On Sat, 15 Nov 2025, David Malcolm wrote:
> 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)").
>
> Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
>
> OK for trunk?
> Dave
The OOP style of this patch deviates from
>
> 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: Define INCLUDE_VECTOR. Include "gcc-rich-location.h"
> and "tree-pretty-print-markup.h".
> (struct fndecl_signature): New.
> (class parm_rich_location): New.
> (class fndecl_comparison): 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.
>
> Signed-off-by: David Malcolm <[email protected]>
> ---
> gcc/cp/call.cc | 3 +
> gcc/cp/cp-tree.h | 14 +-
> gcc/cp/decl2.cc | 192 +++++++++++++++++-
> gcc/cp/pt.cc | 17 +-
> 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 ++
> 10 files changed, 323 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
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index fc4e6da1f9a20..ab37530a42917 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 d0d057c927017..f5fc8bab6eaeb 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7891,7 +7891,19 @@ 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 (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 (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 9e135af41b340..1a123c8cda311 100644
> --- a/gcc/cp/decl2.cc
> +++ b/gcc/cp/decl2.cc
> @@ -27,6 +27,7 @@ along with GCC; see the file COPYING3. If not see
> line numbers. For example, the CONST_DECLs for enum values. */
>
> #include "config.h"
> +#define INCLUDE_VECTOR
> #include "system.h"
> #include "coretypes.h"
> #include "memmodel.h"
> @@ -53,6 +54,8 @@ 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 +773,170 @@ 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))
> + {
> + function_args_iterator iter;
> + tree argtype;
> + FOREACH_FUNCTION_ARGS (TREE_TYPE (fndecl), argtype, iter)
I was not familiar with this macro and looks like it'd be the first use
of it within the C++ frontend. I'd prefer to iterate over the
TYPE_ARG_TYPES list directly for simplicity.
How does this handle difference in variadicness (i.e. missing , ...)?
Non-variadic functions have void_list_node as the last parm type.
> + 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;
> +};
> +
> +/* 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
> + {
> + std::vector<parm_difference> differences = get_differences ();
Just curious, why std::vector instead of auto_vec is in other parts of
this patch and previous patch?
> + if (differences.size () == 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. */
> + std::vector<parm_difference>
> + get_differences () const
> + {
> + std::vector<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,
> + std::vector<parm_difference> &differences) const
> + {
> + if (m_decl_a.m_parm_types[idx_a]
> + != m_decl_b.m_parm_types[idx_b])
> + differences.push_back (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;
> +};
> +
> /* 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 +1084,30 @@ 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 (fns);
> + {
> + 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;
> + } ctxt (function);
> + print_candidates (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 59e9585aac94d..9a94ddf7ccf44 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -2060,10 +2060,13 @@ 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 (tree fns)
> +print_candidates (tree fns, candidate_context *cand_ctxt)
> {
> auto_vec<tree> candidates;
> flatten_candidates (fns, candidates);
> @@ -2078,13 +2081,19 @@ print_candidates (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 (auto 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 1156072b11f16..60d6c5baa0bb9 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 0000000000000..068b392928dc0
> --- /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 0000000000000..452334dc5728c
> --- /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 0000000000000..afbcb76cb28af
> --- /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 0000000000000..498395f4e1525
> --- /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 0000000000000..5a34395180fa5
> --- /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 }
> --
> 2.26.3
>
>
