_a) != RECORD_TYPE)
+    return false;
+  if (TREE_CODE (type_b) != RECORD_TYPE)
+    return false;
+
+  tree tinfo_a = TYPE_TEMPLATE_INFO (type_a);
+  tree tinfo_b = TYPE_TEMPLATE_INFO (type_b);
+  if (!tinfo_a || !tinfo_b)
+    return false;
+
+  return TI_TEMPLATE (tinfo_a) == TI_TEMPLATE (tinfo_b);
+}
+
+/* Start a new line indented by SPC spaces on PP.  */
+
+static void
+newline_and_indent (pretty_printer *pp, int spc)
+{
+  pp_newline (pp);
+  for (int i = 0; i < spc; i++)
+    pp_space (pp);
+}
+
+/* Generate a GC-allocated string for ARG, an expression or type.  */
+
+static const char *
+arg_to_string (tree arg, bool verbose)
+{
+  if (EXPR_P (arg))
+    return expr_to_string (arg);
+  return type_to_string (arg, verbose);
+}
+
+/* Subroutine to type_to_string_with_compare and
+   print_template_tree_comparison.
+
+   Print a representation of ARG (an expression or type) to PP,
+   colorizing it as "type-diff" if PP->show_color.  */
+
+static void
+print_nonequal_arg (pretty_printer *pp, tree arg, bool verbose)
+{
+  pp_printf (pp, "%r%s%R",
+	     "type-diff",
+	     (arg
+	      ? arg_to_string (arg, verbose)
+	      : G_("(no argument)")));
+}
+
+/* As type_to_string, but for a template, potentially colorizing/eliding
+   in comparison with PEER.
+   For example, if TYPE is map<int,double> and PEER is map<int,int>,
+   then the resulting string would be:
+     map<[...],double>
+   with type elision, and:
+     map<int,double>
+   without type elision.
+
+   In both cases the parts of TYPE that differ from PEER will be colorized
+   if SHOW_COLOR is true.  In the above example, this would be "double".
+
+   Only non-default template arguments are printed.
+
+   The resulting string is in a GC-allocated buffer.  */
+
+static const char *
+type_to_string_with_compare (tree type, tree peer, bool verbose,
+			     bool show_color)
+{
+  pretty_printer inner_pp;
+  pretty_printer *pp = &inner_pp;
+  pp_show_color (pp) = show_color;
+
+  tree tinfo = TYPE_TEMPLATE_INFO (type);
+  tree tinfo_peer = TYPE_TEMPLATE_INFO (peer);
+
+  pp_printf (pp, "%s<",
+	     IDENTIFIER_POINTER (DECL_NAME (TI_TEMPLATE (tinfo))));
+  tree args_a = TI_ARGS (tinfo);
+  tree args_b = TI_ARGS (tinfo_peer);
+  gcc_assert (TREE_CODE (args_a) == TREE_VEC);
+  gcc_assert (TREE_CODE (args_b) == TREE_VEC);
+  int flags = 0;
+  int len_a = get_non_default_template_args_count (args_a, flags);
+  args_a = INNERMOST_TEMPLATE_ARGS (args_a);
+  int len_b = get_non_default_template_args_count (args_b, flags);
+  args_b = INNERMOST_TEMPLATE_ARGS (args_b);
+  int len_max = MAX (len_a, len_b);
+  for (int idx = 0; idx < len_max; idx++)
+    {
+      tree arg_a = idx < len_a ? TREE_VEC_ELT (args_a, idx) : NULL;
+      tree arg_b = idx < len_b ? TREE_VEC_ELT (args_b, idx) : NULL;
+      if (arg_a == arg_b)
+	{
+	  if (flag_elide_type)
+	    pp_string (pp, G_("[...]"));
+	  else
+	    pp_string (pp, arg_to_string (arg_a, verbose));
+	}
+      else
+	{
+	  if (comparable_template_types_p (arg_a, arg_b))
+	    pp_string (pp, type_to_string_with_compare (arg_a, arg_b, verbose,
+							show_color));
+	  else
+	    print_nonequal_arg (pp, arg_a, verbose);
+	}
+      if (idx + 1 < len_max)
+	pp_character (pp, ',');
+    }
+  pp_printf (pp, ">");
+  return pp_ggc_formatted_text (pp);
+}
+
+/* Recursively print a tree-like comparison of TYPE_A and TYPE_B to PP,
+   indented by INDENT spaces.
+
+   For example given types:
+
+     vector<map<int,double>>
+
+   and
+
+     vector<map<double,float>>
+
+   the output with type elision would be:
+
+     vector<
+       map<
+         [...],
+         [double != float]>>
+
+   and without type-elision would be:
+
+     vector<
+       map<
+         int,
+         [double != float]>>
+
+   TYPE_A and TYPE_B must both be comparable template types
+   (as per comparable_template_types_p).
+
+   Only non-default template arguments are printed.  */
+
+static void
+print_template_tree_comparison (pretty_printer *pp, tree type_a, tree type_b,
+				bool verbose, int indent)
+{
+  tree tinfo_a = TYPE_TEMPLATE_INFO (type_a);
+  tree tinfo_b = TYPE_TEMPLATE_INFO (type_b);
+
+  newline_and_indent (pp, indent);
+  pp_printf (pp, "%s<",
+	     IDENTIFIER_POINTER (DECL_NAME (TI_TEMPLATE (tinfo_a))));
+  tree args_a = TI_ARGS (tinfo_a);
+  tree args_b = TI_ARGS (tinfo_b);
+  int flags = 0;
+  int len_a = get_non_default_template_args_count (args_a, flags);
+  args_a = INNERMOST_TEMPLATE_ARGS (args_a);
+  int len_b = get_non_default_template_args_count (args_b, flags);
+  args_b = INNERMOST_TEMPLATE_ARGS (args_b);
+  int len_max = MAX (len_a, len_b);
+  gcc_assert (TREE_CODE (args_a) == TREE_VEC);
+  gcc_assert (TREE_CODE (args_b) == TREE_VEC);
+  for (int idx = 0; idx < len_max; idx++)
+    {
+      tree arg_a = idx < len_a ? TREE_VEC_ELT (args_a, idx) : NULL;
+      tree arg_b = idx < len_b ? TREE_VEC_ELT (args_b, idx) : NULL;
+      if (arg_a == arg_b)
+	{
+	  newline_and_indent (pp, indent + 2);
+	  /* Can do elision here, printing "[...]".  */
+	  if (flag_elide_type)
+	    pp_string (pp, G_("[...]"));
+	  else
+	    pp_string (pp, arg_to_string (arg_a, verbose));
+	}
+      else
+	{
+	  if (comparable_template_types_p (arg_a, arg_b))
+	    print_template_tree_comparison (pp, arg_a, arg_b, verbose,
+					    indent + 2);
+	  else
+	    {
+	      newline_and_indent (pp, indent + 2);
+	      pp_character (pp, '[');
+	      print_nonequal_arg (pp, arg_a, verbose);
+	      pp_string (pp, " != ");
+	      print_nonequal_arg (pp, arg_b, verbose);
+	      pp_character (pp, ']');
+	    }
+	}
+      if (idx + 1 < len_max)
+	pp_character (pp, ',');
+    }
+  pp_printf (pp, ">");
+}
+
+/* Subroutine for use in a format_postprocessor::handle
+   implementation.  Adds a chunk to the end of
+   formatted output, so that it will be printed
+   by pp_output_formatted_text.  */
