fixes #9149 [backport]

lib/system.nim

@@ -4307,15 +4307,17 @@ proc `==`*(x, y: cstring): bool {.magic: "EqCString", noSideEffect,
 when defined(nimNoNilSeqs2):
   when not compileOption("nilseqs"):
     when defined(nimHasUserErrors):
-      proc `==`*(x: string; y: type(nil)): bool {.
+      # bug #9149; ensure that 'type(nil)' does not match *too* well by using 'type(nil) | type(nil)'.
+      # Eventually (in 0.20?) we will be able to remove this hack completely.
+      proc `==`*(x: string; y: type(nil) | type(nil)): bool {.
           error: "'nil' is now invalid for 'string'; compile with --nilseqs:on for a migration period".} =
         discard
-      proc `==`*(x: type(nil); y: string): bool {.
+      proc `==`*(x: type(nil) | type(nil); y: string): bool {.
           error: "'nil' is now invalid for 'string'; compile with --nilseqs:on for a migration period".} =
         discard
     else:
-      proc `==`*(x: string; y: type(nil)): bool {.error.} = discard
+      proc `==`*(x: string; y: type(nil) | type(nil)): bool {.error.} = discard
-      proc `==`*(x: type(nil); y: string): bool {.error.} = discard
+      proc `==`*(x: type(nil) | 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

tests/overload/tconverter_to_string.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''123
+c is not nil'''
+"""
+
+# bug #9149
+
+type
+  Container = ref object
+    data: int
+
+converter containerToString*(x: Container): string = $x.data
+
+var c = Container(data: 123)
+var str = string c
+echo str
+
+if c == nil: # this line can compile on v0.18, but not on 0.19
+  echo "c is nil"
+
+if not c.isNil:
+  echo "c is not nil"