[2/2,v2] c++: UX improvements for close matches in print_candidates

  Thanks Patrick and Jason.  Here's an updated version, incorporating
your feedback.

Changed in v2:
* be consistent about using C-style comments
* use same_type_p rather than pointer comparison
* move class decl_mismatch_context out of function
* use auto_vec rather than std::vector
* handle void_type in param list by setting m_variadic, rather
  than adding it to vec
* add test cases of variadic vs non-variadic

Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
OK for trunk?

Thanks
Dave


Blurb from v1:

This patch improves the UX for various cases of "no declaration matches"
where print_candidates encounters a close match.

For example, consider the const vs non-const here:

class foo
{
public:
  void test (int i, int j, void *ptr, int k);
};

// Wrong "const"-ness of param 3.
void foo::test (int i, int j, const void *ptr, int k)
{
}

where we emit (with indentation provided by the prior patch):

test.cc:8:6: error: no declaration matches ‘void foo::test(int, int, const void*, int)’
    8 | void foo::test (int i, int j, const void *ptr, int k)
      |      ^~~
  • there is 1 candidate
    • candidate is: ‘void foo::test(int, int, void*, int)’
      test.cc:4:8:
          4 |   void test (int i, int j, void *ptr, int k);
            |        ^~~~
test.cc:1:7: note: ‘class foo’ defined here
    1 | class foo
      |       ^~~

which requires the user to look through the pairs of parameters
and try to find the mismatch by eye.

This patch adds notes identifying that parameter 3 has the mismatch, and
what the mismatch is, using a pair of colors to highlight and contrast
the type mismatch.

test.cc:8:6: error: no declaration matches ‘void foo::test(int, int, const void*, int)’
    8 | void foo::test (int i, int j, const void *ptr, int k)
      |      ^~~
  • there is 1 candidate
    • candidate is: ‘void foo::test(int, int, void*, int)’
      test.cc:4:8:
          4 |   void test (int i, int j, void *ptr, int k);
            |        ^~~~
      • parameter 3 of candidate has type ‘void*’...
        test.cc:4:34:
            4 |   void test (int i, int j, void *ptr, int k);
              |                            ~~~~~~^~~
      • ...which does not match type ‘const void*’
        test.cc:8:43:
            8 | void foo::test (int i, int j, const void *ptr, int k)
              |                               ~~~~~~~~~~~~^~~
test.cc:1:7: note: ‘class foo’ defined here
    1 | class foo
      |       ^~~

This also works for the "this" case, improving the UX for messing up
const vs non-const between decls and defns of member functions; see
bad-fndef-2.C for an example.

For screenshots showing the colorization effect, see slides 9-12 of my
Cauldron talk:
https://gcc.gnu.org/wiki/cauldron2025#What.27s_new_with_diagnostics_in_GCC_16
("Hierarchical diagnostics (not yet in trunk)").

gcc/cp/ChangeLog:
	* call.cc (get_fndecl_argument_location): Use DECL_SOURCE_LOCATION
	for "this".
	* cp-tree.h (class candidate_context): New.
	(print_candidates): Add optional candidate_context param.
	* decl2.cc: Include "gcc-rich-location.h" and
	"tree-pretty-print-markup.h".
	(struct fndecl_signature): New.
	(class parm_rich_location): New.
	(class fndecl_comparison): New.
	(class decl_mismatch_context): New.
	(check_classfn): For the "no declaration matches" case, pass an
	instance of a custom candidate_context subclass to
	print_candidates, using fndecl_comparison to report on close
	matches.
	* pt.cc (print_candidates): Add optional candidate_context param.
	Use it if provided to potentially emit per-candidate notes.

