fix: extract retType from KnownType.Func when type-checking function bodies

src/main/wacc/typeChecker.scala

@@ -74,10 +74,13 @@ object typeChecker {
       ctx: TypeCheckerCtx
   ): Unit = {
     prog.funcs.foreach { case FuncDecl(_, name, _, stmts) =>
-      val retType = ctx.typeOf(name)
+      ctx.typeOf(name) match {
+        case KnownType.Func(retType, _) =>
           stmts.toList.foreach(
             checkStmt(_, Constraint.Is(retType, s"function ${name.v} must return $retType"))
           )
+        case _ => ctx.error(Error.InternalError(name.pos, "function declaration with non-function"))
+      }
     }
     prog.main.toList.foreach(checkStmt(_, Constraint.Never("main function must not return")))
   }