Allow debugmode control over whether defn(undef) warns.

* ltdl/m4/gnulib-cache.m4: Import verify module.
* m4/m4module.h (M4_DEBUG_TRACE_DEREF, M4_DEBUG_TRACE_INITIAL):
New values.
(M4_DEBUG_TRACE_VERBOSE, M4_DEBUG_TRACE_DEFAULT): Adjust.
* m4/m4.c (m4_create): Start with debugmode = d.
* src/main.c (usage): Mention this.
(main): Let -E impact debug mode.
* m4/utility.c (m4_symbol_value_lookup): Squelch undefined warning
if flag is clear.
* modules/gnu.c (builtin, inder): Likewise.
* m4/debug.c (m4_debug_decode): Parse new mode.
* src/freeze.c (produce_debugmode_state): Accomodate new mode.
* doc/m4.texinfo (Debugging options): Mention change in the
default behavior.
(Debugmode): Mention new flag d.
(Operation modes): Mention interaction with -E.
(Undefine, Defn, Pushdef, Indir, Builtin, Dumpdef): Document and
test its effect.
* tests/options.at (--debugmode): Update tests.
* tests/freeze.at (reloading debugmode): Likewise.
* NEWS: Document this.
Reported by Ralf Wildenhues; without this patch, M4 1.6+ would be
incompatible with Autoconf 2.62.

Signed-off-by: Eric Blake <ebb9@byu.net>

13 files changed, 241 insertions, 94 deletions

diff --git a/ChangeLog b/ChangeLog
index f21cbd8c..9fe67363 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,30 @@
+2008-08-23  Eric Blake  <ebb9@byu.net>
+
+	Allow debugmode control over whether defn(undef) warns.
+	* ltdl/m4/gnulib-cache.m4: Import verify module.
+	* m4/m4module.h (M4_DEBUG_TRACE_DEREF, M4_DEBUG_TRACE_INITIAL):
+	New values.
+	(M4_DEBUG_TRACE_VERBOSE, M4_DEBUG_TRACE_DEFAULT): Adjust.
+	* m4/m4.c (m4_create): Start with debugmode = d.
+	* src/main.c (usage): Mention this.
+	(main): Let -E impact debug mode.
+	* m4/utility.c (m4_symbol_value_lookup): Squelch undefined warning
+	if flag is clear.
+	* modules/gnu.c (builtin, inder): Likewise.
+	* m4/debug.c (m4_debug_decode): Parse new mode.
+	* src/freeze.c (produce_debugmode_state): Accomodate new mode.
+	* doc/m4.texinfo (Debugging options): Mention change in the
+	default behavior.
+	(Debugmode): Mention new flag d.
+	(Operation modes): Mention interaction with -E.
+	(Undefine, Defn, Pushdef, Indir, Builtin, Dumpdef): Document and
+	test its effect.
+	* tests/options.at (--debugmode): Update tests.
+	* tests/freeze.at (reloading debugmode): Likewise.
+	* NEWS: Document this.
+	Reported by Ralf Wildenhues; without this patch, M4 1.6+ would be
+	incompatible with Autoconf 2.62.
+
 2008-08-22  Eric Blake  <ebb9@byu.net>
 
 	Improve --debugmode behavior.
diff --git a/NEWS b/NEWS
index e8b08d07..4420c83e 100644
--- a/NEWS
+++ b/NEWS
@@ -29,21 +29,12 @@ promoted to 2.0.
 *** New `-B'/`--prepend-include' command-line option allows prepending to
     the include path, rather than always searching `.' first.
 
-*** The `-d'/`--debug' command-line option now understands `-' and `+'
-    modifiers, the way the builtin `debugmode' has always done; this allows
-    `-d-V' to disable prior debug settings from the command line, similar
-    to using the builtin `debugmode' without arguments.  The option
-    `--debugmode' is added as an alias for `-d'.  When given the empty
-    string, the mode is treated as `+aeq' instead of `aeq'.  Also, the
-    position of `-d' with respect to files on the command line is now
-    significant.
-
 *** New `--debuglen' command-line option matches the spelling of a new
     macro, and the old spelling `--arglength' now issues a warning that it
     might be withdrawn in the future.
 
-*** New `-g'/`--gnu' command-line option overrides `-G'/`--traditional',
-    allowing the GNU module to be selected even when POSIXLY_CORRECT.
+*** The `-g'/`--gnu' command-line option is now required to allow all GNU
+    extensions when POSIXLY_CORRECT is set.
 