gcc/testsuite/ChangeLog:
	* g++.dg/diagnostic/bad-fndef-1.C: Add directives to expect
	"void *" vs "const void *" notes about parameter 3 of the close
	candidate.
	* g++.dg/diagnostic/bad-fndef-2.C: New test.
	* g++.dg/diagnostic/bad-fndef-3.C: New test.
	* g++.dg/diagnostic/bad-fndef-4.C: New test.
	* g++.dg/diagnostic/bad-fndef-5.C: New test.
	* g++.dg/diagnostic/bad-fndef-6.C: New test.
	* g++.dg/diagnostic/bad-fndef-7.C: New test.
	* g++.dg/diagnostic/bad-fndef-8.C: New test.
	* g++.dg/diagnostic/bad-fndef-9.C: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>
---
 gcc/cp/call.cc                                |   3 +
 gcc/cp/cp-tree.h                              |  15 +-
 gcc/cp/decl2.cc                               | 198 +++++++++++++++++-
 gcc/cp/pt.cc                                  |  19 +-
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-1.C |   2 +
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C |  15 ++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C |  16 ++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C |  38 ++++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C |  15 ++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C |  17 ++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C |  14 ++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C |  14 ++
 gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C |  14 ++
 13 files changed, 374 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C
  

On 1/11/26 6:54 AM, David Malcolm wrote:
> Thanks Patrick and Jason.  Here's an updated version, incorporating
> your feedback.
> 
> Changed in v2:
> * be consistent about using C-style comments
> * use same_type_p rather than pointer comparison
> * move class decl_mismatch_context out of function
> * use auto_vec rather than std::vector
> * handle void_type in param list by setting m_variadic, rather
>    than adding it to vec

This is backwards: Ending with void_list_node indicates non-variadic.

