stricter var type conversions, subranges under preview define

closes nim-lang#24032

changelog.md

@@ -28,6 +28,21 @@
   errors for ambiguous type symbols, and macros operating on generic proc AST
   may encounter symchoice nodes instead of the arbitrarily resolved type symbol nodes.
 
+- With `-d:nimPreviewStrictVarRange`, variables of range types do not match
+  `var` parameters of their base type. This is to prevent implicit operations
+  that can cause values to escape their range, i.e.:
+
+  ```nim
+  type Foo = range[0..5]
+  var foo: Foo = 5
+  proc double(x: var int) =
+    x = x * 2
+  double(foo)
+  echo foo # 10
+  ```
+
+  will give a type mismatch error.
+
 ## Standard library additions and changes
 
 [//]: # "Changes:"

compiler/sigmatch.nim

@@ -1017,9 +1017,22 @@ proc inferStaticsInRange(c: var TCandidate,
     doInferStatic(lowerBound, getInt(upperBound) + 1 - lengthOrd(c.c.config, concrete))
 
 template subtypeCheck() =
-  if result <= isSubrange and f.last.skipTypes(abstractInst).kind in {
-      tyRef, tyPtr, tyVar, tyLent, tyOwned}:
+  case result
+  of isIntConv:
     result = isNone
+  of isSubrange:
+    if c.c.config.isDefined("nimPreviewStrictVarRange"):
+      result = isNone
+  of isConvertible:
+    if f.last.skipTypes(abstractInst).kind != tyOpenArray:
+      # exclude var openarray which compiler supports
+      result = isNone
+  of isSubtype:
+    if f.last.skipTypes(abstractInst).kind in {
+        tyRef, tyPtr, tyVar, tyLent, tyOwned}:
+      # compiler can't handle subtype conversions with pointer indirection
+      result = isNone
+  else: discard
 
 proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
   # this proc is always called for a pair of matching types

lib/pure/json.nim

@@ -1094,6 +1094,8 @@ proc initFromJson[T](dst: var OrderedTable[string, T]; jsonNode: JsonNode; jsonP
 proc initFromJson[T](dst: var ref T; jsonNode: JsonNode; jsonPath: var string)
 proc initFromJson[T](dst: var Option[T]; jsonNode: JsonNode; jsonPath: var string)
 proc initFromJson[T: distinct](dst: var T; jsonNode: JsonNode; jsonPath: var string)
+when defined(nimPreviewStrictVarRange):
+  proc initFromJson[T: range](dst: var T; jsonNode: JsonNode; jsonPath: var string)
 proc initFromJson[T: object|tuple](dst: var T; jsonNode: JsonNode; jsonPath: var string)
 
 # initFromJson definitions
@@ -1224,6 +1226,24 @@ macro assignDistinctImpl[T: distinct](dst: var T;jsonNode: JsonNode; jsonPath: v
 proc initFromJson[T: distinct](dst: var T; jsonNode: JsonNode; jsonPath: var string) =
   assignDistinctImpl(dst, jsonNode, jsonPath)
 
+when defined(nimPreviewStrictVarRange):
+  # doesn't work without #24037
+  macro assignRangeImpl[T: range](dst: var T;jsonNode: JsonNode; jsonPath: var string) =
+    let typImpl = getTypeImpl(dst)
+    assert typImpl.kind == nnkBracketExpr and typImpl[0].eqIdent"range"
+    let lowNode =
+      if typImpl[^1].kind == nnkInfix:
+        typImpl[^1][1]
+      else:
+        typImpl[1]
+    let baseTyp = getType(lowNode)
+
+    result = quote do:
+      initFromJson(`baseTyp`(`dst`), `jsonNode`, `jsonPath`)
+
+  proc initFromJson[T: range](dst: var T; jsonNode: JsonNode; jsonPath: var string) =
+    assignRangeImpl(dst, jsonNode, jsonPath)
+
 proc detectIncompatibleType(typeExpr, lineinfoNode: NimNode) =
   if typeExpr.kind == nnkTupleConstr:
     error("Use a named tuple instead of: " & typeExpr.repr, lineinfoNode)

lib/pure/unicode.nim

@@ -331,7 +331,7 @@ proc runeOffset*(s: openArray[char], pos: Natural, start: Natural = 0): int =
     i = 0
     o = start
   while i < pos:
-    o += runeLenAt(s, o)
+    inc o, runeLenAt(s, o)
     if o >= s.len:
       return -1
     inc i

tests/errmsgs/t22097.nim

@@ -1,9 +1,9 @@
 discard """
-  errormsg: "for a 'var' type a variable needs to be passed; but 'uint16(x)' is immutable"
+  errormsg: "type mismatch: got <uint8>"
 """
 
 proc toUInt16(x: var uint16) =
   discard
 
 var x = uint8(1)
-toUInt16 x
+toUInt16 x

tests/int/twrongexplicitvarconv.nim

@@ -0,0 +1,16 @@
+discard """
+  action: reject
+  nimout: '''
+  but expression 'int(a)' is immutable, not 'var'
+'''
+"""
+
+proc `++`(n: var int) =
+  n += 1
+
+var a: int32 = 15
+
+++int(a) #[tt.Error
+^ type mismatch: got <int>]#
+
+echo a

tests/int/twrongvarconv.nim

@@ -0,0 +1,9 @@
+proc `++`(n: var int) =
+  n += 1
+
+var a: int32 = 15
+
+++a #[tt.Error
+^ type mismatch: got <int32>]#
+
+echo a

tests/iter/titer12.nim

@@ -49,7 +49,7 @@ proc filter[T](it: (iterator : T), f: proc(x: T): bool): (iterator : T) =
 
 proc len[T](it : iterator : T) : Natural =
   for i in it():
-    result += 1
+    result.inc 1
 
 proc simpleSeqIterator(s :seq[int]) : iterator : int =
   iterator it: int {.closure.} =

tests/overflow/trangechecks.nim

@@ -12,7 +12,7 @@ var expected: int
 var x: range[1..10] = 10
 
 try:
-  x += 1
+  inc x, 1
   echo x
 except OverflowDefect, RangeDefect:
   expected += 1
@@ -27,7 +27,7 @@ except OverflowDefect, RangeDefect:
 
 x = 1
 try:
-  x -= 1
+  dec x, 1
   echo x
 except OverflowDefect, RangeDefect:
   expected += 1

tests/range/twrongvarconv.nim

@@ -0,0 +1,18 @@
+discard """
+  matrix: "-d:nimPreviewStrictVarRange"
+"""
+
+# issue #24032
+
+proc `++`(n: var int) =
+  n += 1
+
+type
+  r = range[ 0..15 ]
+
+var a: r = 15
+
+++a #[tt.Error
+^ type mismatch: got <r>]#
+
+echo a