Fix for issue #10342. better message for generic subclass instantiation (#10354)

* Fix for issue #10342. better message for generic subclass instantiation errors.

compiler/semtypinst.nim

@@ -490,7 +490,12 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
       result.kind = tyUserTypeClassInst
 
   of tyGenericBody:
-    localError(cl.c.config, cl.info, "cannot instantiate: '" & typeToString(t) & "'")
+    localError(
+      cl.c.config,
+      cl.info,
+      "cannot instantiate: '" &
+      typeToString(t, preferDesc) &
+      "'; Maybe generic arguments are missing?")
     result = errorType(cl.c)
     #result = replaceTypeVarsT(cl, lastSon(t))
 
@@ -555,6 +560,14 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
 
       for i in countup(0, sonsLen(result) - 1):
         if result.sons[i] != nil:
+          if result.sons[i].kind == tyGenericBody:
+            localError(
+              cl.c.config,
+              t.sym.info,
+              "cannot instantiate '" &
+              typeToString(result.sons[i], preferDesc) &
+              "' inside of type definition: '" &
+              t.owner.name.s & "'; Maybe generic arguments are missing?")
           var r = replaceTypeVarsT(cl, result.sons[i])
           if result.kind == tyObject:
             # carefully coded to not skip the precious tyGenericInst:

compiler/types.nim

@@ -456,12 +456,18 @@ proc typeToString(typ: PType, prefer: TPreferedDesc = preferName): string =
         result = $t.n.intVal
       else:
         result = "int literal(" & $t.n.intVal & ")"
-  of tyGenericBody, tyGenericInst, tyGenericInvocation:
+  of tyGenericInst, tyGenericInvocation:
     result = typeToString(t.sons[0]) & '['
     for i in countup(1, sonsLen(t)-1-ord(t.kind != tyGenericInvocation)):
       if i > 1: add(result, ", ")
       add(result, typeToString(t.sons[i], preferGenericArg))
     add(result, ']')
+  of tyGenericBody:
+    result = typeToString(t.lastSon) & '['
+    for i in countup(0, sonsLen(t)-2):
+      if i > 0: add(result, ", ")
+      add(result, typeToString(t.sons[i], preferTypeName))
+    add(result, ']')
   of tyTypeDesc:
     if t.sons[0].kind == tyNone: result = "typedesc"
     else: result = "type " & typeToString(t.sons[0])
@@ -612,7 +618,6 @@ proc typeToString(typ: PType, prefer: TPreferedDesc = preferName): string =
     result = typeToStr[t.kind]
   result.addTypeFlags(t)
 
-
 proc firstOrd*(conf: ConfigRef; t: PType): BiggestInt =
   case t.kind
   of tyBool, tyChar, tySequence, tyOpenArray, tyString, tyVarargs, tyProxy:

tests/generics/tsubclassgenericerror.nim

@@ -0,0 +1,11 @@
+discard """
+  errormsg: "cannot instantiate 'GenericParentType[T]' inside of type definition: 'GenericChildType'; Maybe generic arguments are missing?"
+  line: 8
+"""
+
+type
+  GenericParentType[T] = ref object of RootObj
+  GenericChildType[T] = ref object of GenericParentType # missing the [T]
+    val: T
+
+var instance : GenericChildType[int] = nil

tests/generics/twrong_generic_object.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "cannot instantiate: 'GenericNodeObj'"
+  errormsg: "cannot instantiate: 'GenericNodeObj[T]'; Maybe generic arguments are missing?"
   line: 21
 """
 # bug #2509