Reverts 1446dc87c3. Fixes #4333. Fixes #4170.

tests/async/tasyncexceptions.nim

@@ -5,6 +5,8 @@ discard """
 """
 import asyncdispatch
 
+# Note: This is a test case for a bug.
+
 proc accept(): Future[int] {.async.} =
   await sleepAsync(100)
   result = 4

tests/async/tawaitsemantics.nim

@@ -0,0 +1,59 @@
+discard """
+  file: "tawaitsemantics.nim"
+  exitcode: 0
+  output: '''
+Error caught
+Test infix
+Test call
+'''
+"""
+
+import asyncdispatch
+
+# This tests the behaviour of 'await' under different circumstances.
+# For example, when awaiting Future variable and this future has failed the
+# exception shouldn't be raised as described here
+# https://github.com/nim-lang/Nim/issues/4170
+
+proc thrower(): Future[void] =
+  result = newFuture[void]()
+  result.fail(newException(Exception, "Test"))
+
+proc dummy: Future[void] =
+  result = newFuture[void]()
+  result.complete()
+
+proc testInfix() {.async.} =
+  # Test the infix operator semantics.
+  var fut = thrower()
+  var fut2 = dummy()
+  await fut or fut2 # Shouldn't raise.
+  # TODO: what about: await thrower() or fut2?
+
+proc testCall() {.async.} =
+  await thrower()
+
+proc tester() {.async.} =
+  # Test that we can handle exceptions without 'try'
+  var fut = thrower()
+  doAssert fut.finished
+  doAssert fut.failed
+  doAssert fut.error.msg == "Test"
+  await fut # We are awaiting a 'Future', so no `read` occurs.
+  doAssert fut.finished
+  doAssert fut.failed
+  doAssert fut.error.msg == "Test"
+  echo("Error caught")
+
+  fut = testInfix()
+  await fut
+  doAssert fut.finished
+  doAssert(not fut.failed)
+  echo("Test infix")
+
+  fut = testCall()
+  await fut
+  doAssert fut.failed
+  echo("Test call")
+
+waitFor(tester())