remove a special case in sigmatch; distinct pointer types no longer match nil type (#20251)

* remove a special case in sigmatch; distinct pointer types no longer match `nil` type

* add tests

* fixes tests

* Update changelog.md

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

changelog.md

@@ -29,6 +29,8 @@
 
 - `nimPreviewDotLikeOps` is going to be removed or deprecated.
 
+- `nil` is no longer a valid value for distinct pointer types.
+
 ## Standard library additions and changes
 
 [//]: # "Changes:"

compiler/sigmatch.nim

@@ -1305,8 +1305,6 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
       if sameDistinctTypes(f, a): result = isEqual
       #elif f.base.kind == tyAnything: result = isGeneric  # issue 4435
       elif c.coerceDistincts: result = typeRel(c, f.base, a, flags)
-    elif a.kind == tyNil and f.base.kind in NilableTypes:
-      result = f.allowsNil # XXX remove this typing rule, it is not in the spec
     elif c.coerceDistincts: result = typeRel(c, f.base, a, flags)
   of tySet:
     if a.kind == tySet:

tests/distinct/tnil.nim

@@ -1,29 +1,21 @@
-discard """
-output: '''
-1
-0
-0
-'''
-"""
 {.experimental: "notnil".}
 type
   MyPointer = distinct pointer
   MyString = distinct string
   MyInt = distinct int
 
-proc foo(a: MyPointer) =
+proc foo(a: MyPointer): int =
   # workaround a Windows 'repr' difference:
-  echo cast[int](a)
+  cast[int](a)
 
-foo(cast[MyPointer](1))
-foo(cast[MyPointer](nil))
-foo(nil)
+doAssert foo(cast[MyPointer](1)) == 1
+doAssert foo(cast[MyPointer](nil)) == 0
+doAssert foo(MyPointer(nil)) == 0
 
 var p: MyPointer
 p = cast[MyPointer](1)
 p = cast[MyPointer](nil)
 p = nil.MyPointer
-p = nil
 
 var i: MyInt
 i = 1.MyInt

tests/errmsgs/tdistinct_nil.nim

@@ -0,0 +1,23 @@
+discard """
+  cmd: "nim check $file"
+  action: reject
+  nimout: '''
+tdistinct_nil.nim(23, 4) Error: type mismatch: got <typeof(nil)>
+but expected one of:
+proc foo(x: DistinctPointer)
+  first type mismatch at position: 1
+  required type for x: DistinctPointer
+  but expression 'nil' is of type: typeof(nil)
+
+expression: foo(nil)
+'''
+"""
+
+type
+  DistinctPointer = distinct pointer
+
+proc foo(x: DistinctPointer) =
+  discard
+
+foo(DistinctPointer(nil))
+foo(nil)