try

compiler/semstmts.nim

@@ -1602,7 +1602,9 @@ proc typeSectionRightSidePass(c: PContext, n: PNode) =
       # we fill it out later. For magic generics like 'seq', it won't be filled
       # so we use tyNone instead of nil to not crash for strange conversions
       # like: mydata.seq
-      if s.typ.kind in {tyOpenArray, tyVarargs} and s.typ.len == 1:
+      if s.typ.kind == tyArray and s.typ.len == 2:
+        discard
+      elif s.typ.kind in {tyOpenArray, tyVarargs} and s.typ.len == 1:
         # XXX investigate why `tySequence` cannot be added here for now.
         discard
       else:
@@ -1717,18 +1719,16 @@ proc typeSectionRightSidePass(c: PContext, n: PNode) =
   c.skipTypes = @[]
 
 proc checkForMetaFields(c: PContext; n: PNode; hasError: var bool) =
-  proc checkMeta(c: PContext; n: PNode; t: PType; hasError: var bool; parent: PType) =
-    if t != nil and (t.isMetaType or t.kind == tyNone) and tfGenericTypeParam notin t.flags:
-      if t.kind == tyBuiltInTypeClass and t.len == 1 and t.elementType.kind == tyProc:
-        localError(c.config, n.info, ("'$1' is not a concrete type; " &
-          "for a callback without parameters use 'proc()'") % t.typeToString)
-      elif t.kind == tyNone and parent != nil:
-        # TODO: openarray has the `tfGenericTypeParam` flag & generics
-        # TODO: handle special cases (sink etc.) and views
-        localError(c.config, n.info, errTIsNotAConcreteType % parent.typeToString)
-      else:
-        localError(c.config, n.info, errTIsNotAConcreteType % t.typeToString)
-      hasError = true
+  proc errMsg(c: PContext; n: PNode; t: PType; parent: PType)=
+    if t.kind == tyBuiltInTypeClass and t.len == 1 and t.elementType.kind == tyProc:
+      localError(c.config, n.info, ("'$1' is not a concrete type; " &
+        "for a callback without parameters use 'proc()'") % t.typeToString)
+    elif t.kind == tyNone and parent != nil:
+      # TODO: openarray has the `tfGenericTypeParam` flag & generics
+      # TODO: handle special cases (sink etc.) and views
+      localError(c.config, n.info, errTIsNotAConcreteType % parent.typeToString)
+    else:
+      localError(c.config, n.info, errTIsNotAConcreteType % t.typeToString)
 
   if n.isNil: return
   case n.kind
@@ -1738,14 +1738,10 @@ proc checkForMetaFields(c: PContext; n: PNode; hasError: var bool) =
     checkForMetaFields(c, n.lastSon, hasError)
   of nkSym:
     let t = n.sym.typ
-    case t.kind
-    of tySequence, tySet, tyArray, tyOpenArray, tyVar, tyLent, tyPtr, tyRef,
-       tyProc, tyGenericInvocation, tyGenericInst, tyAlias, tySink, tyOwned:
-      let start = ord(t.kind in {tyGenericInvocation, tyGenericInst})
-      for i in start..<t.len:
-        checkMeta(c, n, t[i], hasError, t)
-    else:
-      checkMeta(c, n, t, hasError, nil)
+    let comp = findUnspecifiedGenericsOrNil(t)
+    if comp != nil:
+      errMsg(c, n, comp, t)
+      hasError = true
   else:
     internalAssert c.config, false
 

compiler/types.nim

@@ -2030,3 +2030,30 @@ proc genericRoot*(t: PType): PType =
       result = t.sym.typ
     else:
       result = nil
+
+proc findUnspecifiedGenericsOrNil*(t: PType): PType=
+  result = nil
+  if t == nil:
+    return nil
+  if tfHasMeta in t.flags:
+    result = t
+  if t.size == szIllegalRecursion:
+    return nil
+  let tmp = t.size
+  t.size = szIllegalRecursion
+  case t.kind
+  of tyMetaTypes, tyNone:
+    result = t
+  of tyStatic:
+    if t.n == nil:
+      result = t
+  of tySequence, tySet, tyArray, tyOpenArray, tyVar, tyLent, tyPtr, tyRef,
+       tyProc, tyGenericInvocation, tyGenericInst, tyAlias, tySink, tyOwned:
+    let start = ord(t.kind in {tyGenericInvocation, tyGenericInst})
+    for i in start..<t.len:
+      result = findUnspecifiedGenericsOrNil(t[i])
+      if result != nil:
+        break
+  else:
+    discard
+  t.size = tmp

tests/concepts/tconceptsv2.nim

@@ -0,0 +1,22 @@
+discard """
+action: "run"
+output: '''
+A[array[0..0, int]]
+A[seq[int]]
+'''
+"""
+type
+  SomethingLike[T] = concept
+    proc len(s: Self): int
+    proc `[]`(s: Self; index: int): T
+
+  A[T] = object
+    x: T
+
+proc initA*(x: SomethingLike): auto =
+  A[type x](x: x)
+
+var a: array[1, int]
+var s: seq[int]
+echo typeof(initA(a))
+echo typeof(initA(s))