 *** The `-H'/`--hashsize' command-line options, which were made no-ops in
     a previous beta, now issue a deprecation warning.
@@ -253,9 +244,42 @@ promoted to 2.0.
    then apply this patch:
      http://git.sv.gnu.org/gitweb/?p=autoconf.git;a=commitdiff;h=56d42fa71
 
-** The `defn' builtin now warns when operating on an undefined macro name.
-   To simulate 1.4.x behavior, use:
-     pushdef(`defn', `ifdef(`$1', `builtin(`defn', `$1')')')
+** The `-d'/`--debug' command-line option now understands `-' and `+'
+   modifiers, the way the builtin `debugmode' has always done; this allows
+   `-d-V' to disable prior debug settings from the command line, similar to
+   using the builtin `debugmode' without arguments.  The option
+   `--debugmode' is added as an alias for `-d'.  The new flag `d' is added
+   to control whether dereferncing an undefined macro causes a warning;
+   this flag is enabled by default if neither `-d' nor `-E' are specified.
+   When given the empty string, the mode is treated as `+adeq' instead of
+   `aeq'.  Also, the position of `-d' with respect to files on the command
+   line is now significant.
+
+** New `-g'/`--gnu' command-line option overrides `-G'/`--traditional'.
+   For now, the environment variable POSIXLY_CORRECT has no effect on M4
+   behavior; but a future release of M4 will behave as though --traditional
+   is implied if POSIXLY_CORRECT is set (this future change is necessary,
+   because in the current release, there is no way to disable GNU
+   extensions that conflict with POSIX without the use of a non-POSIX
+   command-line argument).  Clients of M4 that want to use GNU extensions,
+   even when POSIXLY_CORRECT is set, should start using the -g command-line
+   argument, even though it is currently a no-op if -G did not appear
+   earlier in the command line, so that the client will not break in the
+   face of an upgraded m4 and a POSIXLY_CORRECT execution environment.
+
+** A new predefined text macro, `__m4_version__', expands to the unquoted
+   version number of M4, if GNU extensions are enabled.  While you should
+   generally favor feature tests over version number checks, this macro can
+   be used, via `defn', to determine whether the version of m4 processing
+   your file is adequate.
+
+** The `defn', `popdef', and `undefine' builtins gained a new warning when
+   operating on an undefined macro name, to match the warning already
+   present in `builtin', `indir', and `dumpdef'.  For backwards
+   compatibility, the warning can be disabled by using `debugmode(`-d')'
+   (or the command line option `--debug=-d').  The flag is also cleared by
+   the command line option `-E'/`--fatal-warnings', so that scripts written
+   for 1.4.x do not cause the script to fail because of new warnings.
 
 ** Enhance the `indir' builtin to trace indirect macros, where the trace
    is requested via `traceon' or the command-line option `-t'.  Previously,
diff --git a/doc/m4.texinfo b/doc/m4.texinfo
index 0b84d089..54ecf8f3 100644
--- a/doc/m4.texinfo
+++ b/doc/m4.texinfo
@@ -644,6 +644,57 @@ a warning is issued.  The introduction of behavior levels is new to M4
 1.4.9; for behavior consistent with earlier versions, you should specify
 @option{-E} twice.
 
+
+For backwards compatibility reasons, using @option{-E} behaves as if an
+implicit @option{--debug=-d} option is also present.  This is so that
+scripts written for older M4 versions will not fail if they used
+constructs that were previously silently allowed, but would now trigger
+a warning.
+
+@example
+$ @kbd{m4}
+defn(`oops')
+@error{}m4:stdin:1: Warning: defn: undefined macro `oops'
+@result{}
+^D
+@end example
+
+@comment ignore
+@example
+$ @kbd{echo $?}
+@result{}0
+@end example
+
+@comment options: -E
+@example
+$ @kbd{m4 -E}
+defn(`oops')
+@result{}
+^D
+@end example
+
+@comment ignore
+@example
+$ @kbd{echo $?}
+@result{}0
+@end example
+
+@comment options: -E -d
+@comment status: 1
+@example
+$ @kbd{m4 -E -d}
+defn(`oops')
+@error{}m4:stdin:1: Warning: defn: undefined macro `oops'
+@result{}
+^D
+@end example
+
+@comment ignore
+@example
+$ @kbd{echo $?}
+@result{}1
+@end example
+
 @item -i
 @itemx --interactive
 @itemx -e
@@ -944,17 +995,20 @@ Set the debug-level according to the flags @var{FLAGS}.  The debug-level
 controls the format and amount of information presented by the debugging
 functions.  @xref{Debugmode}, for more details on the format and
 meaning of @var{FLAGS}.  If omitted, @var{FLAGS} defaults to
-@samp{+aeq}.  If the option occurs multiple times, @var{FLAGS} starting
+@samp{+adeq}.  If the option occurs multiple times, @var{FLAGS} starting
 with @samp{-} or @samp{+} are cumulative, while @var{FLAGS} starting
-with a letter override all earlier settings.  Therefore, to disable all
-previously set flags, specify an explicit @var{FLAGS} of @samp{-V}.  The
-spelling @option{--debug} is recognized as an unambiguous option for
+with a letter override all earlier settings.  The debug-level starts
+with @samp{d} enabled and all other flags disabled.  To disable all
+previously set flags, specify an explicit @var{FLAGS} of @samp{-V}.  For
+backward compatibility reasons, the option @option{--fatal-warnings}
+implies @samp{--debug=-d} as part of its effects.  The spelling
+@option{--debug} is recognized as an unambiguous option for
 compatibility with earlier versions of @acronym{GNU} M4, but for
 consistency with the builtin name, you can also use the spelling
 @option{--debugmode}.  Order is significant with respect to file names.
 