+
+static void
+append_formatted_chunk (pretty_printer *pp, const char *content)
+{
+  output_buffer *buffer = pp_buffer (pp);
+  struct chunk_info *chunk_array = buffer->cur_chunk_array;
+  const char **args = chunk_array->args;
+
+  unsigned int chunk_idx;
+  for (chunk_idx = 0; args[chunk_idx]; chunk_idx++)
+    ;
+  args[chunk_idx++] = content;
+  args[chunk_idx] = NULL;
+}
+
+/* Create a copy of CONTENT, with quotes added, and,
+   potentially, with colorization.
+   No escaped is performed on CONTENT.
+   The result is in a GC-allocated buffer. */
+
+static const char *
+add_quotes (const char *content, bool show_color)
+{
+  pretty_printer tmp_pp;
+  pp_show_color (&tmp_pp) = show_color;
+
+  /* We have to use "%<%s%>" rather than "%qs" here in order to avoid
+     quoting colorization bytes within the results.  */
+  pp_printf (&tmp_pp, "%<%s%>", content);
+
+  return pp_ggc_formatted_text (&tmp_pp);
+}
+
+/* If we had %H and %I, and hence deferred printing them,
+   print them now, storing the result into the chunk_info
+   for pp_format.
+   Also print the difference in tree form, adding it as
+   an additional chunk.  */
+
+void
+cxx_format_postprocessor::handle (pretty_printer *pp)
+{
+  /* If we have one of %H and %I, the other should have
+     been present.  */
+  if (m_type_a.m_tree || m_type_b.m_tree)
+    {
+      gcc_assert (m_type_a.m_tree);
+      gcc_assert (m_type_b.m_tree);
+    }
+
+  if (m_type_a.m_tree && m_type_b.m_tree)
+    {
+      /* Avoid reentrancy issues by working with a copy of
+	 m_type_a and m_type_b, resetting them now.  */
+      deferred_printed_type type_a = m_type_a;
+      deferred_printed_type type_b = m_type_b;
+      m_type_a = deferred_printed_type ();
+      m_type_b = deferred_printed_type ();
+
+      gcc_assert (type_a.m_tree);
+      gcc_assert (type_a.m_buffer_ptr);
+      gcc_assert (type_b.m_tree);
+      gcc_assert (type_b.m_buffer_ptr);
+
+      bool show_color = pp_show_color (pp);
+
+      const char *type_a_text;
+      const char *type_b_text;
+
+      if (comparable_template_types_p (type_a.m_tree, type_b.m_tree))
+	{
+	  type_a_text
+	    = type_to_string_with_compare (type_a.m_tree, type_b.m_tree,
+					   type_a.m_verbose, show_color);
+	  type_b_text
+	    = type_to_string_with_compare (type_b.m_tree, type_a.m_tree,
+					   type_b.m_verbose, show_color);
+
+	  if (flag_diagnostics_show_template_tree)
+	    {
+	      pretty_printer inner_pp;
+	      pp_show_color (&inner_pp) = pp_show_color (pp);
+	      print_template_tree_comparison
+		(&inner_pp, type_a.m_tree, type_b.m_tree, type_a.m_verbose, 2);
+	      append_formatted_chunk (pp, pp_ggc_formatted_text (&inner_pp));
+	    }
+	}
+      else
+	{
+	  /* If the types were not comparable, they are printed normally,
+	     and no difference tree is printed.  */
+	  type_a_text = type_to_string (type_a.m_tree, type_a.m_verbose);
+	  type_b_text = type_to_string (type_b.m_tree, type_b.m_verbose);
+	}
+
+      *type_a.m_buffer_ptr = add_quotes (type_a_text, show_color);
+      *type_b.m_buffer_ptr = add_quotes (type_b_text, show_color);
+   }
+}
+
+/* Subroutine for handling %H and %I, to support i18n of messages like:
+
+    error_at (loc, "could not convert %qE from %H to %I",
+	       expr, type_a, type_b);
+
+   so that we can print things like:
+
+     could not convert 'foo' from 'map<int,double>' to 'map<int,int>'
+
+   and, with type-elision:
+
+     could not convert 'foo' from 'map<[...],double>' to 'map<[...],int>'
+
+   (with color-coding of the differences between the types).
+
+   The %H and %I format codes are peers: both must be present,
+   and they affect each other.  Hence to handle them, we must
+   delay printing until we have both, deferring the printing to
+   pretty_printer's m_format_postprocessor hook.
+
+   This is called in phase 2 of pp_format, when it is accumulating
+   a series of formatted chunks.  We stash the location of the chunk
+   we're meant to have written to, so that we can write to it in the
+   m_format_postprocessor hook.  */
+
+static void
+defer_half_of_type_diff (pretty_printer *pp, char spec, tree type,
+			 const char **buffer_ptr, bool verbose)
+{
+  gcc_assert (pp->m_format_postprocessor);
+  cxx_format_postprocessor *postprocessor
+    = static_cast <cxx_format_postprocessor *> (pp->m_format_postprocessor);
+
+  switch (spec)
+    {
+    case 'H':
+      gcc_assert (postprocessor->m_type_a.m_tree == NULL_TREE);
+      gcc_assert (postprocessor->m_type_a.m_buffer_ptr == NULL);
+      postprocessor->m_type_a
+	= deferred_printed_type (type, buffer_ptr, verbose);
+      break;
+
+    case 'I':
+      gcc_assert (postprocessor->m_type_b.m_tree == NULL_TREE);
+      gcc_assert (postprocessor->m_type_b.m_buffer_ptr == NULL);
+      postprocessor->m_type_b
+	= deferred_printed_type (type, buffer_ptr, verbose);
+      break;
+
+    default:
+      gcc_unreachable ();
+    }
+}
+
+
 /* Called from output_format -- during diagnostic message processing --
    to handle C++ specific format specifier with the following meanings:
    %A   function argument-list.
@@ -3579,10 +3984,13 @@  maybe_print_constexpr_context (diagnostic_context *context)
    %S   substitution (template + args)
    %T   type.
    %V   cv-qualifier.
-   %X   exception-specification.  */
+   %X   exception-specification.
+   %H   type difference (from)
+   %I   type difference (to).  */
 static bool
 cp_printer (pretty_printer *pp, text_info *text, const char *spec,
-	    int precision, bool wide, bool set_locus, bool verbose)
+	    int precision, bool wide, bool set_locus, bool verbose,
+	    const char **buffer_ptr)
 {
   const char *result;
   tree t = NULL;
@@ -3629,6 +4037,13 @@  cp_printer (pretty_printer *pp, text_info *text, const char *spec,
       percent_K_format (text);
       return true;
 
