changes how the now illegal 'string == nil' comparison is detected

compiler/condsyms.nim

@@ -75,3 +75,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimIncrSeqV3")
   defineSymbol("nimAshr")
   defineSymbol("nimNoNilSeqs")
+  defineSymbol("nimNoNilSeqs2")

compiler/sigmatch.nim

@@ -1355,11 +1355,7 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
       else:
         result = isEqual
     of tyNil: result = f.allowsNil
-    of tyString:
+    of tyString: result = isConvertible
-      if optNilSeqs in c.c.config.options or c.magic != mEqCString:
-        result = isConvertible
-      else:
-        result = isNone
     of tyPtr:
       # ptr[Tag, char] is not convertible to 'cstring' for now:
       if a.len == 1:
@@ -1819,7 +1815,7 @@ proc userConvMatch(c: PContext, m: var TCandidate, f, a: PType,
     # see tests/tgenericconverter:
     let srca = typeRel(m, src, a)
     if srca notin {isEqual, isGeneric, isSubtype}: continue
-   
+
     let constraint = c.converters[i].typ.n[1].sym.constraint
     if not constraint.isNil and not matchNodeKinds(constraint, arg):
       continue

compiler/types.nim

@@ -1200,7 +1200,7 @@ proc typeAllowedAux(marker: var IntSet, typ: PType, kind: TSymKind,
      tyNone, tyForward, tyFromExpr:
     result = t
   of tyNil:
-    if kind != skConst: result = t
+    if kind != skConst and kind != skParam: result = t
   of tyString, tyBool, tyChar, tyEnum, tyInt..tyUInt64, tyCString, tyPointer:
     result = nil
   of tyOrdinal:

lib/system.nim

@@ -4048,6 +4048,10 @@ proc `==`*(x, y: cstring): bool {.magic: "EqCString", noSideEffect,
   elif x.isNil or y.isNil: result = false
   else: result = strcmp(x, y) == 0
 
+when defined(nimNoNilSeqs2):
+  proc `==`*(x: string; y: type(nil)): bool {.error.} = discard
+  proc `==`*(x: type(nil); y: string): bool {.error.} = discard
+
 template closureScope*(body: untyped): untyped =
   ## Useful when creating a closure in a loop to capture local loop variables by
   ## their current iteration values. Example:

tests/stdlib/tnilecho.nim

@@ -1,2 +1,6 @@
+discard """
+  output: ""
+"""
+
 var x = @["1", "", "3"]
 doAssert $x == """@["1", "", "3"]"""