remove DOES's usage of SvSCREAM

Currently the SvSCREAM flag is set on a temporary SV whose string value
is "isa", but where for the purposes of printing

    Can't call method "XXX"

its name is treated as "DOES" rather than "isa".

Instead, set the temp SV's PVX buffer to point to a special static
string (PL_isa_DOES) whose value is "isa", but the where the error
reporting code can compare the address with PL_isa_DOES and if so, print
"DOES" instead.

This is to reduce the number of odd special cases for the SvSCREAM flag.

Author: iabyn

pp_hot.c

@@ -4365,6 +4365,8 @@ Perl_vivify_ref(pTHX_ SV *sv, U32 to_what)
     return sv;
 }
 
+extern char PL_isa_DOES[];
+
 PERL_STATIC_INLINE HV *
 S_opmethod_stash(pTHX_ SV* meth)
 {
@@ -4443,7 +4445,7 @@ S_opmethod_stash(pTHX_ SV* meth)
 		     && SvOBJECT(ob))))
     {
 	Perl_croak(aTHX_ "Can't call method \"%"SVf"\" on unblessed reference",
-		   SVfARG((SvSCREAM(meth) && strEQ(SvPV_nolen_const(meth),"isa"))
+		   SVfARG((SvPVX(meth) == PL_isa_DOES)
                                         ? newSVpvs_flags("DOES", SVs_TEMP)
                                         : meth));
     }

sv.h

@@ -369,7 +369,7 @@ perform the upgrade if necessary.  See C<L</svtype>>.
 #define SVp_IOK		0x00001000  /* has valid non-public integer value */
 #define SVp_NOK		0x00002000  /* has valid non-public numeric value */
 #define SVp_POK		0x00004000  /* has valid non-public pointer value */
-#define SVp_SCREAM	0x00008000  /* method name is DOES */
+#define SVp_SCREAM	0x00008000  /* currently unused on plain scalars */
 #define SVphv_CLONEABLE	SVp_SCREAM  /* PVHV (stashes) clone its objects */
 #define SVpgv_GP	SVp_SCREAM  /* GV has a valid GP */
 #define SVprv_PCS_IMPORTED  SVp_SCREAM  /* RV is a proxy for a constant
@@ -443,7 +443,6 @@ perform the upgrade if necessary.  See C<L</svtype>>.
 
    SVf_POK, SVp_POK also set:
    0x00004400   Normal
-   0x0000C400   method name for DOES (SvSCREAM)
    0x40004400   FBM compiled (SvVALID)
    0x4000C400   *** Formerly used for pad names ***
 

universal.c

@@ -184,6 +184,10 @@ The SV can be a Perl object or the name of a Perl class.
 
 #include "XSUB.h"
 
+/* a special string address whose value is "isa", but whicb perl knows
+ * to treat as if it were really "DOES" */
+char PL_isa_DOES[] = "isa";
+
 bool
 Perl_sv_does_sv(pTHX_ SV *sv, SV *namesv, U32 flags)
 {
@@ -222,11 +226,14 @@ Perl_sv_does_sv(pTHX_ SV *sv, SV *namesv, U32 flags)
     PUSHs(namesv);
     PUTBACK;
 
-    methodname = newSVpvs_flags("isa", SVs_TEMP);
-    /* ugly hack: use the SvSCREAM flag so S_method_common
-     * can figure out we're calling DOES() and not isa(),
-     * and report eventual errors correctly. --rgs */
-    SvSCREAM_on(methodname);
+    /* create a PV with value "isa", but with a special address
+     * so that perl knows were' realling doing "DOES" instead */
+    methodname = newSV_type(SVt_PV);
+    SvLEN(methodname) = 0;
+    SvCUR(methodname) = strlen(PL_isa_DOES);
+    SvPVX(methodname) = PL_isa_DOES;
+    SvPOK_on(methodname);
+    sv_2mortal(methodname);
     call_sv(methodname, G_SCALAR | G_METHOD);
     SPAGAIN;