 The cumulative effect of the various options in this example is
-equivalent to a single invocation of @code{debugmode(alqx)}:
+equivalent to a single invocation of @code{debugmode(`adlqx')}:
 
 @comment options: -d-V -d+lx --debug --debugmode=-e
 @example
@@ -2142,7 +2196,8 @@ A macro definition can be removed with @code{undefine}:
 @deffn {Builtin (m4)} undefine (@var{name}@dots{})
 For each argument, remove the macro @var{name}.  The macro names must
 necessarily be quoted, since they will be expanded otherwise.  If an
-argument is not a defined macro, a warning is issued.
+argument is not a defined macro, then the @samp{d} debug level controls
+whether a warning is issued (@pxref{Debugmode}).
 
 The expansion of @code{undefine} is void.
 The macro @code{undefine} is recognized only with parameters.
@@ -2177,30 +2232,19 @@ f(`bye')
 @result{}f(bye)
 @end example
 
-As of M4 1.6, @code{undefine} will warn if @var{name} is not a macro.
-The following example shows how to recover earlier behavior that
-silently ignores undefined names, using concepts that will be explained
-later in the manual.  Or, you could use the command line option
-@option{-Q} (@option{--quiet}, @pxref{Operation modes, , Invoking m4}).
+As of M4 1.6, @code{undefine} can warn if @var{name} is not a macro, by
+using @code{debugmode} (@pxref{Debugmode}) or the command line option
+@option{-d} (@option{--debugmode}, @pxref{Debugging options, , Invoking
+m4}).
 
 @example
-define(`a', `1')
-@result{}
-undefine
-@result{}undefine
-undefine(`a', `a')
-@error{}m4:stdin:3: Warning: undefine: undefined macro `a'
-@result{}
-define(`a', `1')
-@result{}
-pushdef(`undefine', `ifelse(`$#', `0', ``undefine'', `_$0($@@)')')
+$ @kbd{m4}
+undefine(`a')
+@error{}m4:stdin:1: Warning: undefine: undefined macro `a'
 @result{}
-define(`_undefine', `ifdef(`$1', `builtin(`undefine',
-  `$1')')ifelse(`$#', `1', `', `$0(shift($@@))')')
+debugmode(`-d')
 @result{}
-undefine
-@result{}undefine
-undefine(`a', `a')
+undefine(`a')
 @result{}
 @end example
 
@@ -2217,7 +2261,8 @@ the builtin @code{defn}:
 @deffn {Builtin (m4)} defn (@var{name}@dots{})
 Expands to the @emph{quoted definition} of each @var{name}.  If an
 argument is not a defined macro, the expansion for that argument is
-empty and triggers a warning.
+empty, and the @samp{d} debug level controls whether a warning is issued
+(@pxref{Debugmode}).
 
