Re-add ifexpr, but require 'end' to terminate it

src/lparser.cpp

@@ -3121,6 +3121,31 @@ static void expsuffix (LexState *ls, expdesc *v, int line, int flags, TypeHint *
 }
 
 
+static int cond (LexState *ls, bool for_while_loop = false);
+static void ifexpr (LexState *ls, expdesc *v) {
+  throw_warn(ls, "'if a then b else c' is deprecated", "use 'a ? b : c' instead", WT_DEPRECATED);
+  /*
+  ** 'if' expressions are based on a patch published by by Ryota Hirose.
+  */
+  FuncState *fs = ls->fs;
+  int condition;
+  int escape = NO_JUMP;
+  int reg;
+  luaX_next(ls);
+  condition = cond(ls);
+  checknext(ls, TK_THEN);
+  expr(ls, v);
+  reg = luaK_exp2anyreg(fs, v);
+  luaK_concat(fs, &escape, luaK_jump(fs));
+  luaK_patchtohere(fs, condition);
+  checknext(ls, TK_ELSE);
+  expr(ls, v);
+  checknext(ls, TK_END);
+  luaK_exp2reg(fs, v, reg);
+  luaK_patchtohere(fs, escape);
+}
+
+
 static void newexpr (LexState *ls, expdesc *v) {
   FuncState *fs = ls->fs;
 
@@ -3638,6 +3663,7 @@ static BinOpr subexpr (LexState *ls, expdesc *v, int limit, TypeHint *prop, int
     subexpr(ls, v, UNARY_PRIORITY, nullptr, flags);
     luaK_prefix(ls->fs, uop, v, line);
   }
+  else if (ls->t.token == TK_IF) ifexpr(ls, v);
   else if (ls->t.token == '+') {
     int line = ls->getLineNumber();
     luaX_next(ls); /* skip '+' */
@@ -3948,7 +3974,7 @@ static void restassign (LexState *ls, struct LHS_assign *lh, int nvars) {
   luaK_storevar(ls->fs, &lh->v, &e);
 }
 
-static int cond (LexState *ls, bool for_while_loop = false) {
+static int cond (LexState *ls, bool for_while_loop) {
   /* cond -> exp */
   expdesc v;
   ls->used_walrus = false;

testes/pluto/basic.pluto

@@ -773,6 +773,13 @@ do
     }
     assert(true ? (t:t()) : false)
 
+    -- Longhand: 'if' expression
+    assert(if a == 3 then
+        "yes"
+    else
+        "no"
+    end == "yes")
+
     -- @pluto_warnings: enable-unreachable-code
 end