enable experimental:strictDefs (#24225)

tests/stdlib/config.nims

@@ -3,3 +3,5 @@ switch("styleCheck", "error")
 switch("define", "nimPreviewSlimSystem")
 switch("define", "nimPreviewCstringConversion")
 switch("define", "nimPreviewProcConversion")
+switch("warningAserror", "ProveInit")
+switch("warningAserror", "Uninit")

tests/stdlib/talgorithm.nim

@@ -124,7 +124,7 @@ proc main() =
     var x = @[1, 7, 8, 11, 21, 33, 45, 99]
     var y = @[6, 7, 9, 12, 57, 66]
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y)
     doAssert merged.isSorted
     doAssert merged == sorted(x & y)
@@ -133,7 +133,7 @@ proc main() =
     var x = @[111, 88, 76, 56, 45, 31, 22, 19, 11, 3]
     var y = @[99, 85, 83, 82, 69, 64, 48, 42, 33, 31, 26, 13]
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y, (x, y) => -system.cmp(x, y))
     doAssert merged.isSorted((x, y) => -system.cmp(x, y))
     doAssert merged == sorted(x & y, SortOrder.Descending)
@@ -142,7 +142,7 @@ proc main() =
     var x: seq[int] = @[]
     var y = @[1]
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y)
     doAssert merged.isSorted
     doAssert merged.isSorted(SortOrder.Descending)
@@ -152,7 +152,7 @@ proc main() =
     var x = [1, 3, 5, 5, 7]
     var y: seq[int] = @[]
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y)
     doAssert merged.isSorted
     doAssert merged == @x
@@ -167,26 +167,26 @@ proc main() =
 
 
   block:
-    var x: array[0, int]
+    var x = default(array[0, int])
     var y = [1, 4, 6, 7, 9]
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y)
     doAssert merged.isSorted
     doAssert merged == @y
 
   block:
-    var x: array[0, int]
-    var y: array[0, int]
+    var x: array[0, int] = []
+    var y: array[0, int] = []
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y)
     doAssert merged.isSorted
     doAssert merged.len == 0
 
   block:
-    var x: array[0, int]
-    var y: array[0, int]
+    var x: array[0, int] = []
+    var y: array[0, int] = []
 
     var merged: seq[int] = @[99, 99, 99]
     merged.setLen(0)
@@ -195,10 +195,10 @@ proc main() =
     doAssert merged.len == 0
 
   block:
-    var x: seq[int]
-    var y: seq[int]
+    var x: seq[int] = @[]
+    var y: seq[int] = @[]
 
-    var merged: seq[int]
+    var merged: seq[int] = @[]
     merged.merge(x, y)
     doAssert merged.isSorted
     doAssert merged.len == 0
@@ -245,7 +245,7 @@ proc main() =
     var x = @[r(-12), r(1), r(3), r(8), r(13), r(88)]
     var y = @[r(4), r(7), r(12), r(13), r(77), r(99)]
 
-    var merged: seq[Record]
+    var merged: seq[Record] = @[]
     merged.setLen(0)
     merged.merge(x, y, ascendingCmp)
     doAssert merged.isSorted(ascendingCmp)
@@ -267,7 +267,7 @@ proc main() =
     doAssert merged == sorted(x & y, ascendingCmp)
 
 
-  var x: seq[(int, int)]
+  var x: seq[(int, int)] = @[]
   x.merge([(1,1)], [(1,2)], (a,b) => a[0] - b[0])
   doAssert x == @[(1, 1), (1, 2)]
 