+    case 'H':
+    case 'I':
+      {
+	defer_half_of_type_diff (pp, *spec, next_tree, buffer_ptr, verbose);
+	return true;
+      }
+
     default:
       return false;
     }
diff --git a/gcc/diagnostic-color.c b/gcc/diagnostic-color.c
index 8353fe0..6adb872 100644
--- a/gcc/diagnostic-color.c
+++ b/gcc/diagnostic-color.c
@@ -174,6 +174,7 @@  static struct color_cap color_dict[] =
   { "diff-hunk", SGR_SEQ (COLOR_FG_CYAN), 9, false },
   { "diff-delete", SGR_SEQ (COLOR_FG_RED), 11, false },
   { "diff-insert", SGR_SEQ (COLOR_FG_GREEN), 11, false },
+  { "type-diff", SGR_SEQ (COLOR_BOLD COLOR_SEPARATOR COLOR_FG_GREEN), 9, false },
   { NULL, NULL, 0, false }
 };
 
@@ -204,8 +205,9 @@  colorize_stop (bool show_color)
 /* Parse GCC_COLORS.  The default would look like:
    GCC_COLORS='error=01;31:warning=01;35:note=01;36:\
    range1=32:range2=34:locus=01:quote=01:\
-   fixit-insert=32:fixit-delete=31'\
-   diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32'
+   fixit-insert=32:fixit-delete=31:'\
+   diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
+   type-diff=01;32'
    No character escaping is needed or supported.  */
 static bool
 parse_gcc_colors (void)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 65308c9..9966fa0 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -251,6 +251,7 @@  Objective-C and Objective-C++ Dialects}.
 -fdiagnostics-color=@r{[}auto@r{|}never@r{|}always@r{]}  @gol
 -fno-diagnostics-show-option  -fno-diagnostics-show-caret @gol
 -fdiagnostics-parseable-fixits  -fdiagnostics-generate-patch @gol
+-fdiagnostics-show-template-tree -fno-elide-type @gol
 -fno-show-column}
 
