Bug 1304638 - Don't return "(intermediate value)" while decompiling SETELEM/STRICTSETELEM. r=nbp

This caused us to go to the fallback, and call `ValueToSource` to set the
message exception[1].

[1]: http://searchfox.org/mozilla-central/rev/c635b8c61d648bb8a0317c19f8905b3be8132a8a/js/src/jsopcode.cpp#1536

MozReview-Commit-ID: J7tfxoDnUUr
Signed-off-by: Emilio Cobos Álvarez <ecoal95@gmail.com>

js/src/jit-test/tests/basic/expression-autopsy.js

@@ -16,7 +16,7 @@ function check_one(expected, f, err) {
         throw new Error("didn't fail");
 }
 ieval = eval;
-function check(expr, expected=expr) {
+function check(expr, expected=expr, testStrict=true) {
     var end, err;
     for ([end, err] of [[".random_prop", " is undefined"], ["()", " is not a function"]]) {
         var statement = "o = {};" + expr + end, f;
@@ -47,6 +47,11 @@ function check(expr, expected=expr) {
             Function("with ({}) {} var undef, o; for (let z in [1, 2]) { " + statement + " }"),
         ];
 
+        if (testStrict) {
+            // Strict mode.
+            cases.push(Function("o", "undef", "\"use strict\";\n" + statement));
+        }
+
         for (var f of cases) {
             check_one(expected, f, err);
         }
@@ -67,12 +72,15 @@ check("o[65536]");
 check("o[268435455]");
 check("o['1.1']");
 check("o[4 + 'h']", "o['4h']");
-check("this.x");
 check("ieval(undef)", "ieval(...)");
 check("ieval.call()", "ieval.call(...)");
 check("ieval(...[])", "ieval(...)");
 check("ieval(...[undef])", "ieval(...)");
 check("ieval(...[undef, undef])", "ieval(...)");
+check("(o[0] = 4).foo", "o[0].foo");
+// NOTE: This one produces different output in strict mode since "this" is
+// undefined in that case.
+check("this.x", "this.x", false);
 
 for (let tok of ["|", "^", "&", "==", "!==", "===", "!==", "<", "<=", ">", ">=",
                  ">>", "<<", ">>>", "+", "-", "*", "/", "%"]) {
@@ -110,5 +118,20 @@ catch (e)
   assertEq(e.message, "can't convert null to object");
 }
 
+try {
+  (function() {
+    "use strict";
+    var o = [];
+    Object.freeze(o);
+    o[0] = "foo";
+  }());
+  throw new Error("didn't throw");
+} catch (e) {
+  assertEq(e instanceof TypeError, true,
+           "expected TypeError, got " + e);
+  assertEq(e.message,
+           "can't define array index property past the end of an array with non-writable length");
+}
+
 // Check fallback behavior
 assertThrowsInstanceOf(function () { for (let x of undefined) {} }, TypeError);

js/src/jsopcode.cpp

@@ -1265,6 +1265,16 @@ ExpressionDecompiler::decompilePC(jsbytecode* pc)
         return write("super.") &&
                quote(prop, '\0');
       }
+      case JSOP_SETELEM:
+      case JSOP_STRICTSETELEM:
+        // NOTE: We don't show the right hand side of the operation because
+        // it's used in error messages like: "a[0] is not readable".
+        //
+        // We could though.
+        return decompilePCForStackOperand(pc, -3) &&
+               write("[") &&
+               decompilePCForStackOperand(pc, -2) &&
+               write("]");
       case JSOP_GETELEM:
       case JSOP_CALLELEM:
         return decompilePCForStackOperand(pc, -2) &&