kconfig: remove wrong expr_trans_bool()

expr_trans_bool() performs an incorrect transformation.

[Test Code]

    config MODULES
            def_bool y
            modules

    config A
            def_bool y
            select C if B != n

    config B
            def_tristate m

    config C
            tristate

[Result]

    CONFIG_MODULES=y
    CONFIG_A=y
    CONFIG_B=m
    CONFIG_C=m

This output is incorrect because CONFIG_C=y is expected.

Documentation/kbuild/kconfig-language.rst clearly explains the function
of the '!=' operator:

    If the values of both symbols are equal, it returns 'n',
    otherwise 'y'.

Therefore, the statement:

    select C if B != n

should be equivalent to:

    select C if y

Or, more simply:

    select C

Hence, the symbol C should be selected by the value of A, which is 'y'.

However, expr_trans_bool() wrongly transforms it to:

    select C if B

Therefore, the symbol C is selected by (A && B), which is 'm'.

The comment block of expr_trans_bool() correctly explains its intention:

  * bool FOO!=n => FOO
    ^^^^

If FOO is bool, FOO!=n can be simplified into FOO. This is correct.

However, the actual code performs this transformation when FOO is
tristate:

    if (e->left.sym->type == S_TRISTATE) {
                             ^^^^^^^^^^

While it can be fixed to S_BOOLEAN, there is no point in doing so
because expr_tranform() already transforms FOO!=n to FOO when FOO is
bool. (see the "case E_UNEQUAL" part)

expr_trans_bool() is wrong and unnecessary.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>

scripts/kconfig/expr.c

@@ -396,35 +396,6 @@ static struct expr *expr_eliminate_yn(struct expr *e)
 	return e;
 }
 
-/*
- * bool FOO!=n => FOO
- */
-struct expr *expr_trans_bool(struct expr *e)
-{
-	if (!e)
-		return NULL;
-	switch (e->type) {
-	case E_AND:
-	case E_OR:
-	case E_NOT:
-		e->left.expr = expr_trans_bool(e->left.expr);
-		e->right.expr = expr_trans_bool(e->right.expr);
-		break;
-	case E_UNEQUAL:
-		// FOO!=n -> FOO
-		if (e->left.sym->type == S_TRISTATE) {
-			if (e->right.sym == &symbol_no) {
-				e->type = E_SYMBOL;
-				e->right.sym = NULL;
-			}
-		}
-		break;
-	default:
-		;
-	}
-	return e;
-}
-
 /*
  * e1 || e2 -> ?
  */

scripts/kconfig/expr.h

@@ -284,7 +284,6 @@ void expr_free(struct expr *e);
 void expr_eliminate_eq(struct expr **ep1, struct expr **ep2);
 int expr_eq(struct expr *e1, struct expr *e2);
 tristate expr_calc_value(struct expr *e);
-struct expr *expr_trans_bool(struct expr *e);
 struct expr *expr_eliminate_dups(struct expr *e);
 struct expr *expr_transform(struct expr *e);
 int expr_contains_symbol(struct expr *dep, struct symbol *sym);

scripts/kconfig/menu.c

@@ -398,8 +398,6 @@ static void _menu_finalize(struct menu *parent, bool inside_choice)
 				dep = expr_transform(dep);
 				dep = expr_alloc_and(expr_copy(basedep), dep);
 				dep = expr_eliminate_dups(dep);
-				if (menu->sym && menu->sym->type != S_TRISTATE)
-					dep = expr_trans_bool(dep);
 				prop->visible.expr = dep;
 
 				/*