make range type checking more restrictive, see tn8vsint16 test case; minor breaking change

compiler/sigmatch.nim

@@ -390,7 +390,16 @@ proc isConvertibleToRange(f, a: PType): bool =
   # be less picky for tyRange, as that it is used for array indexing:
   if f.kind in {tyInt..tyInt64, tyUInt..tyUInt64} and
      a.kind in {tyInt..tyInt64, tyUInt..tyUInt64}:
-    result = true
+    case f.kind
+    of tyInt, tyInt64: result = true
+    of tyInt8: result = a.kind in {tyInt8, tyInt}
+    of tyInt16: result = a.kind in {tyInt8, tyInt16, tyInt}
+    of tyInt32: result = a.kind in {tyInt8, tyInt16, tyInt32, tyInt}
+    of tyUInt, tyUInt64: result = true
+    of tyUInt8: result = a.kind in {tyUInt8, tyUInt}
+    of tyUInt16: result = a.kind in {tyUInt8, tyUInt16, tyUInt}
+    of tyUInt32: result = a.kind in {tyUInt8, tyUInt16, tyUInt32, tyUInt}
+    else: result = false
   elif f.kind in {tyFloat..tyFloat128} and
        a.kind in {tyFloat..tyFloat128}:
     result = true

tests/range/tn8vsint16.nim

@@ -0,0 +1,18 @@
+discard """
+  output: '''9'''
+"""
+
+type
+  n32 = range[0..high(int)]
+  n8* = range[0'i8..high(int8)]
+
+proc `+`*(a: n32, b: n32{nkIntLit}): n32 = discard
+
+proc `-`*(a: n8, b: n8): n8 = n8(system.`-`(a, b))
+
+var x, y: n8
+var z: int16
+
+# ensure this doesn't call our '-' but system.`-` for int16:
+echo z - n8(9)
+