 @item Warning Options
@@ -3442,7 +3443,8 @@  The default @env{GCC_COLORS} is
 @smallexample
 error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
 quote=01:fixit-insert=32:fixit-delete=31:\
-diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32
+diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
+type-diff=01;32
 @end smallexample
 @noindent
 where @samp{01;31} is bold red, @samp{01;35} is bold magenta,
@@ -3506,6 +3508,11 @@  SGR substring for deleted lines within generated patches.
 @item diff-insert=
 @vindex diff-insert GCC_COLORS @r{capability}
 SGR substring for inserted lines within generated patches.
+
+@item type-diff=
+@vindex type-diff GCC_COLORS @r{capability}
+SGR substring for highlighting mismatching types within template
+arguments in the C++ frontend.
 @end table
 
 @item -fno-diagnostics-show-option
@@ -3578,6 +3585,47 @@  are printed.  For example:
 The diff may or may not be colorized, following the same rules
 as for diagnostics (see @option{-fdiagnostics-color}).
 
+@item -fdiagnostics-show-template-tree
+@opindex fdiagnostics-show-template-tree
+
+In the C++ frontend, when printing diagnostics showing mismatching
+template types, such as:
+
+@smallexample
+  could not convert 'std::map<int, std::vector<double> >()'
+    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
+@end smallexample
+
+the @option{-fdiagnostics-show-template-tree} flag enables printing a
+tree-like structure showing the common and differing parts of the types,
+such as:
+
+@smallexample
+  map<
+    [...],
+    vector<
+      [double != float]>>
+@end smallexample
+
+The parts that differ are highlighted with color (``double'' and
+``float'' in this case).
+
+@item -fno-elide-type
+@opindex fno-elide-type
+@opindex felide-type
+By default when the C++ frontend prints diagnostics showing mismatching
+template types, common parts of the types are printed as ``[...]'' to
+simplify the error message.  For example:
+
+@smallexample
+  could not convert 'std::map<int, std::vector<double> >()'
+    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
+@end smallexample
+
+Specifying the @option{-fno-elide-type} flag suppresses that behavior.
+This flag also affects the output of the
+@option{-fdiagnostics-show-template-tree} flag.
+
 @item -fno-show-column
 @opindex fno-show-column
 Do not print column numbers in diagnostics.  This may be necessary if
diff --git a/gcc/fortran/error.c b/gcc/fortran/error.c
index 0312499..a5a324c 100644
--- a/gcc/fortran/error.c
+++ b/gcc/fortran/error.c
@@ -917,7 +917,8 @@  gfc_notify_std (int std, const char *gmsgid, ...)
 */
 static bool
 gfc_format_decoder (pretty_printer *pp, text_info *text, const char *spec,
-		    int precision, bool wide, bool set_locus, bool hash)
+		    int precision, bool wide, bool set_locus, bool hash,
+		    const char **buffer_ptr)
 {
   switch (*spec)
     {
@@ -948,7 +949,7 @@  gfc_format_decoder (pretty_printer *pp, text_info *text, const char *spec,
 	 etc. diagnostics can use the FE printer while the FE is still
 	 active.  */
       return default_tree_printer (pp, text, spec, precision, wide,
-				   set_locus, hash);
+				   set_locus, hash, buffer_ptr);
     }
 }
 
diff --git a/gcc/pretty-print.c b/gcc/pretty-print.c
index bcb1a70..e70a0ed 100644
--- a/gcc/pretty-print.c
+++ b/gcc/pretty-print.c
@@ -677,7 +677,8 @@  pp_format (pretty_printer *pp, text_info *text)
 
 	    gcc_assert (pp_format_decoder (pp));
 	    ok = pp_format_decoder (pp) (pp, text, p,
-					 precision, wide, plus, hash);
+					 precision, wide, plus, hash,
+					 formatters[argno]);
 	    gcc_assert (ok);
 	  }
 	}
@@ -696,6 +697,11 @@  pp_format (pretty_printer *pp, text_info *text)
     for (; argno < PP_NL_ARGMAX; argno++)
       gcc_assert (!formatters[argno]);
 
