kconfig: allow symbols implied by y to become m

The 'imply' keyword restricts a symbol to y or n, excluding m
when it is implied by y. This is the original behavior since
commit 237e3ad ("Kconfig: Introduce the "imply" keyword").

However, the author of this feature, Nicolas Pitre, stated that
the 'imply' keyword should not impose any restrictions.
(https://lkml.org/lkml/2020/2/19/714)

I agree, and want to get rid of this tricky behavior.

Suggested-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Pitre <nico@fluxnic.net>

Documentation/kbuild/kconfig-language.rst

@@ -173,14 +173,24 @@ applicable everywhere (see syntax).
 	===		===		=============	==============
 	n		y		n		N/m/y
 	m		y		m		M/y/n
-	y		y		y		Y/n
+	y		y		y		Y/m/n
 	y		n		*		N
 	===		===		=============	==============
 
   This is useful e.g. with multiple drivers that want to indicate their
   ability to hook into a secondary subsystem while allowing the user to
   configure that subsystem out without also having to unset these drivers.
 
+  Note: If the combination of FOO=y and BAR=m causes a link error,
+  you can guard the function call with IS_REACHABLE()::
+
+	foo_init()
+	{
+		if (IS_REACHABLE(CONFIG_BAZ))
+			baz_register(&foo);
+		...
+	}
+
 - limiting menu display: "visible if" <expr>
 
   This attribute is only applicable to menu blocks, if the condition is

scripts/kconfig/symbol.c

@@ -401,8 +401,7 @@ void sym_calc_value(struct symbol *sym)
 				sym_warn_unmet_dep(sym);
 			newval.tri = EXPR_OR(newval.tri, sym->rev_dep.tri);
 		}
-		if (newval.tri == mod &&
-		    (sym_get_type(sym) == S_BOOLEAN || sym->implied.tri == yes))
+		if (newval.tri == mod && sym_get_type(sym) == S_BOOLEAN)
 			newval.tri = yes;
 		break;
 	case S_STRING:
@@ -484,8 +483,6 @@ bool sym_tristate_within_range(struct symbol *sym, tristate val)
 		return false;
 	if (sym->visible <= sym->rev_dep.tri)
 		return false;
-	if (sym->implied.tri == yes && val == mod)
-		return false;
 	if (sym_is_choice_value(sym) && sym->visible == yes)
 		return val == yes;
 	return val >= sym->rev_dep.tri && val <= sym->visible;