> * add test cases of variadic vs non-variadic
> 
> Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
> OK for trunk?
> 
> Thanks
> Dave
> 
> 
> Blurb from v1:
> 
> This patch improves the UX for various cases of "no declaration matches"
> where print_candidates encounters a close match.
> 
> For example, consider the const vs non-const here:
> 
> class foo
> {
> public:
>    void test (int i, int j, void *ptr, int k);
> };
> 
> // Wrong "const"-ness of param 3.
> void foo::test (int i, int j, const void *ptr, int k)
> {
> }
> 
> where we emit (with indentation provided by the prior patch):
> 
> test.cc:8:6: error: no declaration matches ‘void foo::test(int, int, const void*, int)’
>      8 | void foo::test (int i, int j, const void *ptr, int k)
>        |      ^~~
>    • there is 1 candidate
>      • candidate is: ‘void foo::test(int, int, void*, int)’
>        test.cc:4:8:
>            4 |   void test (int i, int j, void *ptr, int k);
>              |        ^~~~
> test.cc:1:7: note: ‘class foo’ defined here
>      1 | class foo
>        |       ^~~
> 
> which requires the user to look through the pairs of parameters
> and try to find the mismatch by eye.
> 
> This patch adds notes identifying that parameter 3 has the mismatch, and
> what the mismatch is, using a pair of colors to highlight and contrast
> the type mismatch.
> 
> test.cc:8:6: error: no declaration matches ‘void foo::test(int, int, const void*, int)’
>      8 | void foo::test (int i, int j, const void *ptr, int k)
>        |      ^~~
>    • there is 1 candidate
>      • candidate is: ‘void foo::test(int, int, void*, int)’
>        test.cc:4:8:
>            4 |   void test (int i, int j, void *ptr, int k);
>              |        ^~~~
>        • parameter 3 of candidate has type ‘void*’...
>          test.cc:4:34:
>              4 |   void test (int i, int j, void *ptr, int k);
>                |                            ~~~~~~^~~
>        • ...which does not match type ‘const void*’
>          test.cc:8:43:
>              8 | void foo::test (int i, int j, const void *ptr, int k)
>                |                               ~~~~~~~~~~~~^~~
> test.cc:1:7: note: ‘class foo’ defined here
>      1 | class foo
>        |       ^~~
> 
> This also works for the "this" case, improving the UX for messing up
> const vs non-const between decls and defns of member functions; see
> bad-fndef-2.C for an example.
> 
> For screenshots showing the colorization effect, see slides 9-12 of my
> Cauldron talk:
> https://gcc.gnu.org/wiki/cauldron2025#What.27s_new_with_diagnostics_in_GCC_16
> ("Hierarchical diagnostics (not yet in trunk)").
> 
> gcc/cp/ChangeLog:
> 	* call.cc (get_fndecl_argument_location): Use DECL_SOURCE_LOCATION
> 	for "this".
> 	* cp-tree.h (class candidate_context): New.
> 	(print_candidates): Add optional candidate_context param.
> 	* decl2.cc: Include "gcc-rich-location.h" and
> 	"tree-pretty-print-markup.h".
> 	(struct fndecl_signature): New.
> 	(class parm_rich_location): New.
> 	(class fndecl_comparison): New.
> 	(class decl_mismatch_context): New.
> 	(check_classfn): For the "no declaration matches" case, pass an
> 	instance of a custom candidate_context subclass to
> 	print_candidates, using fndecl_comparison to report on close
> 	matches.
> 	* pt.cc (print_candidates): Add optional candidate_context param.
> 	Use it if provided to potentially emit per-candidate notes.
> 
> gcc/testsuite/ChangeLog:
> 	* g++.dg/diagnostic/bad-fndef-1.C: Add directives to expect
> 	"void *" vs "const void *" notes about parameter 3 of the close
> 	candidate.
> 	* g++.dg/diagnostic/bad-fndef-2.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-3.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-4.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-5.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-6.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-7.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-8.C: New test.
> 	* g++.dg/diagnostic/bad-fndef-9.C: New test.
> 
> Signed-off-by: David Malcolm <dmalcolm@redhat.com>
> ---
>   gcc/cp/call.cc                                |   3 +
>   gcc/cp/cp-tree.h                              |  15 +-
>   gcc/cp/decl2.cc                               | 198 +++++++++++++++++-
>   gcc/cp/pt.cc                                  |  19 +-
