Fix #8404 JS backend doesn't handle float->int type conversion (#15950) [backport]

* Fix #8404 JS backend doesn't handle float->int type conversion
* handle conv to uint as cast, discard other cases
* limit to int32, times use int64
* toInt including tyInt64 break times timezones lib, ignore for now
* also affect to vm
* move to tests/misc/t8404.nim

(cherry picked from commit 797cb2e67b)

compiler/jsgen.nim

@@ -2124,11 +2124,17 @@ proc genConv(p: PProc, n: PNode, r: var TCompRes) =
   if dest.kind == src.kind:
     # no-op conversion
     return
-  case dest.kind:
-  of tyBool:
+  let toInt = (dest.kind in tyInt..tyInt32)
+  let fromInt = (src.kind in tyInt..tyInt32)
+  let toUint = (dest.kind in tyUInt..tyUInt32)
+  let fromUint = (src.kind in tyUInt..tyUInt32)
+  if toUint and (fromInt or fromUint):
+    let trimmer = unsignedTrimmer(dest.size)
+    r.res = "($1 $2)" % [r.res, trimmer]
+  elif dest.kind == tyBool:
     r.res = "(!!($1))" % [r.res]
     r.kind = resExpr
-  of tyInt:
+  elif toInt:
     r.res = "(($1)|0)" % [r.res]
   else:
     # TODO: What types must we handle here?

tests/misc/t8404.nim

@@ -0,0 +1,33 @@
+discard """
+  targets: "c cpp js"
+"""
+template main() =
+  block: # bug #8404
+    # can conv
+    template float2int(T) =
+      var a = -1.0
+      let b = T(a)
+      doAssert b < 0
+      let c = b + 1
+      doAssert c is T
+      doAssert c == 0
+
+    float2int(int8)
+    float2int(int16)
+    float2int(int32)
+    float2int(int64)
+
+  block:
+    # can handle middle conv
+    # `/` can trigger int to float
+    template float2int(T) =
+      let n = T(1 / 256)
+      doAssert n == 0
+
+    float2int(int8)
+    float2int(int16)
+    float2int(int32)
+    # float2int(int64)
+main()
+static:
+  main()