Fix anchoring bug in conditionals with only one branch.

ChangeLog

@@ -174,6 +174,11 @@ not used in PCRE2.
 from distribution tarballs, owing to a typo in Makefile.am which had 
 testoutput8-16-3 twice. Now fixed.
 
+39. If the only branch in a conditional subpattern was anchored, the whole 
+subpattern was treated as anchored, when it should not have been, since the 
+assumed empty second branch cannot be anchored. Demonstrated by test patterns 
+such as /(?(1)^())b/ or /(?(?=^))b/.
+
 
 Version 10.31 12-February-2018
 ------------------------------

src/pcre2_compile.c

@@ -1454,8 +1454,8 @@ else if ((i = escapes[c - ESCAPES_FIRST]) != 0)
       /* \N{U+ can be handled by the \x{ code. However, this construction is
       not valid in EBCDIC environments because it specifies a Unicode
       character, not a codepoint in the local code. For example \N{U+0041}
-      must be "A" in all environments. Also, in Perl, \N{U+ forces Unicode 
-      casing semantics for the entire pattern, so allow it only in UTF (i.e. 
+      must be "A" in all environments. Also, in Perl, \N{U+ forces Unicode
+      casing semantics for the entire pattern, so allow it only in UTF (i.e.
       Unicode) mode. */
 
       if (ptrend - p > 1 && *p == CHAR_U && p[1] == CHAR_PLUS)
@@ -1464,12 +1464,12 @@ else if ((i = escapes[c - ESCAPES_FIRST]) != 0)
         *errorcodeptr = ERR93;
 #else
         if (utf)
-          { 
+          {
           ptr = p + 1;
           escape = 0;   /* Not a fancy escape after all */
           goto COME_FROM_NU;
           }
-        else *errorcodeptr = ERR93;   
+        else *errorcodeptr = ERR93;
 #endif
         }
 
@@ -7864,10 +7864,11 @@ do {
      if (!is_anchored(scode, bracket_map, cb, atomcount, TRUE)) return FALSE;
      }
 
-   /* Condition */
+   /* Condition. If there is no second branch, it can't be anchored. */
 
    else if (op == OP_COND)
      {
+     if (scode[GET(scode,1)] != OP_ALT) return FALSE;
      if (!is_anchored(scode, bracket_map, cb, atomcount, inassert))
        return FALSE;
      }

testdata/testinput2

@@ -5459,4 +5459,19 @@ a)"xI
 
 /(?x-i-i)/
 
+/(?(?=^))b/I
+    abc
+
+/(?(?=^)|)b/I
+    abc
+
+/(?(?=^)|^)b/I
+    bbc
+\= Expect no match
+    abc     
+
+/(?(1)^|^())/I
+
+/(?(1)^())b/I
+
 # End of testinput2

testdata/testoutput2

@@ -16631,6 +16631,46 @@ Failed: error 194 at offset 3: invalid hyphen in option setting
 /(?x-i-i)/
 Failed: error 194 at offset 5: invalid hyphen in option setting
 
+/(?(?=^))b/I
+Capturing subpattern count = 0
+Last code unit = 'b'
+Subject length lower bound = 1
+    abc
+ 0: b
+
+/(?(?=^)|)b/I
+Capturing subpattern count = 0
+First code unit = 'b'
+Subject length lower bound = 1
+    abc
+ 0: b
+
+/(?(?=^)|^)b/I
+Capturing subpattern count = 0
+Compile options: <none>
+Overall options: anchored
+First code unit = 'b'
+Subject length lower bound = 1
+    bbc
+ 0: b
+\= Expect no match
+    abc     
+No match
+
+/(?(1)^|^())/I
+Capturing subpattern count = 1
+Max back reference = 1
+May match empty string
+Compile options: <none>
+Overall options: anchored
+Subject length lower bound = 0
+
+/(?(1)^())b/I
+Capturing subpattern count = 1
+Max back reference = 1
+Last code unit = 'b'
+Subject length lower bound = 1
+
 # End of testinput2
 Error -70: PCRE2_ERROR_BADDATA (unknown error number)
 Error -62: bad serialized data