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-1.C |   2 +
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C |  15 ++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C |  16 ++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C |  38 ++++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C |  15 ++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C |  17 ++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C |  14 ++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C |  14 ++
>   gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C |  14 ++
>   13 files changed, 374 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C
>   create mode 100644 gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index fa8281f01ba..66c2dbe90d1 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -8593,6 +8593,9 @@ get_fndecl_argument_location (tree fndecl, int argnum)
>     if (DECL_ARTIFICIAL (fndecl))
>       return DECL_SOURCE_LOCATION (fndecl);
>   
> +  if (argnum == -1)
> +    return DECL_SOURCE_LOCATION (fndecl);
> +
>     int i;
>     tree param;
>   
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 01843f7fc92..931d2e5ddfd 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7942,7 +7942,20 @@ extern tree most_specialized_instantiation	(tree);
>   extern tree most_specialized_partial_spec       (tree, tsubst_flags_t, bool = false);
>   extern tree most_constrained_function		(tree);
>   extern void inform_num_candidates		(location_t, int);
> -extern void print_candidates			(location_t, tree);
> +
> +/* Abstract base class for optionally providing extra diagnostic note(s)
> +   about a candidate in calls to print_candidates.  */
> +
> +class candidate_context
> +{
> +public:
> +  virtual ~candidate_context () {}
> +  virtual void emit_any_notes_for_candidate (tree cand_fndecl) = 0;
> +};
> +
> +extern void print_candidates			(location_t, tree,
> +						 candidate_context * = nullptr);
> +
>   extern void instantiate_pending_templates	(int);
>   extern tree tsubst_default_argument		(tree, int, tree, tree,
>   						 tsubst_flags_t);
> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
> index 5eb2ed42732..c891c589d74 100644
> --- a/gcc/cp/decl2.cc
> +++ b/gcc/cp/decl2.cc
> @@ -53,6 +53,8 @@ along with GCC; see the file COPYING3.  If not see
>   #include "tree-inline.h"
>   #include "escaped_string.h"
>   #include "contracts.h"
> +#include "gcc-rich-location.h"
> +#include "tree-pretty-print-markup.h"
>   
>   /* Id for dumping the raw trees.  */
>   int raw_dump_id;
> @@ -770,6 +772,197 @@ check_member_template (tree tmpl)
>       error ("template declaration of %q#D", decl);
>   }
>   
> +/* A struct for conveniently working with the parameter types of a
> +   fndecl.  */
> +
> +struct fndecl_signature
> +{
> +  fndecl_signature (tree fndecl)
> +  : m_fndecl (fndecl),
> +    m_num_artificial_parms (num_artificial_parms_for (fndecl)),
> +    m_variadic (false)
> +  {
> +    function_args_iterator iter;
> +    tree argtype;
> +    FOREACH_FUNCTION_ARGS (TREE_TYPE (fndecl), argtype, iter)
> +      if (VOID_TYPE_P (argtype))
> +	m_variadic = true;
> +      else
> +	m_parm_types.safe_push (argtype);
> +    gcc_assert (m_parm_types.length () >= (size_t)m_num_artificial_parms);
> +    m_num_user_parms = m_parm_types.length () - m_num_artificial_parms;
> +  }
> +
> +  /* Convert from index within DECL_ARGUMENTS to 0-based user-written
> +     parameter, or -1 for "this".  */
> +  int
> +  get_argno_from_idx (size_t idx) const
> +  {
> +    return static_cast<int> (idx) - m_num_artificial_parms;
> +  }
> +
> +  location_t
> +  get_argno_location (int argno) const
> +  {
> +    return get_fndecl_argument_location (m_fndecl, argno);
> +  }
> +
> +  tree m_fndecl;
> +  int m_num_artificial_parms;
> +  auto_vec<tree> m_parm_types;
> +  size_t m_num_user_parms;
> +  bool m_variadic;
> +};
> +
> +/* A rich_location subclass that highlights a given argno
> +   within FNDECL_SIG using HIGHLIGHT_COLOR in the quoted source.  */
> +
> +class parm_rich_location : public gcc_rich_location
> +{
> +public:
> +  parm_rich_location (const fndecl_signature &fndecl_sig,
> +		      int argno,
> +		      const char *highlight_color)
> +  : gcc_rich_location (fndecl_sig.get_argno_location (argno),
> +		       nullptr,
> +		       highlight_color)
> +  {
> +  }
> +};
> +
> +/* A class for comparing a pair of fndecls and emitting diagnostic notes