 If @var{name} is a user-defined macro, the quoted definition is simply
 the quoted expansion text.  If, instead, @var{name} is a builtin, the
@@ -2386,9 +2431,11 @@ bar
 @result{}0
 @end example
 
-A warning is issued if @var{name} is undefined.  Also note that as of M4
-1.6, @code{defn} with multiple arguments can join text with builtin
-tokens.  However, when defining a macro via @code{define} or
+As of M4 1.6, @code{defn} can warn if @var{name} is not a macro, by
+using @code{debugmode} (@pxref{Debugmode}) or the command line option
+@option{-d} (@option{--debugmode}, @pxref{Debugging options, , Invoking
+m4}).  Also, @code{defn} with multiple arguments can join text with
+builtin tokens.  However, when defining a macro via @code{define} or
 @code{pushdef}, a warning is issued and the builtin token ignored if the
 builtin token does not occur in isolation.  A future version of
 @acronym{GNU} M4 may lift this restriction.
@@ -2398,6 +2445,10 @@ $ @kbd{m4 -d}
 defn(`foo')
 @error{}m4:stdin:1: Warning: defn: undefined macro `foo'
 @result{}
+debugmode(`-d')
+@result{}
+defn(`foo')
+@result{}
 define(`a', `A')define(`AA', `b')
 @result{}
 traceon(`defn', `define')
@@ -2407,25 +2458,25 @@ defn(`a', `divnum', `a')
 @result{}AA
 define(`mydivnum', defn(`divnum', `divnum'))mydivnum
 @error{}m4trace: -2- defn(`divnum', `divnum') -> `<divnum><divnum>'
-@error{}m4:stdin:5: Warning: define: cannot concatenate builtins
+@error{}m4:stdin:7: Warning: define: cannot concatenate builtins
 @error{}m4trace: -1- define(`mydivnum', `<divnum><divnum>') -> `'
 @result{}
 traceoff(`defn', `define')dumpdef(`mydivnum')
 @error{}mydivnum:@tabchar{}`'
 @result{}
 define(`mydivnum', defn(`divnum')defn(`divnum'))mydivnum
-@error{}m4:stdin:7: Warning: define: cannot concatenate builtins
+@error{}m4:stdin:9: Warning: define: cannot concatenate builtins
 @result{}
 define(`mydivnum', defn(`divnum')`a')mydivnum
-@error{}m4:stdin:8: Warning: define: cannot concatenate builtins
+@error{}m4:stdin:10: Warning: define: cannot concatenate builtins
 @result{}A
 define(`mydivnum', `a'defn(`divnum'))mydivnum
-@error{}m4:stdin:9: Warning: define: cannot concatenate builtins
+@error{}m4:stdin:11: Warning: define: cannot concatenate builtins
 @result{}A
 define(`q', ``$@@'')
 @result{}
 define(`foo', q(`a', defn(`divnum')))foo
-@error{}m4:stdin:11: Warning: define: cannot concatenate builtins
+@error{}m4:stdin:13: Warning: define: cannot concatenate builtins
 @result{}a,
 ifdef(`foo', `yes', `no')
 @result{}yes
@@ -2456,7 +2507,8 @@ exactly like @code{define}.
 If a macro has several definitions (of which only one is accessible),
 the topmost definition can be removed with @code{popdef}.  If there is
 no previous definition, @code{popdef} behaves like @code{undefine}, and
-if there is no definition at all, @code{popdef} issues a warning.
+if there is no definition at all, the @samp{d} debug level controls
+whether a warning is issued (@pxref{Debugmode}).
 
 The expansion of both @code{pushdef} and @code{popdef} is void.
 The macros @code{pushdef} and @code{popdef} are recognized only with
@@ -2529,11 +2581,10 @@ revealing the former definition.
 It is possible to temporarily redefine a builtin with @code{pushdef}
 and @code{defn}.
 
-As of M4 1.6, @code{popdef} will warn if @var{name} is not a macro.
-The following example shows how to recover earlier behavior that
-silently ignores undefined names, using concepts that will be explained
-later in the manual.  Or, you could use the command line option
-@option{-Q} (@option{--quiet}, @pxref{Operation modes, , Invoking m4}).
+As of M4 1.6, @code{popdef} can warn if @var{name} is not a macro, by
+using @code{debugmode} (@pxref{Debugmode}) or the command line option
+@option{-d} (@option{--debugmode}, @pxref{Debugging options, , Invoking
+m4}).
 
 @example
 define(`a', `1')
@@ -2543,16 +2594,9 @@ popdef
 popdef(`a', `a')
 @error{}m4:stdin:3: Warning: popdef: undefined macro `a'
 @result{}
-define(`a', `1')
-@result{}
-pushdef(`popdef', `ifelse(`$#', `0', ``popdef'', `_$0($@@)')')
-@result{}
-define(`_popdef', `ifdef(`$1', `builtin(`popdef',
-  `$1')')ifelse(`$#', `1', `', `$0(shift($@@))')')
+debugmode(`-d')
 @result{}
-popdef
-@result{}popdef
-popdef(`a', `a')
+popdef(`a')
 @result{}
 @end example
 
@@ -2683,9 +2727,10 @@ popdef(`bar')bar
 Any macro can be called indirectly with @code{indir}:
 
 @deffn {Builtin (gnu)} indir (@var{name}, @ovar{args@dots{}})
-Results in a call to the macro @var{name}, which is passed the
-rest of the arguments @var{args}.  If @var{name} is not defined, an
-error message is printed, and the expansion is void.
+Results in a call to the macro @var{name}, which is passed the rest of
+the arguments @var{args}.  If @var{name} is not defined, the expansion
+is void, and the @samp{d} debug level controls whether a warning is
+issued (@pxref{Debugmode}).
 
