Allow type constraints on opaque type aliases

Allows some syntax that is ignored at runtime; only used
ahead of time by the type checker.

hphp/test/quick/typedef_constraint.php

@@ -0,0 +1,10 @@
+<?hh
+
+type Foo = int;
+newtype fbid as int = int;
+
+function f(fbid $x) {
+  echo $x;
+  echo "\n";
+}
+f(4);

hphp/test/quick/typedef_constraint.php.expect

@@ -0,0 +1 @@
+4

hphp/test/quick/typedef_constraint2.php

@@ -0,0 +1,3 @@
+<?hh
+
+type Foo as int = int;

hphp/test/quick/typedef_constraint2.php.expectf

@@ -0,0 +1 @@
+HipHop Fatal error: syntax error, unexpected T_AS, expecting '=' in %s on line 3

hphp/util/parser/hphp.ll

@@ -407,7 +407,7 @@ BACKQUOTE_CHARS     ("{"*([^$`\\{]|("\\"{ANY_CHAR}))|{BACKQUOTE_LITERAL_DOLLAR})
 
 <ST_IN_SCRIPTING>"shape"              { HH_ONLY_KEYWORD(T_SHAPE); }
 <ST_IN_SCRIPTING>"type"               { HH_ONLY_KEYWORD(T_UNRESOLVED_TYPE); }
-<ST_IN_SCRIPTING>"newtype"            { HH_ONLY_KEYWORD(T_UNRESOLVED_TYPE); }
+<ST_IN_SCRIPTING>"newtype"            { HH_ONLY_KEYWORD(T_UNRESOLVED_NEWTYPE); }
 
 <ST_IN_SCRIPTING>">>" {
   if (_scanner->getLookaheadLtDepth() < 2) {

hphp/util/parser/hphp.tab.hpp

@@ -190,7 +190,9 @@
      YYTOKEN(402, T_SHAPE),
      YYTOKEN(403, T_TYPE),
      YYTOKEN(404, T_UNRESOLVED_TYPE),
-     YYTOKEN(405, T_COMPILER_HALT_OFFSET)
+     YYTOKEN(405, T_NEWTYPE),
+     YYTOKEN(406, T_UNRESOLVED_NEWTYPE),
+     YYTOKEN(407, T_COMPILER_HALT_OFFSET)
    };
 #endif
 
@@ -224,5 +226,5 @@ typedef struct YYLTYPE
 #define YYTOKEN_MIN 258
 #endif
 #ifndef YYTOKEN_MAX
-#define YYTOKEN_MAX 405
+#define YYTOKEN_MAX 407
 #endif

hphp/util/parser/hphp.y

@@ -793,6 +793,8 @@ static int yylex(YYSTYPE *token, HPHP::Location *loc, Parser *_p) {
 %token T_SHAPE
 %token T_TYPE
 %token T_UNRESOLVED_TYPE
+%token T_NEWTYPE
+%token T_UNRESOLVED_NEWTYPE
 
 %token T_COMPILER_HALT_OFFSET
 
@@ -813,7 +815,7 @@ top_statement:
   | function_declaration_statement     { _p->nns(); $$ = $1;}
   | class_declaration_statement        { _p->nns(); $$ = $1;}
   | trait_declaration_statement        { _p->nns(); $$ = $1;}
-  | hh_typedef_statement               { $$ = $1; }
+  | hh_type_alias_statement            { $$ = $1; }
   | T_HALT_COMPILER '(' ')' ';'        { _p->onHaltCompiler();
                                          _p->finiParseTree();
                                          YYACCEPT;}
@@ -1850,6 +1852,7 @@ xhp_bareword:
   | T_TRAIT_C                          { $$ = $1;}
   | T_INSTEADOF                        { $$ = $1;}
   | T_TYPE                             { $$ = $1;}
+  | T_NEWTYPE                          { $$ = $1;}
   | T_SHAPE                            { $$ = $1;}
 ;
 
@@ -2327,11 +2330,21 @@ class_constant:
  * mode, but simplify down to the original thing
  */
 
-hh_typedef_statement:
+hh_opt_constraint:
+    /* empty */
+  | T_AS hh_type
+  ;
+
+hh_type_alias_statement:
     T_TYPE hh_name_with_typevar
-    '=' hh_type ';'                    { only_in_hh_syntax(_p);
+      '=' hh_type ';'                  { only_in_hh_syntax(_p);
                                          _p->onTypedef($$, $2, $4);
                                          _p->popTypeScope(); }
+  | T_NEWTYPE hh_name_with_typevar
+            hh_opt_constraint
+    '=' hh_type ';'                    { only_in_hh_syntax(_p);
+                                         _p->onTypedef($$, $2, $5);
+                                         _p->popTypeScope(); }
 ;
 
 hh_name_with_type:  /* foo -> int foo */

hphp/util/parser/lex.yy.cpp

@@ -79004,7 +79004,7 @@ YY_RULE_SETUP
 case 112:
 YY_RULE_SETUP
 #line 410 "hphp.ll"
-{ HH_ONLY_KEYWORD(T_UNRESOLVED_TYPE); }
+{ HH_ONLY_KEYWORD(T_UNRESOLVED_NEWTYPE); }
 	YY_BREAK
 case 113:
 YY_RULE_SETUP

hphp/util/parser/scanner.cpp

@@ -380,6 +380,7 @@ bool Scanner::tryParseShapeMemberList(TokenStore::iterator& pos) {
 
 static bool isUnresolved(int tokid) {
   return tokid == T_UNRESOLVED_LT ||
+         tokid == T_UNRESOLVED_NEWTYPE ||
          tokid == T_UNRESOLVED_TYPE;
 }
 
@@ -407,12 +408,13 @@ int Scanner::getNextToken(ScannerToken &t, Location &l) {
   }
 
   switch (tokid) {
+  case T_UNRESOLVED_NEWTYPE:
   case T_UNRESOLVED_TYPE: {
     auto pos = m_lookahead.begin();
     auto typePos = pos;
     nextLookahead(pos);
     if (pos->t == T_STRING) {
-      typePos->t = T_TYPE;
+      typePos->t = tokid == T_UNRESOLVED_TYPE ? T_TYPE : T_NEWTYPE;
     } else {
       typePos->t = T_STRING;
     }