+  /* If the client supplied a postprocessing object, call its "handle"
+     hook here.  */
+  if (pp->m_format_postprocessor)
+    pp->m_format_postprocessor->handle (pp);
+
   /* Revert to normal obstack and wrapping mode.  */
   buffer->obstack = &buffer->formatted_obstack;
   buffer->line_length = 0;
@@ -847,6 +853,7 @@  pretty_printer::pretty_printer (const char *p, int l)
     indent_skip (),
     wrapping (),
     format_decoder (),
+    m_format_postprocessor (NULL),
     emitted_prefix (),
     need_newline (),
     translate_identifiers (true),
@@ -860,6 +867,8 @@  pretty_printer::pretty_printer (const char *p, int l)
 
 pretty_printer::~pretty_printer ()
 {
+  if (m_format_postprocessor)
+    delete m_format_postprocessor;
   buffer->~output_buffer ();
   XDELETE (buffer);
 }
diff --git a/gcc/pretty-print.h b/gcc/pretty-print.h
index 2596678..5635bf8 100644
--- a/gcc/pretty-print.h
+++ b/gcc/pretty-print.h
@@ -180,11 +180,20 @@  struct pp_wrapping_mode_t
    A client-supplied formatter returns true if everything goes well,
    otherwise it returns false.  */
 typedef bool (*printer_fn) (pretty_printer *, text_info *, const char *,
-			    int, bool, bool, bool);
+			    int, bool, bool, bool, const char **);
 
 /* Client supplied function used to decode formats.  */
 #define pp_format_decoder(PP) (PP)->format_decoder
 
+/* Base class for an optional client-supplied object for doing additional
+   processing between stages 2 and 3 of formatted printing.  */
+class format_postprocessor
+{
+ public:
+  virtual ~format_postprocessor () {}
+  virtual void handle (pretty_printer *) = 0;
+};
+
 /* TRUE if a newline character needs to be added before further
    formatting.  */
 #define pp_needs_newline(PP)  (PP)->need_newline
@@ -239,9 +248,16 @@  struct pretty_printer
      If the BUFFER needs additional characters from the format string, it
      should advance the TEXT->format_spec as it goes.  When FORMAT_DECODER
      returns, TEXT->format_spec should point to the last character processed.
-  */
+     The BUFFER_PTR is passed in, to allow for deferring-handling of format
+     codes (e.g. %H and %I in the C++ frontend).  */
   printer_fn format_decoder;
 
+  /* If non-NULL, this is called by pp_format once after all format codes
+     have been processed, to allow for client-specific postprocessing.
+     This is used by the C++ frontend for handling the %H and %I
+     format codes (which interract with each other).  */
+  format_postprocessor *m_format_postprocessor;
+
   /* Nonzero if current PREFIX was emitted at least once.  */
   bool emitted_prefix;
 
diff --git a/gcc/testsuite/g++.dg/plugin/plugin.exp b/gcc/testsuite/g++.dg/plugin/plugin.exp
index 6a0c1aa..94ebe93 100644
--- a/gcc/testsuite/g++.dg/plugin/plugin.exp
+++ b/gcc/testsuite/g++.dg/plugin/plugin.exp
@@ -65,6 +65,9 @@  set plugin_test_list [list \
     { def_plugin.c def-plugin-test.C } \
     { ../../gcc.dg/plugin/diagnostic_plugin_test_tree_expression_range.c \
 	  diagnostic-test-expressions-1.C } \
+    { show_template_tree_color_plugin.c \
+    	  show-template-tree-color.C \
+    	  show-template-tree-color-no-elide-type.C } \
 ]
 