tests/stdlib/tbitops.nim

@@ -173,7 +173,7 @@ proc main() =
 
   block:
     # basic mask operations (mutating)
-    var v: uint8
+    var v: uint8 = 0'u8
     v.setMask(0b1100_0000)
     v.setMask(0b0000_1100)
     doAssert v == 0b1100_1100
@@ -254,7 +254,7 @@ proc main() =
     doAssert a.clearMasked(3 .. 6) == 0b1000_0100
   block:
     # single bit operations
-    var v: uint8
+    var v: uint8 = 0'u8
     v.setBit(0)
     doAssert v == 0x0000_0001
     v.setBit(1)
@@ -269,7 +269,7 @@ proc main() =
     doAssert not v.testBit(6)
   block:
     # multi bit operations
-    var v: uint8
+    var v: uint8 = 0'u8
     v.setBits(0, 1, 7)
     doAssert v == 0b1000_0011
     v.flipBits(2, 3)
@@ -278,11 +278,11 @@ proc main() =
     doAssert v == 0b0000_1100
   block:
     # signed
-    var v: int8
+    var v: int8 = 0'i8
     v.setBit(7)
     doAssert v == -128
   block:
-    var v: uint64
+    var v: uint64 = 0'u64
     v.setBit(63)
     doAssert v == 0b1000_0000_0000_0000_0000_0000_0000_0000_0000_0000_0000_0000_0000_0000_0000_0000'u64
 
@@ -335,7 +335,7 @@ proc main() =
 
 block: # not ready for vm because exception is compile error
   try:
-    var v: uint32
+    var v: uint32 = 0'u32
     var i = 32
     v.setBit(i)
     doAssert false

tests/stdlib/tdeques.nim

@@ -190,7 +190,7 @@ proc main() =
     doAssert $a == "[10, 20, 30]"
 
   block:
-    var a, b: Deque[int]
+    var a, b: Deque[int] = initDeque[int]()
     for i in 1 .. 256:
       a.addLast(i)
     for i in 1 .. 255:

tests/stdlib/tdiff.nim

@@ -8,6 +8,7 @@ import std/strutils
 import std/assertions
 
 proc testHelper(f: seq[Item]): string =
+  result = ""
   for it in f:
     result.add(
       $it.deletedA & "." & $it.insertedB & "." & $it.startA & "." & $it.startB & "*"

tests/stdlib/tfdleak.nim

@@ -25,7 +25,7 @@ proc leakCheck(f: AsyncFD | int | FileHandle | SocketHandle, msg: string,
   var args = @[$f.int, msg, $expectLeak]
 
   when defined(windows):
-    var refFd: Handle
+    var refFd: Handle = default(Handle)
     # NOTE: This function shouldn't be used to duplicate sockets,
     #       as this function may mess with the socket internal refcounting.
     #       but due to the lack of type segmentation in the stdlib for
@@ -51,7 +51,7 @@ proc leakCheck(f: AsyncFD | int | FileHandle | SocketHandle, msg: string,
 proc isValidHandle(f: int): bool =
   ## Check if a handle is valid. Requires OS-native handles.
   when defined(windows):
-    var flags: DWORD
+    var flags: DWORD = default(DWORD)
     result = getHandleInformation(f.Handle, addr flags) != 0
   else:
     result = fcntl(f.cint, F_GETFD) != -1
@@ -88,7 +88,7 @@ proc main() =
     defer: close client
     client.connect("127.0.0.1", port)
 
-    var input: Socket
+    var input: Socket = default(Socket)
     server.accept(input)
 
     leakCheck(input.getFd, "accept()")

tests/stdlib/tfrexp1.nim

@@ -12,12 +12,12 @@ when manualTest:
   import strformat
 
 proc frexp_test(lo, hi, step: float64) =
-  var exp: int
+  var exp: int = 0
   var frac: float64
 
   var eps = 1e-15.float64
 
-  var x:float64 = lo
+  var x: float64 = lo
   while x <= hi:
     frac = frexp(x.float, exp)
     let rslt = pow(2.0, float(exp)) * frac

tests/stdlib/tgenast.nim

@@ -54,7 +54,7 @@ proc main =
           x.a = val
 
     m1()
-    var x0: Foo
+    var x0: Foo = Foo()
     x0.a1 = 10
     doAssert x0 == Foo(a: 10)
 

