fix(stdlib): Correctly promote numbers to bigints when left-shifting (#1354)

Author: peblair

compiler/test/suites/numbers.re

@@ -41,6 +41,11 @@ describe("numbers", ({test, testSkip}) => {
   assertRun("number_syntax8", "print(1.2e2)", "120.0\n");
   assertRun("number_syntax9", "print(1l)", "1\n");
   assertRun("number_syntax10", "print(1L)", "1\n");
+  assertRun(
+    "number_shift_promote",
+    "print(5 << 64)",
+    "92233720368547758080\n",
+  );
   assertRun(
     "number_syntax11",
     "print(987654321987654321987654321)",

stdlib/runtime/numbers.gr

@@ -1833,7 +1833,15 @@ export let (<<) = (x: Number, y: Number) => {
   } else {
     let xval = coerceNumberToWasmI64(x)
     let yval = coerceNumberToWasmI64(y)
-    WasmI32.toGrain(reducedInteger(WasmI64.shl(xval, yval))): Number
+    // if the number will be shifted beyond the end of the i64 range, promote to BigInt
+    // (note that we subtract one leading zero, since the leading bit is the sign bit)
+    if (WasmI64.leU(WasmI64.sub(WasmI64.clz(i64abs(xval)), 1N), yval)) {
+      let xbi = coerceNumberToBigInt(x)
+      let yval = coerceNumberToWasmI32(y)
+      WasmI32.toGrain(reducedBigInteger(BI.shl(xbi, yval))): Number
+    } else {
+      WasmI32.toGrain(reducedInteger(WasmI64.shl(xval, yval))): Number
+    }
   }
 }