 foreach plugin_test $plugin_test_list {
diff --git a/gcc/testsuite/g++.dg/plugin/show-template-tree-color-no-elide-type.C b/gcc/testsuite/g++.dg/plugin/show-template-tree-color-no-elide-type.C
new file mode 100644
index 0000000..cab0359
--- /dev/null
+++ b/gcc/testsuite/g++.dg/plugin/show-template-tree-color-no-elide-type.C
@@ -0,0 +1,30 @@ 
+/* Verify colorization of the output of -fdiagnostics-show-template-tree,
+   and within the %H and %I format codes.
+   Doing so requires a plugin; see the comments in the plugin for the
+   rationale.  */
+
+// { dg-options "-fdiagnostics-show-template-tree -fdiagnostics-color=always -fno-elide-type" }
+
+template<typename> struct vector {};
+template<typename, typename> struct map {};
+
+void fn_1(vector<int>);
+void fn_2(map<int, int>);
+
+void test ()
+{
+  fn_1 (vector<double> ());
+  /* { dg-begin-multiline-output "" }
+could not convert 'vector<double>()' from 'vector<double>' to 'vector<int>'
+  vector<
+    [double != int]>
+     { dg-end-multiline-output "" } */
+
+  fn_2 (map<int, double>());
+  /* { dg-begin-multiline-output "" }
+could not convert 'map<int, double>()' from 'map<int,double>' to 'map<int,int>'
+  map<
+    int,
+    [double != int]>
+     { dg-end-multiline-output "" } */
+}
diff --git a/gcc/testsuite/g++.dg/plugin/show-template-tree-color.C b/gcc/testsuite/g++.dg/plugin/show-template-tree-color.C
new file mode 100644
index 0000000..eb99a3e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/plugin/show-template-tree-color.C
@@ -0,0 +1,30 @@ 
+/* Verify colorization of the output of -fdiagnostics-show-template-tree,
+   and within the %H and %I format codes.
+   Doing so requires a plugin; see the comments in the plugin for the
+   rationale.  */
+
+// { dg-options "-fdiagnostics-show-template-tree -fdiagnostics-color=always" }
+
+template<typename> struct vector {};
+template<typename, typename> struct map {};
+
+void fn_1(vector<int>);
+void fn_2(map<int, int>);
+
+void test ()
+{
+  fn_1 (vector<double> ());
+  /* { dg-begin-multiline-output "" }
+could not convert 'vector<double>()' from 'vector<double>' to 'vector<int>'
+  vector<
+    [double != int]>
+     { dg-end-multiline-output "" } */
+
+  fn_2 (map<int, double>());
+  /* { dg-begin-multiline-output "" }
+could not convert 'map<int, double>()' from 'map<[...],double>' to 'map<[...],int>'
+  map<
+    [...],
+    [double != int]>
+     { dg-end-multiline-output "" } */
+}
diff --git a/gcc/testsuite/g++.dg/plugin/show_template_tree_color_plugin.c b/gcc/testsuite/g++.dg/plugin/show_template_tree_color_plugin.c
new file mode 100644
index 0000000..af568bf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/plugin/show_template_tree_color_plugin.c
@@ -0,0 +1,38 @@ 
+/* We want to verify the colorized output of cxx_format_postprocessor,
+   but turning on colorization for everything confuses "dg-error" etc.
+   The color codes in the generated messages would also need escaping
+   for use within dg-error.
+
+   Hence the simplest approach is to provide a custom diagnostic_starter_fn,
+   which does nothing.
+
+   The resulting messages lack the "FILENAME:LINE:COL: error: " prefix
+   and can thus be tested using dg-begin/end-multiline-output.  */
+
+/* { dg-options "-O" } */
+
+#include "gcc-plugin.h"
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "plugin-version.h"
+#include "diagnostic.h"
+
+int plugin_is_GPL_compatible;
+
+void
+noop_starter_fn (diagnostic_context *, diagnostic_info *)
+{
+}
+
+int
+plugin_init (struct plugin_name_args *plugin_info,
+	     struct plugin_gcc_version *version)
+{
+  if (!plugin_default_version_check (version, &gcc_version))
+    return 1;
+
+  diagnostic_starter (global_dc) = noop_starter_fn;
+
+  return 0;
+}
diff --git a/gcc/testsuite/g++.dg/template/show-template-tree-2.C b/gcc/testsuite/g++.dg/template/show-template-tree-2.C
new file mode 100644
index 0000000..1cd3a06
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/show-template-tree-2.C
@@ -0,0 +1,118 @@ 
+// Tests of -fdiagnostics-show-template-tree with variadic templates
+// { dg-options "-fdiagnostics-show-template-tree -std=c++11" }
+
+template<typename> struct vector {};
+template<typename, typename> struct map {};
+template<typename ... Types> struct var {};
+
+void fn_0(var<>);
+void fn_1(var<int>);
+void fn_2(var<int, int>);
+void fn_3(vector<var<> >);
+void fn_4(vector<var<int> >);
+void fn_5(vector<var<int, int> >);
+
+void test_fn_0 ()
+{
+  fn_0 (var<> ());
+  fn_0 (var<int> ()); // { dg-error "could not convert .* from 'var<int>' to 'var<>'" }
+  /* { dg-begin-multiline-output "" }
+  var<
+    [int != ]>
+     { dg-end-multiline-output "" } */