tests/stdlib/thashes.nim

@@ -112,7 +112,7 @@ proc main() =
 
   when sizeof(int) == 8 or defined(js):
     block:
-      var s: seq[Hash]
+      var s: seq[Hash] = @[]
       for a in [0.0, 1.0, -1.0, 1000.0, -1000.0]:
         let b = hash(a)
         doAssert b notin s
@@ -197,7 +197,7 @@ proc main() =
     doAssert hash(Obj5(t: false, x: 1)) != hash(Obj5(t: true, y: 2))
 
   block: # hash(ref|ptr|pointer)
-    var a: array[10, uint8]
+    var a: array[10, uint8] = default(array[10, uint8])
     # disableVm:
     whenVMorJs:
       # pending fix proposed in https://github.com/nim-lang/Nim/issues/15952#issuecomment-786312417

tests/stdlib/thttpclient_ssl.nim

@@ -45,7 +45,7 @@ when not defined(windows) and not disableSSLTesting():
   proc runServer(port: Port): bool {.thread.} =
     ## Run a trivial HTTPS server in a {.thread.}
     ## Exit after serving one request
-
+    result = false
     var socket = newSocket()
     socket.setSockOpt(OptReusePort, true)
     socket.bindAddr(port)
@@ -55,7 +55,7 @@ when not defined(windows) and not disableSSLTesting():
     ##  Handle one connection
     socket.listen()
 
-    var client: Socket
+    var client: Socket = default(Socket)
     var address = ""
 
     log "server: ready"

tests/stdlib/tisolation.nim

@@ -125,7 +125,7 @@ proc main(moveZeroesOut: static bool) =
     doAssert data.extract.id == 12
 
   block:
-    var x: seq[Isolated[JsonNode]]
+    var x: seq[Isolated[JsonNode]] = @[]
     x.add isolate(newJString("1234"))
 
     doAssert $x == """@[(value: "1234")]"""

tests/stdlib/tjsonutils.nim

@@ -15,7 +15,7 @@ proc testRoundtrip[T](t: T, expected: string) =
   let j = t.toJson
   doAssert $j == expected, "\n" & $j & "\n" & expected
   doAssert j.jsonTo(T).toJson == j
-  var t2: T
+  var t2: T = default(T)
   t2.fromJson(j)
   doAssert t2.toJson == j
 

tests/stdlib/tmacros.nim

@@ -25,6 +25,7 @@ block: # bug #17454
 block: # unpackVarargs
   block:
     proc bar1(a: varargs[int]): string =
+      result = ""
       for ai in a: result.add " " & $ai
     proc bar2(a: varargs[int]) =
       let s1 = bar1(a)

tests/stdlib/tnet.nim

@@ -94,10 +94,10 @@ block: # "IpAddress/Sockaddr conversion"
 
     doAssert($ipaddrstr == $ipaddr_1)
 
-    var sockaddr: Sockaddr_storage
-    var socklen: SockLen
-    var ipaddr_2: IpAddress
-    var port_2: Port
+    var sockaddr: Sockaddr_storage = default(Sockaddr_storage)
+    var socklen: SockLen = default(SockLen)
+    var ipaddr_2: IpAddress = default(IpAddress)
+    var port_2: Port = default(Port)
 
     toSockAddr(ipaddr_1, Port(0), sockaddr, socklen)
     fromSockAddr(sockaddr, socklen, ipaddr_2, port_2)
@@ -108,11 +108,11 @@ block: # "IpAddress/Sockaddr conversion"
     doAssert($ipaddr_1 == $ipaddr_2)
 