 The macro @code{indir} is recognized only with parameters.
 @end deffn
@@ -2724,6 +2769,10 @@ indir(`f', define(`f', `3'))
 indir(`f', undefine(`f'))
 @error{}m4:stdin:4: Warning: indir: undefined macro `f'
 @result{}
+debugmode(`-d')
+@result{}
+indir(`f')
+@result{}
 @end example
 
 When handed the result of @code{defn} (@pxref{Defn}) as one of its
@@ -2775,7 +2824,8 @@ Builtin macros can be called indirectly with @code{builtin}:
 @deffnx {Builtin (gnu)} builtin (@code{defn(`builtin')}, @var{name1})
 Results in a call to the builtin @var{name}, which is passed the
 rest of the arguments @var{args}.  If @var{name} does not name a
-builtin, a warning message is printed, and the expansion is void.
+builtin, the expansion is void, and the @samp{d} debug level controls
+whether a warning is issued (@pxref{Debugmode}).
 
 As a special case, if @var{name} is exactly the special token
 representing the @code{builtin} macro, as obtained by @code{defn}
@@ -2835,6 +2885,10 @@ m4_indir(`divnum')
 @result{}
 m4_indir(`m4_divnum')
 @result{}0
+m4_debugmode(`-d')
+@result{}
+m4_builtin(`m4_divnum')
+@result{}
 @end example
 
 Note that @code{indir} and @code{builtin} can be used to invoke builtins
@@ -3794,14 +3848,15 @@ Fortunately, there is support for macro debugging in @code{m4}.
 If you want to see what a name expands into, you can use the builtin
 @code{dumpdef}:
 
-@deffn {Builtin (m4)} dumpdef (@ovar{names@dots{}})
+@deffn {Builtin (m4)} dumpdef (@ovar{name@dots{}})
 Accepts any number of arguments.  If called without any arguments,
 it displays the definitions of all known names, otherwise it displays
-the definitions of the @var{names} given.  The output is printed
+the definitions of each @var{name} given.  The output is printed
 directly to standard error, independently of the @option{--debugfile}
 option (@pxref{Debugging options, , Invoking m4}), or @code{debugfile} macro.
-The output is sorted by name.  If an unknown name is encountered, a
-warning is printed.
+The output is sorted by name.  If a @var{name} is undefined, the
+@samp{d} debug level controls whether a warning is issued
+(@pxref{Debugmode}).
 
 The expansion of @code{dumpdef} is void.
 @end deffn
@@ -3833,6 +3888,10 @@ f(popdef(`f')dumpdef(`f'))
 f(popdef(`f')dumpdef(`f'))
 @error{}m4:stdin:3: Warning: dumpdef: undefined macro `f'
 @result{}f1
+debugmode(`-d')
+@result{}
+dumpdef(`-f')
+@result{}
 @end example
 
 @xref{Debugmode}, for information on how the @samp{m}, @samp{q}, and
@@ -4171,6 +4230,19 @@ the expansion determined.  The definition is subject to length
 truncation specified by @code{debuglen} (@pxref{Debuglen}).  This is
 often used with the @samp{x} flag.
 
+@item d
+Output a warning on any attempt to dereference an undefined macro via
+@code{builtin}, @code{defn}, @code{dumpdef}, @code{indir},
+@code{popdef}, or @code{undefine}.  Note that @code{indef},
+@code{m4symbols},
+@code{traceon}, and @code{traceoff} do not dereference undefined macros.
+Like any other warning, the warnings enabled by this flag go to standard
+error regardless of the current @code{debugfile} setting, and will
+change exit status if the command line option @option{--fatal-warnings}
+was specified.  This flag is useful in diagnosing spelling mistakes in
+macro names.  It is enabled by default when neither @option{--debug} nor
+@option{--fatal-warnings} are specified on the command line.
+
 @item e
 In trace output, show the expansion of each macro call.  The expansion
 is subject to length truncation specified by @code{debuglen}
@@ -4236,7 +4308,7 @@ the previous setting.
 @comment currently enabled?
 