+  fn_0 (var<int, int> ()); // { dg-error "could not convert .* from 'var<int, int>' to 'var<>'" }
+  /* { dg-begin-multiline-output "" }
+  var<
+    [int, int != ]>
+     { dg-end-multiline-output "" } */
+  fn_0 (vector<var<int> >()); // { dg-error "could not convert .* from 'vector<var<int> >' to 'var<>'" }
+  fn_0 (vector<var<int, int> >());  // { dg-error "could not convert .* from 'vector<var<int, int> >' to 'var<>'" }
+}
+
+void test_fn_1 ()
+{
+  fn_1 (var<> ()); // { dg-error "could not convert .* from 'var<>' to 'var<int>'" }
+  /* { dg-begin-multiline-output "" }
+  var<
+    [ != int]>
+     { dg-end-multiline-output "" } */
+  fn_1 (var<int> ());
+  fn_1 (var<int, int> ()); // { dg-error "could not convert .* from 'var<int, int>' to 'var<int>'" }
+  /* { dg-begin-multiline-output "" }
+  var<
+    [int, int != int]>
+     { dg-end-multiline-output "" } */
+  fn_1 (vector<var<int> >()); // { dg-error "could not convert .* from 'vector<var<int> >' to 'var<int>'" }
+  fn_1 (vector<var<int, int> >()); // { dg-error "could not convert .* from 'vector<var<int, int> >' to 'var<int>'" }
+}
+
+void test_fn_2 ()
+{
+  fn_2 (var<> ()); // { dg-error "could not convert .* from 'var<>' to 'var<int, int>'" }
+  /* { dg-begin-multiline-output "" }
+  var<
+    [ != int, int]>
+     { dg-end-multiline-output "" } */
+  fn_2 (var<int> ()); // { dg-error "could not convert .* from 'var<int>' to 'var<int, int>'" }
+  /* { dg-begin-multiline-output "" }
+  var<
+    [int != int, int]>
+     { dg-end-multiline-output "" } */
+  fn_2 (var<int, int> ());
+  fn_2 (vector<var<int> >()); // { dg-error "could not convert .* from 'vector<var<int> >' to 'var<int, int>'" }
+  fn_2 (vector<var<int, int> >()); // { dg-error "could not convert .* from 'vector<var<int, int> >' to 'var<int, int>'" }
+}
+
+void test_fn_3 ()
+{
+  fn_3 (var<> ()); // { dg-error "could not convert .* from 'var<>' to 'vector<var<> >'" }
+  fn_3 (var<int> ()); // { dg-error "could not convert .* from 'var<int>' to 'vector<var<> >'" }
+  fn_3 (var<int, int> ()); // { dg-error "could not convert .* from 'var<int, int>' to 'vector<var<> >'" }
+  fn_3 (vector<var<> >());
+  fn_3 (vector<var<int> >());  // { dg-error "could not convert .* from 'vector<var<int>>' to 'vector<var<>>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    var<
+      [int != ]>>
+     { dg-end-multiline-output "" } */
+  fn_3 (vector<var<int, int> >()); // { dg-error "could not convert .* from 'vector<var<int, int>>' to 'vector<var<>>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    var<
+      [int, int != ]>>
+     { dg-end-multiline-output "" } */
+}
+
+void test_fn_4 ()
+{
+  fn_4 (var<> ()); // { dg-error "could not convert .* from 'var<>' to 'vector<var<int> >'" }
+  fn_4 (var<int> ()); // { dg-error "could not convert .* from 'var<int>' to 'vector<var<int> >'" }
+  fn_4 (var<int, int> ()); // { dg-error "could not convert .* from 'var<int, int>' to 'vector<var<int> >'" }
+  fn_4 (vector<var<> >()); // { dg-error "could not convert .* from 'vector<var<>>' to 'vector<var<int>>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    var<
+      [ != int]>>
+     { dg-end-multiline-output "" } */
+  fn_4 (vector<var<int> >()); 
+  fn_4 (vector<var<int, int> >()); // { dg-error "could not convert .* from 'vector<var<int, int>>' to 'vector<var<int>>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    var<
+      [int, int != int]>>
+     { dg-end-multiline-output "" } */
+}
+
+void test_fn_5 ()
+{
+  fn_5 (var<> ()); // { dg-error "could not convert .* from 'var<>' to 'vector<var<int, int> >'" }
+  fn_5 (var<int> ());  // { dg-error "could not convert .* from 'var<int>' to 'vector<var<int, int> >'" }
+  fn_5 (var<int, int> ());  // { dg-error "could not convert .* from 'var<int, int>' to 'vector<var<int, int> >'" }
+  fn_5 (vector<var<int> >());  // { dg-error "could not convert .* from 'vector<var<int>>' to 'vector<var<int, int>>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    var<
+      [int != int, int]>>
+     { dg-end-multiline-output "" } */
+  fn_5 (vector<var<int, int> >());
+}
diff --git a/gcc/testsuite/g++.dg/template/show-template-tree-3.C b/gcc/testsuite/g++.dg/template/show-template-tree-3.C
new file mode 100644
index 0000000..0eda40b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/show-template-tree-3.C
@@ -0,0 +1,37 @@ 
+/* End-to-end test of -fdiagnostics-show-template-tree and -felide-type
+   using the STL.
+   In particular, ensure that we don't print the default arguments e.g.
+   rather than printing
+     from 'vector<double,allocator<double>>' to 'vector<float,allocator<float>>'
+   (albeit with differences nicely color-coded), we want to print:
+     from 'vector<double>' to 'vector<float>'
