Introduce a --warn command line option

Replace the singleton --warn-undefined-variables with infrastructure
to manage multiple warnings: the --warn option can take an action
"ignore", "warn", or "error" (which will apply to all warnings), or
a specific warning type and an action for that type.  Multiple
options can be provided and are consolidated.

* NEWS: Announce the new option.
* doc/make.1: Document in the man page.
* doc/make.texi (Warnings): Document in the user's manual.
* Makefile.am: Add new header warning.h.
* src/warning.h: Define enum for actions and warning types, and
macros to test whether they are set.  Keep the default settings
separate so that we can correctly reconstruct MAKEFLAGS.
* src/makeint.h: Remove deprecated warn_undefined_variables_flag.
* src/main.c: Create global variables to hold warning settings.
(switches): Add a new switch for --warn.
(initialize_warnings): Set the default warning actions.
(main): Call initialize_warnings().
(encode_warning_state, decode_warning_state): Convert warning states
between strings and enums.
(encode_warning_name, decode_warning_name): Convert warning names
between strings and enums.
(decode_warn_flags): Convert a --warn option into enum values.  If
deprecated warn_undefined_variables_flag is set convert it to --warn.
(decode_switches): Don't remove duplicates of --warn since order
matters.  Call decode_warn_flags() to handle --warn.
(define_makeflags): Special-case handling of --warn options written
to MAKEFLAGS: write out the current settings.
* src/read.c (tilde_expand): Use new warning control macros.
* src/variable.c (warn_undefined): Ditto.
* src/job.c (construct_command_argv): Ditto.
* tests/scripts/options/warn: Rename from warn-undefined-variables
and add tests for --warn.
* tests/scripts/variables/MAKEFLAGS: Expect the new behavior.

2 files changed, 104 insertions, 7 deletions

diff --git a/doc/make.1 b/doc/make.1
index f02afbee..933a8273 100644
--- a/doc/make.1
+++ b/doc/make.1
@@ -364,8 +364,36 @@ command on the given file before running
 except that the modification time is changed only in the imagination of
 .BR make .
 .TP 0.5i
+\fB\-\-warn\fR[=\fIARG[\fR,\fIARG\fR]]
+Control warning reporting for makefiles.  This option can appear multiple times.
+In case of conflicts, later settings override earlier settings.
+.I ARG
+can be an action; one of
+.IR ignore ,
+.IR warn ,
+or
+.I error
+to set the default action for all warnings, or it can be a specific warning:
+.I undefined-var
+(referencing an undefined variable).  The behavior of each warning can be set
+by adding
+.BI : action
+after the warning name.  If an action is not specified the default is
+.IR warn .
+If no
+.I ARG
+is provided the action for all warnings is
+.IR warn .
+If no
+.B \-\-warn
+option is provided the default action for
+.I undefined-var
+is
+.IR ignore .
+.TP 0.5i
 .B \-\-warn\-undefined\-variables
-Warn when an undefined variable is referenced.
+A deprecated alternative for
+.BR \-\-warn=undefined-var .
 .SH "EXIT STATUS"
 GNU Make exits with a status of zero if all makefiles were successfully parsed
 and no targets that were built failed.  A status of one will be returned
diff --git a/doc/make.texi b/doc/make.texi
index aa488eb7..31db623b 100644
--- a/doc/make.texi
+++ b/doc/make.texi
@@ -310,6 +310,7 @@ How to Run @code{make}
                                   an alternate compiler and other things.
 * Testing::                     How to proceed past some errors, to
                                   test compilation.
+* Warnings::                    How to control reporting of makefile issues.
 * Temporary Files::             Where @code{make} keeps its temporary files.
 * Options Summary::             Summary of Options
 
@@ -5170,7 +5171,7 @@ compatibility.  It has the same value as @code{MAKEFLAGS} except that it
 does not contain the command line variable definitions, and it always
 begins with a hyphen unless it is empty (@code{MAKEFLAGS} begins with a
 hyphen only when it begins with an option that has no single-letter
-version, such as @samp{--warn-undefined-variables}).  @code{MFLAGS} was
+version, such as @samp{--no-print-directory}).  @code{MFLAGS} was
 traditionally used explicitly in the recursive @code{make} command, like
 this:
 
@@ -8873,6 +8874,7 @@ determines that some target is not already up to date.
                                   an alternate compiler and other things.
 * Testing::                     How to proceed past some errors, to
                                   test compilation.
+* Warnings::                    How to control reporting of makefile issues.
 * Temporary Files::             Where @code{make} keeps its temporary files.
 * Options Summary::             Summary of Options
 @end menu
@@ -9265,7 +9267,7 @@ overridden.  This is to use the @code{override} directive, which is a line
 that looks like this: @samp{override @var{variable} = @var{value}}
 (@pxref{Override Directive, ,The @code{override} Directive}).
 