 If no flags are specified with the @option{--debug} option, the default is
-@samp{+aeq}.  Many examples in this manual show their output using
+@samp{+adeq}.  Many examples in this manual show their output using
 default flags.
 
 @cindex @acronym{GNU} extensions
@@ -4248,7 +4320,7 @@ The argument @var{flags} should be a subset of the letters listed above.
 If no argument is present, all debugging flags are cleared (as if
 @var{flags} were an explicit @samp{-V}).  With an empty argument, the
 most common flags are enabled (as if @var{flags} were an explicit
-@samp{+aeq}).  If an unknown flag is encountered, an error is issued.
+@samp{+adeq}).  If an unknown flag is encountered, an error is issued.
 
 The expansion of @code{debugmode} is void.
 @end deffn
@@ -4260,7 +4332,7 @@ define(`foo', `FOO$1')
 @result{}
 traceon(`foo', `divnum')
 @result{}
-debugmode()dnl same as debugmode(`+aeq')
+debugmode()dnl same as debugmode(`+adeq')
 foo
 @error{}m4trace: -1- foo -> `FOO'
 @result{}FOO
diff --git a/ltdl/m4/gnulib-cache.m4 b/ltdl/m4/gnulib-cache.m4
index da4eef3f..f61b8d7d 100644
--- a/ltdl/m4/gnulib-cache.m4
+++ b/ltdl/m4/gnulib-cache.m4
@@ -15,7 +15,7 @@
 
 
 # Specification in the form of a command-line invocation:
-#   gnulib-tool --import --dir=. --local-dir=local --lib=libgnu --source-base=gnu --m4-base=ltdl/m4 --doc-base=doc --aux-dir=build-aux --with-tests --libtool --macro-prefix=M4 assert autobuild avltree-oset binary-io clean-temp cloexec close-stream closein config-h configmake dirname error exit fdl fflush filenamecat flexmember fopen-safer fseeko gendocs gettext git-version-gen gnumakefile gnupload gpl-3.0 intprops memmem mkstemp obstack obstack-printf-posix progname propername quote regex regexprops-generic sprintf-posix stdbool stdlib-safer strnlen strtod strtol tempname unlocked-io vasnprintf-posix verror xalloc xalloc-die xmemdup0 xprintf-posix xstrndup xvasprintf-posix
+#   gnulib-tool --import --dir=. --local-dir=local --lib=libgnu --source-base=gnu --m4-base=ltdl/m4 --doc-base=doc --aux-dir=build-aux --with-tests --libtool --macro-prefix=M4 assert autobuild avltree-oset binary-io clean-temp cloexec close-stream closein config-h configmake dirname error exit fdl fflush filenamecat flexmember fopen-safer fseeko gendocs gettext git-version-gen gnumakefile gnupload gpl-3.0 intprops memmem mkstemp obstack obstack-printf-posix progname propername quote regex regexprops-generic sprintf-posix stdbool stdlib-safer strnlen strtod strtol tempname unlocked-io vasnprintf-posix verify verror xalloc xalloc-die xmemdup0 xprintf-posix xstrndup xvasprintf-posix
 
 # Specification in the form of a few gnulib-tool.m4 macro invocations:
 gl_LOCAL_DIR([local])
@@ -64,6 +64,7 @@ gl_MODULES([
   tempname
   unlocked-io
   vasnprintf-posix
+  verify
   verror
   xalloc
   xalloc-die
diff --git a/m4/debug.c b/m4/debug.c
index d9efc8b8..f4005ebf 100644
--- a/m4/debug.c
+++ b/m4/debug.c
@@ -97,6 +97,10 @@ m4_debug_decode (m4 *context, const char *opts)
 	      level |= M4_DEBUG_TRACE_STACK;
 	      break;
 
+	    case 'd':
+	      level |= M4_DEBUG_TRACE_DEREF;
+	      break;
+
 	    case 'V':
 	      level |= M4_DEBUG_TRACE_VERBOSE;
 	      break;
diff --git a/m4/m4.c b/m4/m4.c
index 1fc1f113..31d977f2 100644
--- a/m4/m4.c
+++ b/m4/m4.c
@@ -37,6 +37,7 @@ m4_create (void)
   obstack_init (&context->trace_messages);
 
   context->nesting_limit = DEFAULT_NESTING_LIMIT;
+  context->debug_level = M4_DEBUG_TRACE_INITIAL;
   context->max_debug_arg_length = SIZE_MAX;
 
   context->search_path =
diff --git a/m4/m4module.h b/m4/m4module.h
index 6c0587b9..1f3b7bcf 100644
--- a/m4/m4module.h
+++ b/m4/m4module.h
@@ -402,14 +402,20 @@ enum {
   M4_DEBUG_TRACE_MODULE		= (1 << 10),
   /* s: trace pushdef stacks */
   M4_DEBUG_TRACE_STACK		= (1 << 11),
+  /* d: warn if dereferencing undefined macro */
+  M4_DEBUG_TRACE_DEREF		= (1 << 12),
 
