add zero values to sets with a flatmap count of 1

If a flatmap value has a count of 1 and no other attributes, it usually
indicates the equivalent configuration of an empty (or default value)
set block. Treat this as containing a single zero value object and
insert that into the set.

config/hcl2shim/flatmap.go

@@ -347,10 +347,8 @@ func hcl2ValueFromFlatmapSet(m map[string]string, prefix string, ty cty.Type) (c
 		return cty.UnknownVal(ty), nil
 	}
 
-	// We actually don't really care about the "count" of a set for our
-	// purposes here, but we do need to check if it _exists_ in order to
-	// recognize the difference between null (not set at all) and empty.
-	if strCount, exists := m[prefix+"#"]; !exists {
+	strCount, exists := m[prefix+"#"]
+	if !exists {
 		return cty.NullVal(ty), nil
 	} else if strCount == UnknownVariableValue {
 		return cty.UnknownVal(ty), nil
@@ -394,7 +392,31 @@ func hcl2ValueFromFlatmapSet(m map[string]string, prefix string, ty cty.Type) (c
 		vals = append(vals, val)
 	}
 
-	if len(vals) == 0 {
+	if len(vals) == 0 && strCount == "1" {
+		// An empty set wouldn't be represented in the flatmap, so this must be
+		// a single empty object since the count is actually 1.
+		// Add an appropriately typed null value to the set.
+		var val cty.Value
+		switch {
+		case ety.IsMapType():
+			val = cty.MapValEmpty(ety)
+		case ety.IsListType():
+			val = cty.ListValEmpty(ety)
+		case ety.IsSetType():
+			val = cty.SetValEmpty(ety)
+		case ety.IsObjectType():
+			// TODO: cty.ObjectValEmpty
+			objectMap := map[string]cty.Value{}
+			for attr, ty := range ety.AttributeTypes() {
+				objectMap[attr] = cty.NullVal(ty)
+			}
+			val = cty.ObjectVal(objectMap)
+		default:
+			val = cty.NullVal(ety)
+		}
+		vals = append(vals, val)
+
+	} else if len(vals) == 0 {
 		return cty.SetValEmpty(ety), nil
 	}
 

config/hcl2shim/flatmap_test.go

@@ -679,10 +679,52 @@ func TestHCL2ValueFromFlatmap(t *testing.T) {
 				}),
 			}),
 		},
+		{
+			Flatmap: map[string]string{
+				"foo.#": "1",
+			},
+			Type: cty.Object(map[string]cty.Type{
+				"foo": cty.Set(cty.Object(map[string]cty.Type{
+					"bar": cty.String,
+				})),
+			}),
+			Want: cty.ObjectVal(map[string]cty.Value{
+				"foo": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"bar": cty.NullVal(cty.String),
+					}),
+				}),
+			}),
+		},
+		{
+			Flatmap: map[string]string{
+				"multi.#":                "1",
+				"multi.2.set.#":          "1",
+				"multi.2.set.3.required": "val",
+			},
+			Type: cty.Object(map[string]cty.Type{
+				"multi": cty.Set(cty.Object(map[string]cty.Type{
+					"set": cty.Set(cty.Object(map[string]cty.Type{
+						"required": cty.String,
+					})),
+				})),
+			}),
+			Want: cty.ObjectVal(map[string]cty.Value{
+				"multi": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"set": cty.SetVal([]cty.Value{
+							cty.ObjectVal(map[string]cty.Value{
+								"required": cty.StringVal("val"),
+							}),
+						}),
+					}),
+				}),
+			}),
+		},
 	}
 
-	for _, test := range tests {
-		t.Run(fmt.Sprintf("%#v as %#v", test.Flatmap, test.Type), func(t *testing.T) {
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%d %#v as %#v", i, test.Flatmap, test.Type), func(t *testing.T) {
 			got, err := HCL2ValueFromFlatmap(test.Flatmap, test.Type)
 
 			if test.WantErr != "" {