Expand tyAlias in typeToString (#8634)

Fixes #8339

compiler/types.nim

@@ -426,6 +426,8 @@ proc typeToString(typ: PType, prefer: TPreferedDesc = preferName): string =
        sfAnon notin t.sym.flags:
     if t.kind == tyInt and isIntLit(t):
       result = t.sym.name.s & " literal(" & $t.n.intVal & ")"
+    elif t.kind == tyAlias:
+      result = typeToString(t.sons[0])
     elif prefer in {preferName, preferTypeName} or t.sym.owner.isNil:
       result = t.sym.name.s
       if t.kind == tyGenericParam and t.sonsLen > 0:

tests/errmsgs/t8339.nim

@@ -0,0 +1,8 @@
+discard """
+  line: 8
+  errormsg: "type mismatch: got <seq[int]> but expected 'seq[float]'"
+"""
+
+import sequtils
+
+var x: seq[float] = @[1].mapIt(it)