> +   about the differences between them, if they are sufficiently close.  */
> +
> +class fndecl_comparison
> +{
> +public:
> +  fndecl_comparison (tree decl_a,
> +		     tree decl_b)
> +  : m_decl_a (decl_a),
> +    m_decl_b (decl_b)
> +  {
> +    gcc_assert (TREE_CODE (decl_a) == FUNCTION_DECL);
> +    gcc_assert (TREE_CODE (decl_b) == FUNCTION_DECL);
> +
> +  }
> +
> +  /* Attempt to emit diagnostic notes describing the differences between
> +     the fndecls, when they are a sufficiently close match.  */
> +  void
> +  maybe_emit_notes_for_close_match () const
> +  {
> +    auto_vec<parm_difference> differences = get_differences ();
> +    if (differences.length () == 1)
> +      {
> +	auto_diagnostic_nesting_level sentinel;
> +	print_parm_type_difference (differences[0]);
> +      }
> +  }
> +
> +private:
> +  struct parm_difference
> +  {
> +    size_t m_parm_idx_a;
> +    size_t m_parm_idx_b;
> +  };
> +
> +  /* If we have a close match, return the differences between the two
> +     fndecls.  Otherwise return an empty vector.  */
> +  auto_vec<parm_difference>
> +  get_differences () const
> +  {
> +    auto_vec<parm_difference> differences;
> +
> +    /* For now, just handle the case where the "user parm" count is
> +       equal.  */
> +    if (m_decl_a.m_num_user_parms != m_decl_b.m_num_user_parms)
> +      return differences;
> +
> +    /* Ideally we'd check the edit distance, and thus report on close
> +       matches which are missing a param, have transposed params, etc.
> +       For now, just iterate through the params, finding differences
> +       elementwise.  */
> +
> +    /* Find differences in artificial params, if they are comparable.
> +       This should find e.g. const vs non-const differences in "this".  */
> +    if (m_decl_a.m_num_artificial_parms == m_decl_b.m_num_artificial_parms)
> +      for (int parm_idx = 0;
> +	   parm_idx < m_decl_a.m_num_artificial_parms;
> +	   ++parm_idx)
> +	compare (parm_idx, parm_idx, differences);
> +
> +    /* Find differences in user-provided params.  */
> +    for (size_t user_parm_idx = 0;
> +	 user_parm_idx < m_decl_a.m_num_user_parms;
> +	 ++user_parm_idx)
> +      {
> +	const size_t idx_a = m_decl_a.m_num_artificial_parms + user_parm_idx;
> +	const size_t idx_b = m_decl_b.m_num_artificial_parms + user_parm_idx;
> +	compare (idx_a, idx_b, differences);
> +      }
> +
> +    return differences;
> +  }
> +
> +  void
> +  compare (size_t idx_a, size_t idx_b,
> +	   auto_vec<parm_difference> &differences) const
> +  {
> +    if (!same_type_p (m_decl_a.m_parm_types[idx_a],
> +		      m_decl_b.m_parm_types[idx_b]))
> +      differences.safe_push (parm_difference {idx_a, idx_b});
> +  }
> +
> +  void
> +  print_parm_type_difference (const parm_difference &diff) const
> +  {
> +    const char * const highlight_a = "highlight-a";
> +    pp_markup::element_quoted_type type_of_parm_a
> +      (m_decl_a.m_parm_types[diff.m_parm_idx_a],
> +       highlight_a);
> +    const int argno_a = m_decl_a.get_argno_from_idx (diff.m_parm_idx_a);
> +    parm_rich_location rich_loc_a (m_decl_a, argno_a, highlight_a);
> +    inform (&rich_loc_a,
> +	    "parameter %P of candidate has type %e...",
> +	    argno_a, &type_of_parm_a);
> +
> +    const char * const highlight_b = "highlight-b";
> +    pp_markup::element_quoted_type type_of_parm_b
> +      (m_decl_b.m_parm_types[diff.m_parm_idx_b],
> +       highlight_b);
> +    const int argno_b = m_decl_b.get_argno_from_idx (diff.m_parm_idx_b);
> +    parm_rich_location rich_loc_b (m_decl_b, argno_b, highlight_b);
> +    inform (&rich_loc_b,
> +	    "...which does not match type %e",
> +	    &type_of_parm_b);
> +  }
> +
> +  fndecl_signature m_decl_a;
> +  fndecl_signature m_decl_b;
> +};
> +
> +class decl_mismatch_context : public candidate_context
> +{
> +public:
> +  decl_mismatch_context (tree function)
> +  : m_function (function)
> +  {
> +    gcc_assert (TREE_CODE (function) == FUNCTION_DECL);
> +  }
> +
> +  void
> +  emit_any_notes_for_candidate (tree cand) final override
> +  {
> +    if (TREE_CODE (cand) == FUNCTION_DECL)
> +      {
> +	fndecl_comparison diff (cand, m_function);
> +	diff.maybe_emit_notes_for_close_match ();
> +      }
> +  }
> +private:
> +  tree m_function;
> +};
> +
>   /* Sanity check: report error if this function FUNCTION is not
