gdb/c++: Improve error messages in overload resolution

When resolving overloaded functions, GDB relies on knowing relationships
between types, i.e. if a type inherits from another. However, some
compilers may not add complete information for given types as a way to
reduce unnecessary debug information. In these cases, GDB would just say
that it couldn't resolve the method or function, with no extra
information.

The problem is that sometimes the user may not know that the type
information is incomplete, and may just assume that there is a bug in
GDB. To improve the user experience, we attempt to detect if the
overload match failed because of an incomplete type, and warn the user
of this.

This commit also adds a testcase confirming that the message is only
triggered in the correct scenario. This test was not developed as an
expansion of gdb.cp/overload.cc because it needed the dwarf assembler,
and porting all of overload.cc seemed unnecessary.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.cp/incomplete-type-overload.cc

@@ -0,0 +1,45 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+class
+base {
+public:
+  int member;
+};
+
+class complete: public base { };
+
+class incomplete: public base { };
+
+complete comp;
+complete *cp = &comp;
+incomplete *inc;
+int *ip;
+
+int
+foo (base* b)
+{
+    asm ("foo_label: .globl foo_label");
+    return 1;
+}
+
+int
+main (void)
+{
+    asm("main_label: .globl main_label");
+    comp.member = 0;
+    return 0;
+}

gdb/testsuite/gdb.cp/incomplete-type-overload.exp

@@ -0,0 +1,183 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of GDB's testsuite.
+
+# This test intends to check the error message that GDB emits when unable
+# to determine the correct overloaded function due to incomplete types.
+
+load_lib dwarf.exp
+
+if { [skip_cplus_tests] } { return }
+
+if { ![dwarf2_support] } { return }
+
+standard_testfile .cc .S
+set asm_file [standard_output_file ${srcfile2}]
+
+if [prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}] {
+    return
+}
+
+if {[test_compiler_info clang-*-*]} {
+    untested "gcc is required for dwarf assembler tests"
+    return
+}
+
+if ![runto_main] {
+    return
+}
+
+# Get important sizes to create fake dwarf for the test
+set int_size [get_sizeof "int" -1]
+set addr_size [get_sizeof "void *" -1]
+set struct_base_size [get_sizeof "base" 4]
+set struct_complete_size [get_sizeof "complete" 4]
+get_func_info foo
+
+# Create fake DWARF for the .cc file.
+# This is the best way to ensure we have an incomplete type.
+Dwarf::assemble ${asm_file} {
+    global srcdir subdir srcfile srcfile2 foo_start foo_end
+    global int_size addr_size struct_base_size struct_complete_size
+    declare_labels L
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	    {name $srcfile}
+	    {stmt_list $L DW_FORM_sec_offset}
+	} {
+	    declare_labels int_label base_label complete_label incomplete_label
+	    declare_labels ptr_base_label ptr_inc_label ptr_comp_label ptr_int_label
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size $int_size DW_FORM_sdata}
+		{DW_AT_encoding @DW_ATE_signed}
+		{DW_AT_name "int"}
+	    }
+
+	    base_label: DW_TAG_class_type {
+		{DW_AT_byte_size $struct_base_size DW_FORM_sdata}
+		{DW_AT_name "base"}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name "member"}
+		    {DW_AT_type :$int_label}
+		    {DW_AT_data_member_location 0 DW_FORM_sdata}
+		}
+	    }
+
+	    complete_label: DW_TAG_class_type {
+		{DW_AT_byte_size $struct_complete_size DW_FORM_sdata}
+		{DW_AT_name "complete"}
+	    } {
+		DW_TAG_inheritance {
+		    {DW_AT_type :$base_label}
+		    {DW_AT_data_member_location 0 DW_FORM_sdata}
+		    {DW_AT_accessibility 1 DW_FORM_data1}
+		}
+	    }
+
+	    incomplete_label: DW_TAG_class_type {
+		{DW_AT_name "incomplete"}
+		{DW_AT_declaration 1 DW_FORM_flag_present}
+	    }
+
+	    ptr_base_label: DW_TAG_pointer_type {
+		{DW_AT_byte_size $addr_size DW_FORM_udata}
+		{DW_AT_type :$base_label}
+	    }
+
+	    ptr_inc_label: DW_TAG_pointer_type {
+		{DW_AT_byte_size $addr_size DW_FORM_udata}
+		{DW_AT_type :$incomplete_label}
+	    }
+
+	    ptr_comp_label: DW_TAG_pointer_type {
+		{DW_AT_byte_size $addr_size DW_FORM_udata}
+		{DW_AT_type :$complete_label}
+	    }
+
+	    ptr_int_label: DW_TAG_pointer_type {
+		{DW_AT_byte_size $addr_size DW_FORM_udata}
+		{DW_AT_type :$int_label}
+	    }
+
+	DW_TAG_variable {
+	    {DW_AT_name "comp"}
+	    {DW_AT_type :$complete_label}
+	    {DW_AT_location {DW_OP_addr [gdb_target_symbol "comp"]} SPECIAL_expr}
+	    {DW_AT_external 1 DW_FORM_flag}
+	}
+
+	DW_TAG_variable {
+	    {DW_AT_name "cp"}
+	    {DW_AT_type :$ptr_comp_label}
+	    {DW_AT_location {DW_OP_addr [gdb_target_symbol "cp"]} SPECIAL_expr}
+	    {DW_AT_external 1 DW_FORM_flag}
+	}
+
+	DW_TAG_variable {
+	    {DW_AT_name "inc"}
+	    {DW_AT_type :$ptr_inc_label}
+	    {DW_AT_location {DW_OP_addr [gdb_target_symbol "inc"]} SPECIAL_expr}
+	    {DW_AT_external 1 DW_FORM_flag}
+	}
+
+	DW_TAG_variable {
+	    {DW_AT_name "ip"}
+	    {DW_AT_type :$ptr_int_label}
+	    {DW_AT_location {DW_OP_addr [gdb_target_symbol "ip"]} SPECIAL_expr}
+	    {DW_AT_external 1 DW_FORM_flag}
+	}
+
+	    DW_TAG_subprogram {
+		{MACRO_AT_func {"main"}}
+		{DW_AT_external 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{MACRO_AT_func {"foo"}}
+		{DW_AT_type :$int_label}
+		{DW_AT_external 1 flag}
+	    } { formal_parameter {
+		    {DW_AT_name "b"}
+		    {DW_AT_type :$ptr_base_label}
+		}
+	    }
+	}
+    }
+
+    lines {version 2} L {
+	include_dir "$srcdir/$subdir"
+	file_name $srcfile 1
+    }
+}
+
+if [prepare_for_testing "failed to prepare" $testfile [list $asm_file $srcfile] {}] {
+    return
+}
+
+if ![runto_main] {
+    return
+}
+
+gdb_test "print foo(cp)" "= 1" "successful invocation"
+gdb_test "print foo(inc)"\
+	 "The type. 'incomplete .' isn't fully known to GDB.*"\
+	 "unsuccessful because declaration"
+gdb_test "print foo(ip)"\
+	 "Cannot resolve function foo to any overloaded instance."\
+	 "unsuccessful because incorrect"