   /* V: very verbose --  print everything */
-  M4_DEBUG_TRACE_VERBOSE	= ((1 << 12) - 1)
+  M4_DEBUG_TRACE_VERBOSE	= ((1 << 13) - 1)
 };
 
-/* default flags -- equiv: aeq */
-#define M4_DEBUG_TRACE_DEFAULT		\
-	(M4_DEBUG_TRACE_ARGS | M4_DEBUG_TRACE_EXPANSION | M4_DEBUG_TRACE_QUOTE)
+/* initial flags, used if no -d or -E -- equiv: d */
+#define M4_DEBUG_TRACE_INITIAL M4_DEBUG_TRACE_DEREF
+
+/* default flags, used by debugmode() -- equiv: +adeq */
+#define M4_DEBUG_TRACE_DEFAULT				\
+	(M4_DEBUG_TRACE_ARGS | M4_DEBUG_TRACE_EXPANSION \
+	 | M4_DEBUG_TRACE_QUOTE | M4_DEBUG_TRACE_DEREF)
 
 #define m4_is_debug_bit(C,B)	((m4_get_debug_level_opt (C) & (B)) != 0)
 
diff --git a/m4/utility.c b/m4/utility.c
index 596c6215..31c17051 100644
--- a/m4/utility.c
+++ b/m4/utility.c
@@ -134,7 +134,8 @@ m4_symbol_value_lookup (m4 *context, m4_macro_args *argv, size_t i,
       const char *name = M4ARG (i);
       size_t len = M4ARGLEN (i);
       result = m4_symbol_lookup (M4SYMTAB, name, len);
-      if (must_exist && !result)
+      if (must_exist && !result
+	  && m4_is_debug_bit (context, M4_DEBUG_TRACE_DEREF))
 	m4_warn (context, 0, argv->info, _("undefined macro %s"),
 		 quotearg_style_mem (locale_quoting_style, name, len));
     }
diff --git a/modules/gnu.c b/modules/gnu.c
index 2e960305..74e1c4a7 100644
--- a/modules/gnu.c
+++ b/modules/gnu.c
@@ -424,14 +424,14 @@ M4BUILTIN_HANDLER (builtin)
 	  len = M4ARGLEN (2);
 	  if (len == strlen (name))
 	    value = m4_builtin_find_by_name (NULL, name);
-	  if (value == NULL)
-	    m4_warn (context, 0, me, _("undefined builtin %s"),
-		     quotearg_style_mem (locale_quoting_style, name, len));
-	  else
+	  if (value)
 	    {
 	      m4_push_builtin (context, obs, value);
 	      free (value);
 	    }
+	  else if (m4_is_debug_bit (context, M4_DEBUG_TRACE_DEREF))
+	    m4_warn (context, 0, me, _("undefined builtin %s"),
+		     quotearg_style_mem (locale_quoting_style, name, len));
 	}
       else
 	m4_warn (context, 0, me, _("invalid macro name ignored"));
@@ -443,8 +443,11 @@ M4BUILTIN_HANDLER (builtin)
       if (len == strlen (name))
 	value = m4_builtin_find_by_name (NULL, name);
       if (value == NULL)
-	m4_warn (context, 0, me, _("undefined builtin %s"),
-		 quotearg_style_mem (locale_quoting_style, name, len));
+	{
+	  if (m4_is_debug_bit (context, M4_DEBUG_TRACE_DEREF))
+	    m4_warn (context, 0, me, _("undefined builtin %s"),
+		     quotearg_style_mem (locale_quoting_style, name, len));
+	}
       else
 	{
 	  const m4_builtin *bp = m4_get_symbol_value_builtin (value);
@@ -692,8 +695,11 @@ M4BUILTIN_HANDLER (indir)
       m4_symbol *symbol = m4_symbol_lookup (M4SYMTAB, name, len);
 
