Ensure that integer values used in SIMD Codable test are representable as JS "integers"

Author: stephentyrone

test/stdlib/SIMD.swift

@@ -7,9 +7,17 @@ import StdlibUnittest
 
 let SIMDCodableTests = TestSuite("SIMDCodable")
 
+// Round an integer to the closest representable JS integer value
+func jsInteger<T>(_ value: T) -> T where T : FixedWidthInteger {
+  // Attempt to round-trip though Double; if that fails it's because the
+  // rounded value is too large to fit in T, so use the largest value that
+  // does fit instead.
+  return T(exactly: Double(value)) ?? T(Double(T.max).nextDown)
+}
+
 func testRoundTrip<T>(_ for: T.Type)
 where T : SIMD, T.Scalar : FixedWidthInteger {
-  let input = T.random(in: T.Scalar.min ... T.Scalar.max)
+  let input = jsInteger(T.random(in: T.Scalar.min ... T.Scalar.max))
   let encoder = JSONEncoder()
   let decoder = JSONDecoder()
   do {