+   (again, with the "double" and "float" highlighted, though we can't test
+   for that in this case).  */
+
+// { dg-options "-fdiagnostics-show-template-tree" }
+
+#include <map>
+#include <vector>
+
+using std::vector;
+using std::map;
+
+void takes_vf (vector<float> v);
+void takes_mivf (map<int, vector<float> > v);
+
+int test ()
+{
+  takes_vf (vector<double> ()); // { dg-error "could not convert '.*' from 'vector<double>' to 'vector<float>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    [double != float]>
+     { dg-end-multiline-output "" } */
+
+  takes_mivf (map<int, vector<double> > ()); // { dg-error "could not convert '.*' from 'map<.\\.\\.\\..,vector<double>>' to 'map<.\\.\\.\\..,vector<float>>'" }
+  /* { dg-begin-multiline-output "" }
+  map<
+    [...],
+    vector<
+      [double != float]>>
+     { dg-end-multiline-output "" } */
+}
diff --git a/gcc/testsuite/g++.dg/template/show-template-tree-no-elide-type.C b/gcc/testsuite/g++.dg/template/show-template-tree-no-elide-type.C
new file mode 100644
index 0000000..d4bfa81
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/show-template-tree-no-elide-type.C
@@ -0,0 +1,24 @@ 
+// { dg-options "-fdiagnostics-show-template-tree -fno-elide-type" }
+
+template<typename> struct vector {};
+template<typename, typename> struct map {};
+
+void fn_1(vector<int>);
+void fn_2(map<int, int>);
+void fn_3(vector<map<int, float> >);
+
+void test ()
+{
+  fn_1 (vector<double> ()); // { dg-error "could not convert .* from 'vector<double>' to 'vector<int>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    [double != int]>
+     { dg-end-multiline-output "" } */
+
+  fn_2 (map<int, double>());  // { dg-error "could not convert .* from 'map<int,double>' to 'map<int,int>'" }
+  /* { dg-begin-multiline-output "" }
+  map<
+    int,
+    [double != int]>
+     { dg-end-multiline-output "" } */
+}
diff --git a/gcc/testsuite/g++.dg/template/show-template-tree.C b/gcc/testsuite/g++.dg/template/show-template-tree.C
new file mode 100644
index 0000000..2567dbf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/show-template-tree.C
@@ -0,0 +1,43 @@ 
+// { dg-options "-fdiagnostics-show-template-tree" }
+
+template<typename> struct vector {};
+template<typename, typename> struct map {};
+
+void fn_1(vector<int>);
+void fn_2(map<int, int>);
+void fn_3(vector<map<int, float> >);
+
+void test ()
+{
+  fn_1 (vector<int> ());
+  fn_1 (42); // { dg-error "could not convert '42' from 'int' to 'vector<int>'" }
+  fn_1 (vector<double> ()); // { dg-error "could not convert .* from 'vector<double>' to 'vector<int>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    [double != int]>
+     { dg-end-multiline-output "" } */
+  fn_1 (map<int, int> ()); // { dg-error "could not convert .* from 'map<int, int>' to 'vector<int>'" }
+
+  fn_2 (map<int, int>());
+  fn_2 (map<int, double>());  // { dg-error "could not convert .* from 'map<.\\.\\.\\..,double>. to .map<.\\.\\.\\..,int>'" }
+  /* { dg-begin-multiline-output "" }
+  map<
+    [...],
+    [double != int]>
+     { dg-end-multiline-output "" } */
+  fn_2 (map<double, double>());  // { dg-error "could not convert .* from .map<double,double>. to .map<int,int>." }
+  /* { dg-begin-multiline-output "" }
+  map<
+    [double != int],
+    [double != int]>
+     { dg-end-multiline-output "" } */
+
+  fn_3 (vector<map<int, float> >());
+  fn_3 (vector<map<int, double> >());  // { dg-error "could not convert .* from 'vector<map<.\\.\\.\\..,double>>' to 'vector<map<.\\.\\.\\..,float>>'" }
+  /* { dg-begin-multiline-output "" }
+  vector<
+    map<
+      [...],
+      [double != float]>>
+     { dg-end-multiline-output "" } */
+}
diff --git a/gcc/tree-diagnostic.c b/gcc/tree-diagnostic.c
index 4f211ed..bc225fb 100644
--- a/gcc/tree-diagnostic.c
+++ b/gcc/tree-diagnostic.c
@@ -245,7 +245,8 @@  virt_loc_aware_diagnostic_finalizer (diagnostic_context *context,
 /* Default tree printer.   Handles declarations only.  */
 bool
 default_tree_printer (pretty_printer *pp, text_info *text, const char *spec,
-		      int precision, bool wide, bool set_locus, bool hash)
+		      int precision, bool wide, bool set_locus, bool hash,
+		      const char **)
 {
   tree t;
 
diff --git a/gcc/tree-diagnostic.h b/gcc/tree-diagnostic.h
index e95183f..03a1e5b 100644
--- a/gcc/tree-diagnostic.h
+++ b/gcc/tree-diagnostic.h
@@ -55,6 +55,6 @@  void virt_loc_aware_diagnostic_finalizer (diagnostic_context *,
 
 void tree_diagnostics_defaults (diagnostic_context *context);
 bool default_tree_printer (pretty_printer *, text_info *, const char *,
-			   int, bool, bool, bool);
+			   int, bool, bool, bool, const char **);
 
 #endif /* ! GCC_TREE_DIAGNOSTIC_H */