>      really a member of the class (CTYPE) it is supposed to belong to.
>      TEMPLATE_PARMS is used to specify the template parameters of a member
> @@ -917,7 +1110,10 @@ check_classfn (tree ctype, tree function, tree template_parms)
>   	  error_at (DECL_SOURCE_LOCATION (function),
>   		    "no declaration matches %q#D", function);
>   	  if (fns)
> -	    print_candidates (DECL_SOURCE_LOCATION (function), fns);
> +	    {
> +	      decl_mismatch_context ctxt (function);
> +	      print_candidates (DECL_SOURCE_LOCATION (function), fns, &ctxt);
> +	    }
>   	  else if (DECL_CONV_FN_P (function))
>   	    inform (DECL_SOURCE_LOCATION (function),
>   		    "no conversion operators declared");
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 9bc15527ea4..0b7cc36ef33 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -2065,10 +2065,15 @@ inform_num_candidates (location_t loc, int num_candidates)
>   }
>   
>   /* Print the list of candidate FNS in an error message.  FNS can also
> -   be a TREE_LIST of non-functions in the case of an ambiguous lookup.  */
> +   be a TREE_LIST of non-functions in the case of an ambiguous lookup.
> +
> +   If CAND_CTXT is non-null, use it for each candidate to allow for
> +   additional per-candidate notes.  */
>   
>   void
> -print_candidates (location_t error_loc, tree fns)
> +print_candidates (location_t error_loc,
> +		  tree fns,
> +		  candidate_context *cand_ctxt)
>   {
>     auto_vec<tree> candidates;
>     flatten_candidates (fns, candidates);
> @@ -2083,13 +2088,19 @@ print_candidates (location_t error_loc, tree fns)
>       {
>         tree cand = candidates[0];
>         inform (DECL_SOURCE_LOCATION (cand), "candidate is: %#qD", cand);
> +      if (cand_ctxt)
> +	cand_ctxt->emit_any_notes_for_candidate (cand);
>       }
>     else
>       {
>         int idx = 0;
>         for (tree cand : candidates)
> -	inform (DECL_SOURCE_LOCATION (cand), "candidate %i: %#qD",
> -		++idx, cand);
> +	{
> +	  inform (DECL_SOURCE_LOCATION (cand), "candidate %i: %#qD",
> +		  ++idx, cand);
> +	  if (cand_ctxt)
> +	    cand_ctxt->emit_any_notes_for_candidate (cand);
> +	}
>       }
>   }
>   
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-1.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-1.C
> index 1156072b11f..60d6c5baa0b 100644
> --- a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-1.C
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-1.C
> @@ -13,3 +13,5 @@ void foo::test (int i, int j, const void *ptr, int k) // { dg-line defn }
>   // { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
>   // { dg-message "8: candidate 1: " "candidate 1" { target *-*-* } other_decl }
>   // { dg-message "8: candidate 2: " "candidate 2" { target *-*-* } close_decl }
> +// { dg-message "34: parameter 3 of candidate has type 'void\\*'" "param of decl" { target *-*-* } close_decl }
> +// { dg-message "43: \\.\\.\\.which does not match type 'const void\\*'" "param of defn" { target *-*-* } defn }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C
> new file mode 100644
> index 00000000000..068b392928d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-2.C
> @@ -0,0 +1,15 @@
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, int j, int k); // { dg-line decl }
> +};
> +
> +// Wrong "const"-ness of this:
> +void foo::test (int i, int j, int k) const // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }
> +// { dg-message "8: parameter 'this' of candidate has type 'foo\\*'" "this of decl" { target *-*-* } decl }
> +// { dg-message "6: \\.\\.\\.which does not match type 'const foo\\*'" "this of defn" { target *-*-* } defn }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C
> new file mode 100644
> index 00000000000..452334dc572
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-3.C
> @@ -0,0 +1,16 @@
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, int j, int k) const; // { dg-line decl }
> +};
> +
> +// Wrong "const"-ness of "this":
> +
> +void foo::test (int i, int j, int k) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }
> +// { dg-message "8: parameter 'this' of candidate has type 'const foo\\*'" "this of decl" { target *-*-* } decl }
> +// { dg-message "6: \\.\\.\\.which does not match type 'foo\\*'" "this of defn" { target *-*-* } defn }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C
> new file mode 100644