gdb/valops.c

@@ -41,6 +41,7 @@
 #include "extension.h"
 #include "gdbtypes.h"
 #include "gdbsupport/byte-vector.h"
+#include "typeprint.h"
 
 /* Local functions.  */
 
@@ -2617,6 +2618,49 @@ value_find_oload_method_list (struct value **argp, const char *method,
 		    basetype, boffset);
 }
 
+/* Helper function for find_overload_match.  If no matches were
+   found, this function may generate a hint for the user that some
+   of the relevant types are incomplete, so GDB can't evaluate
+   type relationships to properly evaluate overloads.
+
+   If no incomplete types are present, an empty string is returned.  */
+static std::string
+incomplete_type_hint (gdb::array_view<value *> args)
+{
+  int incomplete_types = 0;
+  std::string incomplete_arg_names;
+  for (const struct value *arg : args)
+    {
+      struct type *t = value_type (arg);
+      while (t->code () == TYPE_CODE_PTR)
+	t = t->target_type ();
+      if (t->is_stub ())
+	{
+	  string_file buffer;
+	  if (incomplete_types > 0)
+	    incomplete_arg_names += ", ";
+
+	  current_language->print_type (value_type (arg), "", &buffer,
+				       -1, 0, &type_print_raw_options);
+
+	  incomplete_types++;
+	  incomplete_arg_names += buffer.string ();
+	}
+    }
+  std::string hint;
+  if (incomplete_types > 1)
+    hint = string_printf (_("\nThe types: '%s' aren't fully known to GDB."
+			    " Please cast them directly to the desired"
+			    " typed in the function call."),
+			    incomplete_arg_names.c_str ());
+  else if (incomplete_types == 1)
+    hint = string_printf (_("\nThe type: '%s' isn't fully known to GDB."
+			    " Please cast it directly to the desired"
+			    " typed in the function call."),
+			    incomplete_arg_names.c_str ());
+  return hint;
+}
+
 /* Given an array of arguments (ARGS) (which includes an entry for
    "this" in the case of C++ methods), the NAME of a function, and
    whether it's a method or not (METHOD), find the best function that
@@ -2933,14 +2977,15 @@ find_overload_match (gdb::array_view<value *> args,
 
   if (match_quality == INCOMPATIBLE)
     {
+      std::string hint = incomplete_type_hint (args);
       if (method == METHOD)
-	error (_("Cannot resolve method %s%s%s to any overloaded instance"),
+	error (_("Cannot resolve method %s%s%s to any overloaded instance.%s"),
 	       obj_type_name,
 	       (obj_type_name && *obj_type_name) ? "::" : "",
-	       name);
+	       name, hint.c_str ());
       else
-	error (_("Cannot resolve function %s to any overloaded instance"),
-	       func_name);
+	error (_("Cannot resolve function %s to any overloaded instance.%s"),
+	       func_name, hint.c_str ());
     }
   else if (match_quality == NON_STANDARD)
     {