-@node Testing, Temporary Files, Overriding, Running
+@node Testing, Warnings, Overriding, Running
 @section Testing the Compilation of a Program
 @cindex testing compilation
 @cindex compilation, testing
@@ -9303,7 +9305,68 @@ program, perhaps to find several independent problems so that you can
 correct them all before the next attempt to compile.  This is why Emacs'
 @kbd{M-x compile} command passes the @samp{-k} flag by default.
 
-@node Temporary Files, Options Summary, Testing, Running
+@node Warnings, Temporary Files, Testing, Running
+@section Makefile Warnings
+@cindex warnings
+
+GNU Make can detect some types of incorrect usage in makefiles and show
+warnings about them.  Currently these issues can be detected:
+
+@table @samp
+@item undefined-var
+Referencing a variable that has not been defined.
+@end table
+
+When one of these incorrect usages is detected, GNU Make can perform one of
+these actions:
+
+@table @samp
+@item ignore
+Ignore the usage.
+
+@item warn
+Show a warning about the usage and continue processing the makefile.
+
+@item error
+Show an error for the usage and immediately stop processing the makefile.
+@end table
+
+The default action when no warning control options are provided for
+@samp{undefined-var} is @samp{warn}.
+
+To modify this default behavior, you can use the @code{--warn} option.  This
+option can be specified on the command line, or by adding it to the
+@code{MAKEFLAGS} variable (@pxref{Recursion, ,Recursive Use of @code{make}}).
+Settings added to @code{MAKEFLAGS} are only affect after the assignment
+statement.
+
+The @code{--warn} option can be provided multiple times: the effects are
+cumulative with later options overriding over earlier options.  When GNU Make
+provides warning settings to sub-makes, they are all combined into a single
+@code{--warn} option in @code{MAKEFLAGS}.
+
+If @code{--warn} is provided with no arguments then all issues are detected
+and reported at the @samp{warn} level unless otherwise specified.
+
+If one of the actions (@samp{ignore}, @samp{warn}, @samp{error}) is provided
+as an argument to @code{--warn}, then this action becomes the default for all
+warning types.  For example all warnings can be disabled by using
+@code{--warn=ignore}, or all warnings can be considered fatal errors by using
+@code{--warn=error}.
+
+Additionally, warning types can be specified.  If the warning is listed alone,
+then that warning is enabled with the @code{warn} action: e.g.,
+@code{--warn=undefined-var} will enable @samp{undefined-var} warnings with the
+@samp{warn} action.  Alternatively an action can be provided after the warning
+type, separated by a @code{:}.  So @code{--warn=undefined-var:error} enables
+@samp{undefined-var} warnings with an action of @samp{error}.
+
+More specific settings take precedence over the global setting.  For example,
+an option @code{--warn=undefined-var:error,ignore} will set the action for
+@samp{undefined-var} warnings to @samp{error}, and the action for all other
+warnings to @samp{ignore}.
+
+@node Temporary Files, Options Summary, Warnings, Running
 @section Temporary Files
 @cindex temporary files
 
@@ -9756,13 +9819,19 @@ running a @code{touch} command on the given file before running
 imagination of @code{make}.
 @xref{Instead of Execution, ,Instead of Executing Recipes}.
 
+
+@item --warn[=@var{arg}[,@var{arg}]]
+@cindex @code{--warn}
+@cindex warnings
+Specify the handling of @ref{Warnings, ,Makefile Warnings} detected in
+makefiles.
+
 @item --warn-undefined-variables
 @cindex @code{--warn-undefined-variables}
 @cindex variables, warning for undefined
 @cindex undefined variables, warning message
-Issue a warning message whenever @code{make} sees a reference to an
-undefined variable.  This can be helpful when you are trying to debug
-makefiles which use variables in complex ways.
+A deprecated name for @code{--warn=undefined-var}.  @xref{Warnings,
+,Makefile Warnings}.
 @end table
 
 @node Implicit Rules, Archives, Running, Top