Why does defined(X) not work in a preprocessor definition without a space?

Question

A preprocessor definition that includes defined(X) will never evaluate to true, but (defined X) will. This occurs in MSVC9; I have not tested other preprocessors. A simple example:

#define FEATURE0 1
#define FEATURE1 0
#define FEATURE2 1

#define FEATURE3 (FEATURE0 && !FEATURE1 && (defined(FEATURE2)))
#define FEATURE4 (FEATURE0 && !FEATURE1 && (defined FEATURE2))
#define FEATURE5 (FEATURE0 && !FEATURE1 && (defined (FEATURE2)))

#if FEATURE3
#pragma message("FEATURE3 Enabled")
#elif (FEATURE0 && !FEATURE1 && (defined(FEATURE2)))
#pragma message("FEATURE3 Enabled (Fallback)")
#endif

#if FEATURE4
#pragma message("FEATURE4 Enabled")
#elif (FEATURE0 && !FEATURE1 && (defined FEATURE2))
#pragma message("FEATURE4 Enabled (Fallback)")
#endif

#if FEATURE5
#pragma message("FEATURE5 Enabled")
#elif (FEATURE0 && !FEATURE1 && (defined (FEATURE2)))
#pragma message("FEATURE5 Enabled (Fallback)")
#endif

The output from the compiler is:

1>FEATURE3 Enabled (Fallback)
1>FEATURE4 Enabled
1>FEATURE5 Enabled

Working cases: defined (X), defined( X ), and defined X.
Broken case: defined(X)

Why is defined evaluated differently when part of a definition, as in the #if cases in the example, compared to direct evaluation, as in the #elif cases in the example?

Answer 1

defined is specific to #if and #elif. When using it through macro expansion the behavior is undefined.