     if sockaddr.ss_family.cint == AF_INET.toInt:
-      var sockaddr4: Sockaddr_in
+      var sockaddr4: Sockaddr_in = default(Sockaddr_in)
       copyMem(addr sockaddr4, addr sockaddr, sizeof(sockaddr4))
       fromSockAddr(sockaddr4, socklen, ipaddr_2, port_2)
     elif sockaddr.ss_family.cint == AF_INET6.toInt:
-      var sockaddr6: Sockaddr_in6
+      var sockaddr6: Sockaddr_in6 = default(Sockaddr_in6)
       copyMem(addr sockaddr6, addr sockaddr, sizeof(sockaddr6))
       fromSockAddr(sockaddr6, socklen, ipaddr_2, port_2)
 

tests/stdlib/tnetdial.nim

@@ -40,7 +40,7 @@ proc testThread() {.thread.} =
 
 proc test() =
   let serverFd = initIPv6Server("::1", port)
-  var t: Thread[void]
+  var t: Thread[void] = default(Thread[void])
   createThread(t, testThread)
 
   var done = false

tests/stdlib/toptions.nim

@@ -47,6 +47,7 @@ proc main() =
 
     block example:
       proc find(haystack: string, needle: char): Option[int] =
+        result = none(int)
         for i, c in haystack:
           if c == needle:
             return some i
@@ -105,7 +106,7 @@ proc main() =
     block filter:
       doAssert(some(123).filter(proc (v: int): bool = v == 123) == some(123))
       doAssert(some(456).filter(proc (v: int): bool = v == 123).isNone)
-      doAssert(intNone.filter(proc (v: int): bool = doAssert false).isNone)
+      doAssert(intNone.filter(proc (v: int): bool = raiseAssert "false").isNone)
 
     block flatMap:
       proc addOneIfNotZero(v: int): Option[int] =
@@ -136,7 +137,7 @@ proc main() =
       doAssert(some(0).flatMap(maybeToString).flatMap(maybeExclaim) == none(string))
 
     block SomePointer:
-      var intref: ref int
+      var intref: ref int = nil
       doAssert(option(intref).isNone)
       intref.new
       doAssert(option(intref).isSome)

tests/stdlib/tparseutils.nim

@@ -44,12 +44,12 @@ proc test() =
   doAssert(parseSaturatedNatural("1_000_000", value) == 9)
   doAssert value == 1_000_000
 
-  var i64Value: int64
+  var i64Value: int64 = 0'i64
   discard parseBiggestInt("9223372036854775807", i64Value)
   doAssert i64Value == 9223372036854775807
 
   block:
-    var f: float
+    var f: float = 0.0
     let res = collect:
       for x in ["9.123456789012345+","11.123456789012345+","9.123456789012345-","8.123456789012345+","9.12345678901234-","9.123456789012345"]:
         (parseFloat(x, f, 0), $f)

tests/stdlib/tpegs.nim

@@ -192,7 +192,7 @@ block:
     expr.rule = sequence(capture(ident), *sequence(
                   nonterminal(ws), term('+'), nonterminal(ws), nonterminal(expr)))
 
-    var c: Captures
+    var c: Captures = default(Captures)
     var s = "a+b +  c +d+e+f"
     doAssert rawMatch(s, expr.rule, 0, c) == len(s)
     var a = ""
@@ -208,7 +208,7 @@ block:
     doAssert match("_______ana", peg"A <- 'ana' / . A")
     doAssert match("abcs%%%", peg"A <- ..A / .A / '%'")
 
