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From: Marek Polacek <polacek@redhat.com>
To: GCC Patches <gcc-patches@gcc.gnu.org>, Jason Merrill <jason@redhat.com>
Subject: [PATCH] c++: tweaks for explicit conversion fns diagnostic
Date: Fri, 25 Aug 2023 19:37:46 -0400	[thread overview]
Message-ID: <20230825233746.904433-1-polacek@redhat.com> (raw)

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --

1) When saying that a conversion is erroneous because it would use
an explicit constructor, it might be nice to show where exactly
the explicit constructor is located.  For example, with this patch:

[...]
explicit.C:4:12: note: 'S::S(int)' declared here
    4 |   explicit S(int) { }
      |            ^

2) When a conversion doesn't work out merely because the conversion
function necessary to do the conversion couldn't be used because
it was marked explicit, it would be useful to the user to say so,
rather than just saying "cannot convert".  For example, with this patch:

explicit.C:13:12: error: cannot convert 'S' to 'bool' in initialization
   13 |   bool b = S{1};
      |            ^~~~
      |            |
      |            S
explicit.C:5:12: note: explicit conversion function was not considered
    5 |   explicit operator bool() const { return true; }
      |            ^~~~~~~~

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal): Show where the conversion function
	was declared.
	(maybe_show_nonconverting_candidate): New.
	* cp-tree.h (maybe_show_nonconverting_candidate): Declare.
	* typeck.cc (convert_for_assignment): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/explicit.C: New test.
---
 gcc/cp/call.cc                             | 41 +++++++++++++++++++---
 gcc/cp/cp-tree.h                           |  1 +
 gcc/cp/typeck.cc                           |  5 +++
 gcc/testsuite/g++.dg/diagnostic/explicit.C | 16 +++++++++
 4 files changed, 59 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/explicit.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 23e458d3252..09ebcf6a115 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -8459,12 +8459,21 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
 		if (pedwarn (loc, 0, "converting to %qT from initializer list "
 			     "would use explicit constructor %qD",
 			     totype, convfn))
-		  inform (loc, "in C++11 and above a default constructor "
-			  "can be explicit");
+		  {
+		    inform (loc, "in C++11 and above a default constructor "
+			    "can be explicit");
+		    inform (DECL_SOURCE_LOCATION (convfn), "%qD declared here",
+			    convfn);
+		  }
 	      }
 	    else
-	      error ("converting to %qT from initializer list would use "
-		     "explicit constructor %qD", totype, convfn);
+	      {
+		auto_diagnostic_group d;
+		error ("converting to %qT from initializer list would use "
+		       "explicit constructor %qD", totype, convfn);
+		inform (DECL_SOURCE_LOCATION (convfn), "%qD declared here",
+			convfn);
+	      }
 	  }
 
 	/* If we're initializing from {}, it's value-initialization.  */
@@ -14323,4 +14332,28 @@ is_list_ctor (tree decl)
   return true;
 }
 
+/* We know that can_convert_arg_bad already said "no" when trying to convert
+   FROM to TO with ARG and FLAGS.  Try to figure out if it was because
+   an explicit conversion function was skipped when looking for a way to
+   perform the conversion.  At this point we've already printed an error.  */
+
+void
+maybe_show_nonconverting_candidate (tree to, tree from, tree arg, int flags)
+{
+  if (!(flags & LOOKUP_ONLYCONVERTING))
+    return;
+
+  conversion_obstack_sentinel cos;
+  conversion *c = implicit_conversion (to, from, arg, /*c_cast_p=*/false,
+				       flags & ~LOOKUP_ONLYCONVERTING, tf_none);
+  if (c && !c->bad_p && c->user_conv_p)
+    /* Ay, the conversion would have worked in copy-init context.  */
+    for (; c; c = next_conversion (c))
+      if (c->kind == ck_user
+	  && DECL_P (c->cand->fn)
+	  && DECL_NONCONVERTING_P (c->cand->fn))
+	inform (DECL_SOURCE_LOCATION (c->cand->fn), "explicit conversion "
+		"function was not considered");
+}
+
 #include "gt-cp-call.h"
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 608d6310e53..6b225ca182f 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6727,6 +6727,7 @@ extern bool cp_handle_deprecated_or_unavailable (tree, tsubst_flags_t = tf_warni
 extern void cp_warn_deprecated_use_scopes	(tree);
 extern tree get_function_version_dispatcher	(tree);
 extern bool any_template_arguments_need_structural_equality_p (tree);
+extern void maybe_show_nonconverting_candidate	(tree, tree, tree, int);
 
 /* in class.cc */
 extern tree build_vfield_ref			(tree, tree);
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index d5c0c85ed51..bf9963db3ba 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10262,6 +10262,8 @@ convert_for_assignment (tree type, tree rhs,
 		{
 		  range_label_for_type_mismatch label (rhstype, type);
 		  gcc_rich_location richloc (rhs_loc, has_loc ? &label : NULL);
+		  auto_diagnostic_group d;
+
 		  switch (errtype)
 		    {
 		    case ICR_DEFAULT_ARGUMENT:
@@ -10295,6 +10297,9 @@ convert_for_assignment (tree type, tree rhs,
 		    default:
 		      gcc_unreachable();
 		  }
+
+		  /* See if we can be more helpful.  */
+		  maybe_show_nonconverting_candidate (type, rhstype, rhs, flags);
 		}
 	      if (TYPE_PTR_P (rhstype)
 		  && TYPE_PTR_P (type)
diff --git a/gcc/testsuite/g++.dg/diagnostic/explicit.C b/gcc/testsuite/g++.dg/diagnostic/explicit.C
new file mode 100644
index 00000000000..fde582b8e1c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/explicit.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+
+struct S {
+  explicit S(int) { }
+  explicit operator bool() const { return true; } // { dg-message "explicit conversion function was not considered" }
+  explicit operator int() const { return 42; } // { dg-message "explicit conversion function was not considered" }
+};
+
+void
+g ()
+{
+  S s = {1}; // { dg-error "would use explicit constructor" }
+  bool b = S{1}; // { dg-error "cannot convert .S. to .bool. in initialization" }
+  int i;
+  i = S{2}; // { dg-error "cannot convert .S. to .int. in assignment" }
+}

base-commit: e1f096a3cc96c71907cfbc7b8baf67a3d863cb6d
-- 
2.41.0


             reply	other threads:[~2023-08-25 23:37 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-25 23:37 Marek Polacek [this message]
2023-08-26  0:34 ` Jason Merrill
2023-08-28 23:24   ` [PATCH v2] " Marek Polacek
2023-08-29 20:42     ` Jason Merrill
2023-08-29 21:45       ` Marek Polacek

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