Boolean literals are only sometimes of type BOOL.

whatsabool.m

- (void)testBooleanLiteralsAreOfTypeBool {
    NSNumber *testBoolNumber = @YES;
    const char *typeString = [testBoolNumber objCType];
    STAssertTrue('c' == *typeString, @"Boxed BOOL is of type %s, not of type 'c' (signed char, aka BOOL).", typeString);
}

- (void)testBoxedBooleanLiteralsAreOfTypeBool {
    // Boxing works fine if a BOOL value is boxed directly...
    BOOL testBool = YES;
    NSNumber *testBoolNumber = @(testBool);
    const char *typeString = [testBoolNumber objCType];
    STAssertTrue('c' == *typeString, @"Boxed BOOL is of type %s, not of type 'c' (signed char, aka BOOL).", typeString);
    
    // ""
    BOOL testBool2 = !testBool;
    NSNumber *testBoolNumber2 = @(testBool2);
    const char *typeString2 = [testBoolNumber2 objCType];
    STAssertTrue('c' == *typeString2, @"Boxed BOOL is of type %s, not of type 'c' (signed char, aka BOOL).", typeString2);
    
    // ""
    NSNumber *testBoolNumber3 = [self testBoxedBooleanLiteralsAreOfTypeBoolMethodBoxingABoolLArgument:testBool];
    const char *typeString3 = [testBoolNumber3 objCType];
    STAssertTrue('c' == *typeString3, @"Boxed BOOL is of type %s, not of type 'c' (signed char, aka BOOL).", typeString3);
    
    // ""
    NSNumber *testBoolNumber4 = @([testBoolNumber3 boolValue]);
    const char *typeString4 = [testBoolNumber4 objCType];
    STAssertTrue('c' == *typeString4, @"Boxed BOOL is of type '%s', not of type 'c' (signed char, aka BOOL).", typeString4);
    
    
    // .. but not if a "raw Boolean" is boxed.
    // Note: YES is here typedef'd to __objc_yes.
    NSNumber *testBoolNumber5 = @(YES);
    const char *typeString5 = [testBoolNumber5 objCType];
    STAssertFalse('c' == *typeString5, @"This is not broken, as expected: Boxed BOOL is of type 'c' (signed char, aka BOOL), not of type 'i'.");
    
    // ... ditto the result of a "boolean expression".
    NSNumber *testBoolNumber6 = @(!testBool);
    const char *typeString6 = [testBoolNumber6 objCType];
    STAssertFalse('c' == *typeString6, @"This is not broken, as expected: Boxed BOOL is of type 'c' (signed char, aka BOOL), not of type 'i'.");
    
    
    // If we cast the failing cases to BOOLs prior to boxing, we're alright.
    NSNumber *testBoolNumber7 = @((BOOL)YES);
    const char *typeString7 = [testBoolNumber7 objCType];
    STAssertTrue('c' == *typeString7, @"Boxed BOOL is of type %s, not of type 'c' (signed char, aka BOOL).", typeString7);

    NSNumber *testBoolNumber8 = @((BOOL)!testBool);
    const char *typeString8 = [testBoolNumber8 objCType];
    STAssertTrue('c' == *typeString8, @"Boxed BOOL is of type %s, not of type 'c' (signed char, aka BOOL).", typeString8);
}

- (NSNumber *)testBoxedBooleanLiteralsAreOfTypeBoolMethodBoxingABoolLArgument:(BOOL)testBool {
    return @(testBool);
}

Full explanation: in cases 5 and 6, the NSNumbers produced do not have objCType == 'c', but rather 'i'. This implies that the compiler is generating +[NSNumber numberWithInt:] from the literal syntax in cases 5 and 6, not +[NSNumber numberWithBOOL:] like in the others.

This can be a problem if you're serializing these NSNumbers into JSON, where (depending on your library, at least SBJSON, but likely the others -- they probably all rely upon -[NSNumber objCType] to serialize numeric vs. Boolean values) the Boolean values will be represented by "1" or "0" instead of by "true" and "false".