-    var matches: array[0..MaxSubpatterns-1, string]
+    var matches: array[0..MaxSubpatterns-1, string] = default(array[0..MaxSubpatterns-1, string])
     if "abc" =~ peg"{'a'}'bc' 'xyz' / {\ident}":
       doAssert matches[0] == "abc"
     else:
@@ -325,7 +325,7 @@ block:
   call()
 call()
 """
-      var c: Captures
+      var c: Captures = default(Captures)
       doAssert program.len == program.rawMatch(grammar, 0, c)
       doAssert c.ml == 1
 

tests/stdlib/trandom.nim

@@ -11,7 +11,7 @@ when not defined(js):
 randomize(233)
 
 proc main() =
-  var occur: array[1000, int]
+  var occur: array[1000, int] = default(array[1000, int])
 
   for i in 0..100_000:
     let x = rand(high(occur))

tests/stdlib/tre.nim

@@ -23,7 +23,7 @@ proc testAll() =
   doAssert find("_____abc_______", re"abc") == 5
   doAssert findBounds("_____abc_______", re"abc") == (5,7)
 
-  var matches: array[6, string]
+  var matches: array[6, string] = default(array[6, string])
   if match("abcdefg", re"c(d)ef(g)", matches, 2):
     doAssert matches[0] == "d"
     doAssert matches[1] == "g"
@@ -85,7 +85,7 @@ proc testAll() =
   doAssert("XYZ".match(re"^\d*") == true)
 
   block:
-    var matches: array[16, string]
+    var matches: array[16, string] = default(array[16, string])
     if match("abcdefghijklmnop", re"(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)", matches):
       for i in 0..matches.high:
         doAssert matches[i] == $chr(i + 'a'.ord)

tests/stdlib/trst.nim

@@ -41,11 +41,13 @@ proc toAst(input: string,
             warnings: ref seq[string] = nil): string =
   ## If `error` is nil then no errors should be generated.
   ## The same goes for `warnings`.
+  result = ""
+
   proc testMsgHandler(filename: string, line, col: int, msgkind: MsgKind,
                       arg: string) =
     let mc = msgkind.whichMsgClass
     let a = $msgkind % arg
-    var message: string
+    var message: string = ""
     toLocation(message, filename, line, col + ColRstOffset)
     message.add " $1: $2" % [$mc, a]
     if mc == mcError:

tests/stdlib/trstgen.nim

@@ -22,11 +22,13 @@ proc toHtml(input: string,
             warnings: ref seq[string] = nil): string =
   ## If `error` is nil then no errors should be generated.
   ## The same goes for `warnings`.
+  result = ""
+
   proc testMsgHandler(filename: string, line, col: int, msgkind: MsgKind,
                       arg: string) =
     let mc = msgkind.whichMsgClass
     let a = $msgkind % arg
-    var message: string
+    var message: string = ""
     toLocation(message, filename, line, col + ColRstOffset)
     message.add " $1: $2" % [$mc, a]
     if mc == mcError:

tests/stdlib/tsequtils.nim

@@ -275,7 +275,10 @@ block: # toSeq test
       numeric = @[1, 2, 3, 4, 5, 6, 7, 8, 9]
       oddNumbers = toSeq(filter(numeric) do (x: int) -> bool:
         if x mod 2 == 1:
-          result = true)
+          result = true
+        else:
+          result = false
+      )
     doAssert oddNumbers == @[1, 3, 5, 7, 9]
 
   block:

tests/stdlib/tsortcall.nim

@@ -15,6 +15,7 @@ suite "test sort, sorted, and isSorted procs":
       age: int
 
   func newUser(name: string, age: int): User =
+    result = default(User)
     result.name = name
     result.age = age
 

tests/stdlib/tssl.nim

@@ -40,7 +40,7 @@ proc main() =
   when defined(posix):
     var
       ignoreAction = Sigaction(sa_handler: SIG_IGN)
-      oldSigPipeHandler: Sigaction
+      oldSigPipeHandler: Sigaction = default(Sigaction)
     if sigemptyset(ignoreAction.sa_mask) == -1:
       raiseOSError(osLastError(), "Couldn't create an empty signal set")
     if sigaction(SIGPIPE, ignoreAction, oldSigPipeHandler) == -1:
@@ -58,10 +58,10 @@ proc main() =
     let (_, port) = server.getLocalAddr()
     server.listen()
 
