Add the `isa` operator

Adds a new infix operator named `isa`, with the semantics that

  $x isa SomeClass

is true if and only if `$x` is a blessed object reference that is either
`SomeClass` directly, or includes the class somewhere in its @ISA
hierarchy. It is false without warning or error for non-references or
non-blessed references.

This operator respects `->isa` method overloading, and is intended to
replace boilerplate code such as

  use Scalar::Util 'blessed';

  blessed($x) and $x->isa("SomeClass")

1 files changed, 23 insertions, 8 deletions

diff --git a/feature.h b/feature.h
index 2b5b656bb8..0044b06b42 100644
--- a/feature.h
+++ b/feature.h
@@ -17,14 +17,15 @@
 #define FEATURE_MYREF_BIT           0x0004
 #define FEATURE_EVALBYTES_BIT       0x0008
 #define FEATURE_FC_BIT              0x0010
-#define FEATURE_POSTDEREF_QQ_BIT    0x0020
-#define FEATURE_REFALIASING_BIT     0x0040
-#define FEATURE_SAY_BIT             0x0080
-#define FEATURE_SIGNATURES_BIT      0x0100
-#define FEATURE_STATE_BIT           0x0200
-#define FEATURE_SWITCH_BIT          0x0400
-#define FEATURE_UNIEVAL_BIT         0x0800
-#define FEATURE_UNICODE_BIT         0x1000
+#define FEATURE_ISA_BIT             0x0020
+#define FEATURE_POSTDEREF_QQ_BIT    0x0040
+#define FEATURE_REFALIASING_BIT     0x0080
+#define FEATURE_SAY_BIT             0x0100
+#define FEATURE_SIGNATURES_BIT      0x0200
+#define FEATURE_STATE_BIT           0x0400
+#define FEATURE_SWITCH_BIT          0x0800
+#define FEATURE_UNIEVAL_BIT         0x1000
+#define FEATURE_UNICODE_BIT         0x2000
 
 #define FEATURE_BUNDLE_DEFAULT	0
 #define FEATURE_BUNDLE_510	1
@@ -54,6 +55,12 @@
 	 FEATURE_IS_ENABLED_MASK(FEATURE_FC_BIT)) \
     )
 
+#define FEATURE_ISA_IS_ENABLED \
+    ( \
+	CURRENT_FEATURE_BUNDLE == FEATURE_BUNDLE_CUSTOM && \
+	 FEATURE_IS_ENABLED_MASK(FEATURE_ISA_BIT) \
+    )
+
 #define FEATURE_SAY_IS_ENABLED \
     ( \
 	(CURRENT_FEATURE_BUNDLE >= FEATURE_BUNDLE_510 && \
@@ -236,6 +243,14 @@ S_magic_sethint_feature(pTHX_ SV *keysv, const char *keypv, STRLEN keylen,
             }
             return;
 
+        case 'i':
+            if (keylen == sizeof("feature_isa")-1
+                 && memcmp(subf+1, "sa", keylen - sizeof("feature_")) == 0) {
+                mask = FEATURE_ISA_BIT;
+                break;
+            }
+            return;
+
         case 'm':
             if (keylen == sizeof("feature_myref")-1
                  && memcmp(subf+1, "yref", keylen - sizeof("feature_")) == 0) {