fixes #7179; Floats are not range checked (#25050)

fixes #7179


```nim
var f = 751.0
echo f.int8
```

In this case, `int8(float)` yields different numbers for different
optimization levels, since float to int conversions are undefined
behaviors. In this PR, it mitigates this problem by conversions to same
size integers before converting to the final type: i.e.
`int8(int64(float))`, which has UB problems but is better than before

Author: ringabout

compiler/transf.nim

@@ -552,7 +552,34 @@ proc transformConv(c: PTransf, n: PNode): PNode =
     # we don't include uint and uint64 here as these are no ordinal types ;-)
     if not isOrdinalType(source):
       # float -> int conversions. ugh.
-      result = transformSons(c, n)
+      # generate a range check:
+      if dest.kind in tyInt..tyInt64:
+        if dest.kind == tyInt64 or source.kind == tyInt64:
+          result = newTransNode(nkChckRange64, n, 3)
+        else:
+          result = newTransNode(nkChckRange, n, 3)
+        dest = skipTypes(n.typ, abstractVar)
+
+        if dest.size < source.size:
+          let intType =
+            if source.size == 4:
+              getSysType(c.graph, n.info, tyInt32)
+            else:
+              getSysType(c.graph, n.info, tyInt64)
+          result[0] = 
+            newTreeIT(n.kind, n.info, n.typ, n[0],
+              newTreeIT(nkConv, n.info, intType,
+              newNodeIT(nkType, n.info, intType), transform(c, n[1]))
+            )
+
+        else:
+          result[0] = transformSons(c, n)
+
+        result[1] = newIntTypeNode(firstOrd(c.graph.config, dest), dest)
+        result[2] = newIntTypeNode(lastOrd(c.graph.config, dest), dest)
+      else:
+        result = transformSons(c, n)
+
     elif firstOrd(c.graph.config, n.typ) <= firstOrd(c.graph.config, n[1].typ) and
         lastOrd(c.graph.config, n[1].typ) <= lastOrd(c.graph.config, n.typ):
       # BUGFIX: simply leave n as it is; we need a nkConv node,

tests/stdlib/tsystem.nim

@@ -245,3 +245,32 @@ proc bar2() =
 
 static: bar2()
 bar2()
+
+when not defined(js):
+  proc foo =
+    block:
+      var s1:int = -10
+      doAssertRaises(RangeDefect):
+        var n2:Natural = s1.Natural
+
+    block:
+      var f = 751.0
+      let m = f.int8
+
+    block:
+      var s2:float = -10
+      doAssertRaises(RangeDefect):
+        var n2:Natural = s2.Natural
+
+
+    block:
+      type A = range[0..10]
+
+      let f = 156.0
+
+      doAssertRaises(RangeDefect):
+        let a = f.A
+
+        echo a # 156
+
+  foo()