fixes #4766

compiler/seminst.nim

@@ -326,7 +326,8 @@ proc generateInstance(c: PContext, fn: PSym, pt: TIdTable,
   # no need to instantiate generic templates/macros:
   internalAssert c.config, fn.kind notin {skMacro, skTemplate}
   # generates an instantiated proc
-  if c.instCounter > 1000: internalError(c.config, fn.ast.info, "nesting too deep")
+  if c.instCounter > 50:
+    globalError(c.config, info, "generic instantiation too nested")
   inc(c.instCounter)
   # careful! we copy the whole AST including the possibly nil body!
   var n = copyTree(fn.ast)

tests/errmsgs/tnested_generic_instantiation.nim

@@ -0,0 +1,19 @@
+discard """
+errormsg: "generic instantiation too nested"
+file: "system.nim"
+"""
+
+# bug #4766
+
+type
+  Plain = ref object
+    discard
+
+  Wrapped[T] = object
+    value: T
+
+converter toWrapped[T](value: T): Wrapped[T] =
+  Wrapped[T](value: value)
+
+let result = Plain()
+discard $result