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diff --git a/js/src/jit-test/tests/debug/Frame-onStep-12.js b/js/src/jit-test/tests/debug/Frame-onStep-12.js
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+// Check that stepping doesn't make it look like unreachable code is running.
+
+// Because our script source notes record only those bytecode offsets
+// at which source positions change, the default behavior in the
+// absence of a source note is to attribute a bytecode instruction to
+// the same source location as the preceding instruction. When control
+// flows from the preceding bytecode to the one we're emitting, that's
+// usually plausible. But successors in the bytecode stream are not
+// necessarily successors in the control flow graph. If the preceding
+// bytecode was a back edge of a loop, or the jump at the end of a
+// 'then' clause, its source position can be completely unrelated to
+// that of its successor.
+//
+// We try to avoid showing such nonsense offsets to the user by
+// requiring breakpoints and single-stepping to stop only at a line's
+// entry points, as reported by Debugger.Script.prototype.getLineOffsets;
+// and then ensuring that those entry points are all offsets mentioned
+// explicitly in the source notes, and hence deliberately attributed
+// to the given bytecode.
+//
+// This bit of JavaScript compiles to bytecode ending in a branch
+// instruction whose source position is the body of an unreachable
+// loop. The first instruction of the bytecode we emit following it
+// will inherit this nonsense position, if we have not explicitly
+// emitted a source note for said instruction.
+//
+// This test steps across such code and verifies that control never
+// appears to enter the unreachable loop.
+
+var bitOfCode = `debugger;                    // +0
+                 if(false) {                  // +1
+                   for(var b=0; b<0; b++) {   // +2
+                      c = 2                   // +3
+                    }                         // +4
+                 }`;                          // +5
+
+var g = newGlobal({newCompartment: true});
+var dbg = Debugger(g);
+
+g.eval("function nothing() { }\n");
+
+var log = '';
+dbg.onDebuggerStatement = function(frame) {
+  let debugLine = frame.script.getOffsetLocation(frame.offset).lineNumber;
+  frame.onStep = function() {
+    let foundLine = this.script.getOffsetLocation(this.offset).lineNumber;
+    if (this.script.getLineOffsets(foundLine).indexOf(this.offset) >= 0) {
+      log += (foundLine - debugLine).toString(16);
+    }
+  };
+};
+
+function testOne(name, body, expected) {
+  print(name);
+  log = '';
+  g.eval(`function ${name} () { ${body} }`);
+  g.eval(`${name}();`);
+  assertEq(log, expected);
+}
+
+
+
+// Test the instructions at the end of a "try".
+testOne("testTryFinally",
+        `try {
+           ${bitOfCode}
+         } finally {            // +6
+         }                      // +7
+         nothing();             // +8
+        `, "1689");
+
+// The same but without a finally clause.
+testOne("testTryCatch",
+        `try {
+           ${bitOfCode}
+         } catch (e) {          // +6
+         }                      // +7
+         nothing();             // +8
+        `, "189");
+
+// Test the instructions at the end of a "catch".
+testOne("testCatchFinally",
+        `try {
+           throw new TypeError();
+         } catch (e) {
+           ${bitOfCode}
+         } finally {            // +6
+         }                      // +7
+         nothing();             // +8
+        `, "1689");
+
+// Test the instruction at the end of a "finally" clause.
+testOne("testFinally",
+        `try {
+         } finally {
+           ${bitOfCode}
+         }                      // +6
+         nothing();             // +7
+        `, "178");
+
+// Test the instruction at the end of a "then" clause.
+testOne("testThen",
+        `if (1 === 1) {
+           ${bitOfCode}
+         } else {               // +6
+         }                      // +7
+         nothing();             // +8
+        `, "189");
+
+// Test the instructions leaving a switch block.
+testOne("testSwitch",
+        `var x = 5;
+         switch (x) {
+           case 5:
+             ${bitOfCode}
+         }                      // +6
+         nothing();             // +7
+        `, "178");