title: why does rbind() work and bind_rows() not work in combining these sf objects?
body: <p>I've got some <code>sf</code> objects I want to combine. I've stripped them down to a single polygon each (<code>rtmp</code> and <code>rtmp2</code>) for the purposes of this question. <code>rbind()</code> and <code>do.call(rbind,...)</code> seem to work fine, but <code>bind_rows()</code> doesn't. Obviously I can work around this, but what am I missing?</p>
  
  <p>(Data at bottom of question.)</p>
  
  <p>These work:</p>
  
  <pre><code>library(dplyr)  ## version 0.8.3
  library(sf)     ## version 0.8.0
  r1 &lt;- rbind(rtmp,rtmp2)  ## works
  r2 &lt;- do.call(rbind,list(rtmp,rtmp2)) ## works
  
  identical(r1,r2) ## TRUE
  all.equal(r1,r2) ## fails??
  </code></pre>
  
  <blockquote>
    <p>Error: Can't join on 'geometry' x 'geometry' because of incompatible types (sfc_POLYGON/sfc / sfc_POLYGON/sfc)</p>
  </blockquote>
  
  <p>This fails:</p>
  
  <pre><code>bind_rows(list(rtmp,rtmp2))
  </code></pre>
  
  <blockquote>
    <p>Error in .subset2(x, i) : 
      attempt to select less than one element in get1index<br>
    In addition: Warning messages:
    1: In bind_rows_(x, .id) :
      Vectorizing 'sfc_POLYGON' elements may not preserve their attributes
    2: In bind_rows_(x, .id) :
      Vectorizing 'sfc_POLYGON' elements may not preserve their attributes</p>
  </blockquote>
  
  <h2>data</h2>
  
  <pre><code>rtmp &lt;- structure(list(PROVCODE = "ON", geometry = structure(list(structure(list(
      structure(c(7201954.77714286, 7206422.6, 7206659.96857143, 
      7206183.55714286, 7206456.25714286, 7206918.87142857, 7207300.78857143, 
      7207337.03428571, 7207488.91714286, 7207400.89428571, 7208110.25714286, 
      7208360.52, 7208664.28571429, 7208710.88571429, 7208819.62285714, 
      7209207.96, 7209308.06285714, 7209665.33428571, 7209425.42857143, 
      7209568.68285714, 7209522.08285714, 7209870.72285714, 7210022.60571429, 
      7210017.42857143, 7210321.19428571, 7210507.59714286, 7210630.14, 
      7210407.49142857, 7210535.21142857, 7210378.15142857, 7210921.82285714, 
      7210906.87428571, 7211255.58285714, 7211310.45142857, 7211675.00857143, 
      7212183.14285714, 7208783.07714286, 7209760.22, 7208297.23714286, 
      7207629.9, 7201954.77714286, 935407.037142857, 937804.568571429, 
      937788.902857143, 937063.325714286, 936599.088571429, 936431.897142857, 
      936498.582857143, 936341.52, 936410.56, 936134.408571429, 
      936103.34, 935840.997142857, 935891.048571429, 935715.002857143, 
      935972.168571429, 936172.38, 935989.428571429, 935885.871428571, 
      935349.102857143, 935278.337142857, 934976.297142857, 934831.317142857, 
      934964.217142857, 934770.911428571, 934691.517142857, 934978.022857143, 
      934460.24, 934237.594285714, 933921.745714286, 933302.131428571, 
      933074.305714286, 932540.708571429, 932384.642857143, 931995.7, 
      932099.337142857, 931815.214285714, 929976.52, 928202.56, 
      927651.188571429, 927094.268571429, 935407.037142857), .Dim = c(41L, 
      2L))), class = c("XY", "POLYGON", "sfg"))), class = c("sfc_POLYGON", 
  "sfc"), precision = 0, bbox = structure(c(xmin = 7201954.77714286, 
  ymin = 927094.268571429, xmax = 7212183.14285714, ymax = 937804.568571429
  ), class = "bbox"), crs = structure(list(epsg = NA_integer_, 
      proj4string = NA_character_), class = "crs"), n_empty = 0L)), row.names = c(NA, 
  -1L), sf_column = "geometry", agr = structure(c(PROVCODE = NA_integer_), .Label = c("constant", 
  "aggregate", "identity"), class = "factor"), class = c("sf", 
  "tbl_df", "tbl", "data.frame"))
  
  rtmp2 &lt;- structure(list(PROVCODE = "ON", geometry = structure(list(structure(list(
      structure(c(6914891.13617828, 7022510.44138576, 7130129.74659325, 
      7130129.74659325, 7022510.44138576, 6914891.13617828, 6914891.13617828, 
      896361.565655993, 958495.600487533, 896361.565655993, 772093.495992912, 
      709959.461161371, 772093.495992912, 896361.565655993), .Dim = c(7L, 
      2L), .Dimnames = list(NULL, c("x", "y")))), class = c("XY", 
  "POLYGON", "sfg"))), class = c("sfc_POLYGON", "sfc"), precision = 0, bbox = structure(c(xmin = 6914891.13617828, 
  ymin = 709959.461161371, xmax = 7130129.74659325, ymax = 958495.600487533
  ), class = "bbox"), crs = structure(list(epsg = NA_integer_, 
      proj4string = NA_character_), class = "crs"), n_empty = 0L)), row.names = 277L, class = c("sf", 
  "data.frame"), sf_column = "geometry", agr = structure(c(PROVCODE = NA_integer_), .Label = c("constant", 
  "aggregate", "identity"), class = "factor"))
  </code></pre>