Unit test for over-large address in Select/Unselect (#2765)

Unit test for #2762 fix. Address
register is allowed to be larger than needed to address data.
Fails before the fix, passes after fix.
Includes the fix commit.

---------

Co-authored-by: Mathias Soeken <mathias.soeken@outlook.com>
Co-authored-by: Dmitry Vasilevsky <dmitryv@microsoft.com>

Author: 3 people

library/src/tests/resources/src/select.qs

@@ -60,6 +60,29 @@ namespace Test {
         Fact(equal, "Select+Unselect should be equivalent to identity up to global phase.");
     }
 
+    internal operation TestSelectLongerAddress() : Unit {
+        use addressRegister = Qubit[5];
+        use targetRegister = Qubit[4];
+
+        // Could be random, but fixed for reproducibility
+        let data = [
+            [false, false, false, false],
+            [false, false, true, false],
+            [true, true, false, false],
+            [false, true, false, false],
+        ];
+
+        // Select followed by unselect. This should be equivalent to identity.
+        within {
+            Select(data, addressRegister, targetRegister);
+        } apply {
+            // Do nothing.
+        }
+
+        Fact(CheckAllZero(targetRegister), "Target register must be in |0⟩ state after unlookup.");
+        Fact(CheckAllZero(addressRegister), "Address register must be in |0⟩ state after unlookup.");
+    }
+
     internal operation TestSelectFuzz(rounds : Int) : Unit {
         for _ in 1..rounds {
             let addressBits = DrawRandomInt(2, 6);

library/src/tests/table_lookup.rs

@@ -70,3 +70,12 @@ fn check_select_phase() {
         &Value::Tuple(vec![].into(), None),
     );
 }
+
+#[test]
+fn check_select_longer_address() {
+    test_expression_with_lib(
+        "Test.TestSelectLongerAddress()",
+        SELECT_TEST_LIB,
+        &Value::Tuple(vec![].into(), None),
+    );
+}

library/std/src/Std/TableLookup.qs

@@ -251,7 +251,7 @@ operation Unlookup(
         let phaseData = Padded(-2^addressBitsNeeded, false, phaseData);
 
         // Apply phase lookup to correct phases in the address register
-        PhaseLookup(address, phaseData);
+        PhaseLookup(address[...addressBitsNeeded - 1], phaseData);
     }
 }