-    var clientThread: Thread[Port]
+    var clientThread: Thread[Port] = default(Thread[Port])
     createThread(clientThread, abruptShutdown, port)
 
-    var peer: Socket
+    var peer: Socket = default(Socket)
     try:
       server.accept(peer)
       peer.send(DummyData)
@@ -88,10 +88,10 @@ proc main() =
     let (_, port) = server.getLocalAddr()
     server.listen()
 
-    var clientThread: Thread[Port]
+    var clientThread: Thread[Port] = default(Thread[Port])
     createThread(clientThread, abruptShutdown, port)
 
-    var peer: Socket
+    var peer: Socket = default(Socket)
     try:
       server.accept(peer)
       peer.send(DummyData)
@@ -115,10 +115,10 @@ proc main() =
     let (_, port) = server.getLocalAddr()
     server.listen()
 
-    var clientThread: Thread[Port]
+    var clientThread: Thread[Port] = default(Thread[Port])
     createThread(clientThread, notifiedShutdown, port)
 
-    var peer: Socket
+    var peer: Socket = default(Socket)
     try:
       server.accept(peer)
       peer.send(DummyData)

tests/stdlib/tstdlib_various.nim

@@ -159,7 +159,7 @@ block tsegfaults:
     var crashes = 0
     proc main =
       try:
-        var x: ptr int
+        var x: ptr int = nil
         echo x[]
         try:
           raise newException(ValueError, "not a crash")

tests/stdlib/tstrbasics.nim

@@ -95,6 +95,7 @@ proc main() =
       a0.add b0.toOpenArray(1,3)
       doAssert a0 == "hioob"
     proc fn(c: openArray[char]): string =
+      result = ""
       result.add c
     doAssert fn("def") == "def"
     doAssert fn(['d','\0', 'f'])[2] == 'f'

tests/stdlib/tstrformat.nim

@@ -24,7 +24,7 @@ proc main() =
     template formatValue(result: var string; value: Obj; specifier: string) =
       result.formatValue($value, specifier)
 
-    var o: Obj
+    var o: Obj = default(Obj)
     doAssert fmt"{o}" == "foobar"
     doAssert fmt"{o:10}" == "foobar    "
 
@@ -292,7 +292,7 @@ proc main() =
     let x = 3.14
     doAssert fmt"{(if x!=0: 1.0/x else: 0):.5}" == "0.31847"
     doAssert fmt"""{(block:
-      var res: string
+      var res: string = ""
       for i in 1..15:
         res.add (if i mod 15 == 0: "FizzBuzz"
           elif i mod 5 == 0: "Buzz"
@@ -564,7 +564,7 @@ proc main() =
     doAssert fmt"""{(if true: "'" & ')' else: "")}""" == "')"
 
   block: # issue #20381
-    var ss: seq[string]
+    var ss: seq[string] = @[]
     template myTemplate(s: string) =
       ss.add s
       ss.add s

tests/stdlib/tstrscans.nim

@@ -113,6 +113,7 @@ block:
     while start+result < input.len and input[start+result] in seps: inc result
 
   proc demangle(s: string; res: var string; start: int): int =
+    result = 0
     while result+start < s.len and s[result+start] in {'_', '@'}: inc result
     res = ""
     while result+start < s.len and s[result+start] > ' ' and s[result+start] != '_':

tests/stdlib/tstrset.nim

@@ -22,6 +22,7 @@ type
   PRadixNodeLeaf = ref TRadixNodeLeaf
 
 proc search(r: PRadixNode, s: string): PRadixNode =
+  result = default(PRadixNode)
   var r = r
   var i = 0
   while r != nil:
@@ -55,7 +56,7 @@ proc contains*(r: PRadixNode, s: string): bool =
   return search(r, s) != nil
 
 proc testOrIncl*(r: var PRadixNode, s: string): bool =
-  nil
+  result = false
 
 proc incl*(r: var PRadixNode, s: string) = discard testOrIncl(r, s)
 

