don't allow casting to non-concrete types; fixes #5428 (#5502)

compiler/msgs.nim

@@ -89,6 +89,7 @@ type
     errMainModuleMustBeSpecified,
     errXExpected,
     errTIsNotAConcreteType,
+    errCastToANonConcreteType,
     errInvalidSectionStart, errGridTableNotImplemented, errGeneralParseError,
     errNewSectionExpected, errWhitespaceExpected, errXisNoValidIndexFile,
     errCannotRenderX, errVarVarTypeNotAllowed, errInstantiateXExplicitly,
@@ -326,6 +327,7 @@ const
     errMainModuleMustBeSpecified: "please, specify a main module in the project configuration file",
     errXExpected: "\'$1\' expected",
     errTIsNotAConcreteType: "\'$1\' is not a concrete type.",
+    errCastToANonConcreteType: "cannot cast to a non concrete type: \'$1\'",
     errInvalidSectionStart: "invalid section start",
     errGridTableNotImplemented: "grid table is not implemented",
     errGeneralParseError: "general parse error",

compiler/semexprs.nim

@@ -218,13 +218,16 @@ proc semConv(c: PContext, n: PNode): PNode =
 proc semCast(c: PContext, n: PNode): PNode =
   ## Semantically analyze a casting ("cast[type](param)")
   checkSonsLen(n, 2)
+  let targetType = semTypeNode(c, n.sons[0], nil)
+  let castedExpr = semExprWithType(c, n.sons[1])
+  if tfHasMeta in targetType.flags:
+    localError(n.sons[0].info, errCastToANonConcreteType, $targetType)
+  if not isCastable(targetType, castedExpr.typ):
+    localError(n.info, errExprCannotBeCastToX, $targetType)
   result = newNodeI(nkCast, n.info)
-  result.typ = semTypeNode(c, n.sons[0], nil)
+  result.typ = targetType
   addSon(result, copyTree(n.sons[0]))
-  addSon(result, semExprWithType(c, n.sons[1]))
-  if not isCastable(result.typ, result.sons[1].typ):
-    localError(result.info, errExprCannotBeCastToX,
-               typeToString(result.typ))
+  addSon(result, castedExpr)
 
 proc semLowHigh(c: PContext, n: PNode, m: TMagic): PNode =
   const

compiler/types.nim

@@ -20,6 +20,7 @@ type
     preferName, preferDesc, preferExported, preferModuleInfo, preferGenericArg
 
 proc typeToString*(typ: PType; prefer: TPreferedDesc = preferName): string
+template `$`*(typ: PType): string = typeToString(typ)
 
 proc base*(t: PType): PType =
   result = t.sons[0]