       if (symbol == NULL)
-	m4_warn (context, 0, me, _("undefined macro %s"),
-		 quotearg_style_mem (locale_quoting_style, name, len));
+	{
+	  if (m4_is_debug_bit (context, M4_DEBUG_TRACE_DEREF))
+	    m4_warn (context, 0, me, _("undefined macro %s"),
+		     quotearg_style_mem (locale_quoting_style, name, len));
+	}
       else
 	{
 	  m4_macro_args *new_argv;
diff --git a/src/freeze.c b/src/freeze.c
index 4d3a2b0f..44400f3f 100644
--- a/src/freeze.c
+++ b/src/freeze.c
@@ -27,6 +27,7 @@
 #include "binary-io.h"
 #include "close-stream.h"
 #include "quotearg.h"
+#include "verify.h"
 #include "xmemdup0.h"
 
 static	void  produce_mem_dump		(FILE *, const char *, size_t);
@@ -110,9 +111,9 @@ static void
 produce_debugmode_state (FILE *file, int flags)
 {
   /* This code tracks the number of bits in M4_DEBUG_TRACE_VERBOSE.  */
-  char str[13];
+  char str[14];
   int offset = 0;
-  assert ((1 << (sizeof str - 1)) - 1 == M4_DEBUG_TRACE_VERBOSE);
+  verify ((1 << (sizeof str - 1)) - 1 == M4_DEBUG_TRACE_VERBOSE);
   if (flags & M4_DEBUG_TRACE_ARGS)
     str[offset++] = 'a';
   if (flags & M4_DEBUG_TRACE_EXPANSION)
@@ -137,6 +138,8 @@ produce_debugmode_state (FILE *file, int flags)
     str[offset++] = 'm';
   if (flags & M4_DEBUG_TRACE_STACK)
     str[offset++] = 's';
+  if (flags & M4_DEBUG_TRACE_DEREF)
+    str[offset++] = 'd';
   str[offset] = '\0';
   if (offset)
     xfprintf (file, "d%d\n%s\n", offset, str);
diff --git a/src/main.c b/src/main.c
index 341a9e81..ee72381a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -149,7 +149,7 @@ Frozen state files:\n\
       fputs (_("\
 Debugging:\n\
   -d, --debug[=[-|+]FLAGS], --debugmode[=[-|+]FLAGS]\n\
-                               set debug level (no FLAGS implies `+aeq')\n\
+                               set debug level (no FLAGS implies `+adeq')\n\
       --debugfile=FILE         redirect debug and trace output\n\
   -l, --debuglen=NUM           restrict macro tracing size\n\
   -t, --trace=NAME, --traceon=NAME\n\
@@ -161,6 +161,7 @@ Debugging:\n\
 FLAGS is any of:\n\
   a   show actual arguments in trace\n\
   c   show collection line in trace\n\
+  d   warn when dereferencing undefined macros (default on unless -E)\n\
   e   show expansion in trace\n\
   f   include current input file name in trace and debug\n\
   i   show changes in input files in debug\n\
@@ -459,6 +460,7 @@ main (int argc, char *const *argv, char *const *envp)
 	  break;
 
 	case 'E':
+	  m4_debug_decode (context, "-d");
 	  if (m4_get_fatal_warnings_opt (context))
 	    m4_set_warnings_exit_opt (context, true);
 	  else
diff --git a/tests/freeze.at b/tests/freeze.at
index 995c4a68..9b8c9463 100644
--- a/tests/freeze.at
+++ b/tests/freeze.at
@@ -364,7 +364,7 @@ AT_CHECK_M4([-R frozen.m4f unfrozen.m4], [0], [[1
 ]], [[m4trace:unfrozen.m4:1: -1- len
 ]])
 
-dnl With plain -d after -R, add +aeq to the frozen file
+dnl With plain -d after -R, add +adeq to the frozen file
 AT_CHECK_M4([-R frozen.m4f -d unfrozen.m4], [0], [[1
 ]], [[m4trace:unfrozen.m4:1: -1- len(`a') -> `1'
 ]], [], [ ])
diff --git a/tests/options.at b/tests/options.at
index e220c0a4..904cc7d6 100644
--- a/tests/options.at
+++ b/tests/options.at
@@ -386,7 +386,7 @@ AT_DATA([[nested]],
 ')dnl
 ]])
 
-dnl AT_CHECK_M4 starts life with -d.  Make sure it looks like -d+aeq.
+dnl AT_CHECK_M4 starts life with -d.  Make sure it looks like -d+adeq.
 AT_CHECK_M4([-tlen in], [0], [[3
 0
 ]], [[m4trace: -1- len(`abc') -> `3'