tests/stdlib/tsugar.nim

@@ -291,7 +291,7 @@ template main() =
 
   block: # bug #20704
     proc test() =
-      var xs, ys: seq[int]
+      var xs, ys: seq[int] = @[]
       for i in 0..5:
         xs.add(i)
 

tests/stdlib/ttasks.nim

@@ -513,12 +513,12 @@ block:
 
     proc testReturnValues() =
       let t = toTask returnsSomething(2233, 11)
-      var res: int
+      var res: int = 0
       t.invoke(addr res)
       doAssert res == 2233+11
 
       let tb = toTask noArgsButReturnsSomething()
-      var resB: string
+      var resB: string = ""
       tb.invoke(addr resB)
       doAssert resB == "abcdef"
 

tests/stdlib/ttimes.nim

@@ -12,11 +12,13 @@ proc staticTz(hours, minutes, seconds: int = 0): Timezone {.noSideEffect.} =
   let offset = hours * 3600 + minutes * 60 + seconds
 
   proc zonedTimeFromAdjTime(adjTime: Time): ZonedTime =
+    result = default(ZonedTime)
     result.isDst = false
     result.utcOffset = offset
     result.time = adjTime + initDuration(seconds = offset)
 
   proc zonedTimeFromTime(time: Time): ZonedTime =
+    result = default(ZonedTime)
     result.isDst = false
     result.utcOffset = offset
     result.time = time

tests/stdlib/ttypeinfo.nim

@@ -78,7 +78,7 @@ block:
 block: # bug #23556
   proc test =
     var
-      t: seq[int]
+      t: seq[int] = @[]
       aseq = toAny(t)
 
     invokeNewSeq(aseq, 0)

tests/stdlib/tvarargs.nim

@@ -6,7 +6,7 @@ import std/assertions
 
 template main =
   proc hello(x: varargs[string]): seq[string] =
-    var s: seq[string]
+    var s: seq[string] = @[]
     s.add x
     s
 

tests/stdlib/twrapnils.nim

@@ -33,9 +33,9 @@ proc main() =
 
     proc fun(a: Bar): auto = a.b2
 
-    var a: Foo
+    var a: Foo = nil
 
-    var x6: ptr Bar
+    var x6: ptr Bar = nil
     when nimvm: discard # pending https://github.com/timotheecour/Nim/issues/568
     else:
       x6 = create(Bar)
@@ -107,7 +107,7 @@ proc main() =
         d2: D
     proc identity[T](a: T): T = a
     proc identity2[T](a: T, ignore: int): T = a
-    var a: A
+    var a: A = default(A)
     doAssert ?.a.b.c.d.e.f == 0
     doAssert ?.a.b.c.d.e.d2.d3[].d3.e.d2.e.f == 0
     doAssert ?.a.b.c.d.d3[].e.f == 0
@@ -174,7 +174,7 @@ proc main() =
     doAssert ?.identity(d) == nil
     doAssert ?.identity(d[]) == default(typeof(d[]))
     doAssert ?.identity(d[]).i4 == 0
-    var a: A
+    var a: A = default(A)
     doAssert ?.identity(a) == default(A)
     doAssert ?.identity(a.a0) == 0
     doAssert ?.identity(a.d) == nil
@@ -195,7 +195,7 @@ proc main() =
           b1: float
 
     block:
-      var a: A
+      var a: A = default(A)
       doAssert ?.a.a0.a1[0].a2.addr == nil
       a = A(a2: 3)
       doAssert ?.a.a0.a1[0].a2.addr == nil

tests/stdlib/uselocks.nim

@@ -5,6 +5,7 @@ type MyType* [T] = object
   lock: Lock
 
 proc createMyType*[T]: MyType[T] =
+  result = default(MyType[T])
   initLock(result.lock)
 
 proc use* (m: var MyType): int =