> index 00000000000..afbcb76cb28
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-4.C
> @@ -0,0 +1,38 @@
> +// { dg-additional-options "-fdiagnostics-show-caret" }
> +
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, int j, void *ptr, int k); // { dg-line decl }
> +};
> +
> +// Wrong "const"-ness of a param (param 3).
> +void foo::test (int i, int j, const void *ptr, int k) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +/* { dg-begin-multiline-output "" }
> + void foo::test (int i, int j, const void *ptr, int k)
> +      ^~~
> +   { dg-end-multiline-output "" } */
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }
> +/* { dg-begin-multiline-output "" }
> +   void test (int i, int j, void *ptr, int k);
> +        ^~~~
> +   { dg-end-multiline-output "" } */
> +// { dg-message "34: parameter 3 of candidate has type 'void\\*'" "param of decl" { target *-*-* } decl }
> +/* { dg-begin-multiline-output "" }
> +   void test (int i, int j, void *ptr, int k);
> +                            ~~~~~~^~~
> +   { dg-end-multiline-output "" } */
> +// { dg-message "43: \\.\\.\\.which does not match type 'const void\\*'" "param of defn" { target *-*-* } defn }
> +/* { dg-begin-multiline-output "" }
> + void foo::test (int i, int j, const void *ptr, int k)
> +                               ~~~~~~~~~~~~^~~
> +   { dg-end-multiline-output "" } */
> +
> +/* { dg-begin-multiline-output "" }
> + class foo
> +       ^~~
> +   { dg-end-multiline-output "" } */
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C
> new file mode 100644
> index 00000000000..498395f4e15
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-5.C
> @@ -0,0 +1,15 @@
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  static void test (int i, int j, void *ptr, int k); // { dg-line decl }
> +};
> +
> +// Wrong "const"-ness of a param, for static member fn (param 3).
> +void foo::test (int i, int j, const void *ptr, int k) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "15: candidate is: " "candidate is" { target *-*-* } decl }
> +// { dg-message "41: parameter 3 of candidate has type 'void\\*'" "param of decl" { target *-*-* } decl }
> +// { dg-message "43: \\.\\.\\.which does not match type 'const void\\*'" "param of defn" { target *-*-* } defn }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C
> new file mode 100644
> index 00000000000..5a34395180f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-6.C
> @@ -0,0 +1,17 @@
> +class bar;
> +
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, foo *j, int k); // { dg-line decl }
> +};
> +
> +// Missing '*' on a param (param 2).
> +void foo::test (int i, foo j, int k) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }
> +// { dg-message "26: parameter 2 of candidate has type 'foo\\*'" "param of decl" { target *-*-* } decl }
> +// { dg-message "28: \\.\\.\\.which does not match type 'foo'" "param of defn" { target *-*-* } defn }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C
> new file mode 100644
> index 00000000000..12dee583aa3
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-7.C
> @@ -0,0 +1,14 @@
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, int j, int k, ...);            // { dg-line decl }
> +};
> +
> +// Variadic vs non-variadic
> +
> +void foo::test (int i, int j, int k) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C
> new file mode 100644
> index 00000000000..a484864dbac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-8.C
> @@ -0,0 +1,14 @@
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, int j, int k);            // { dg-line decl }
> +};
> +
> +// Variadic vs non-variadic
> +
> +void foo::test (int i, int j, int k, ...) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }
> diff --git a/gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C
> new file mode 100644
> index 00000000000..cdd691c49c0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/bad-fndef-9.C
> @@ -0,0 +1,14 @@
> +class foo // { dg-message "'class foo' defined here" }
> +{
> +public:
> +  void test (int i, int j, int k);            // { dg-line decl }
> +};
> +
> +// Variadic vs non-variadic
> +
> +void foo::test (int i, int j, ...) // { dg-line defn }
> +{
> +}
> +
> +// { dg-error "6: no declaration matches" "error" { target *-*-* } defn }
> +// { dg-message "8: candidate is: " "candidate is" { target *-*-* } decl }