Check for multi-values maps in output too

A map value read from a config file will be the default
`[]map[string]interface{}` type decoded from HCL. Since this type can't
be applied to a variable, it's likely that it was a simple map. If
there's a single map value, we can pull that out of the slice during
Eval.

terraform/eval_output.go

@@ -98,6 +98,19 @@ func (n *EvalWriteOutput) Eval(ctx EvalContext) (interface{}, error) {
 			Sensitive: n.Sensitive,
 			Value:     valueTyped,
 		}
+	case []map[string]interface{}:
+		// an HCL map is multi-valued, so if this was read out of a config the
+		// map may still be in a slice.
+		if len(valueTyped) == 1 {
+			mod.Outputs[n.Name] = &OutputState{
+				Type:      "map",
+				Sensitive: n.Sensitive,
+				Value:     valueTyped[0],
+			}
+			break
+		}
+		return nil, fmt.Errorf("output %s type (%T) with %d values not valid for type map",
+			n.Name, valueTyped, len(valueTyped))
 	default:
 		return nil, fmt.Errorf("output %s is not a valid type (%T)\n", n.Name, valueTyped)
 	}

terraform/eval_output_test.go

@@ -0,0 +1,56 @@
+package terraform
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestEvalWriteMapOutput(t *testing.T) {
+	ctx := new(MockEvalContext)
+	ctx.StateState = NewState()
+	ctx.StateLock = new(sync.RWMutex)
+
+	cases := []struct {
+		name string
+		cfg  *ResourceConfig
+		err  bool
+	}{
+		{
+			// Eval should recognize a single map in a slice, and collapse it
+			// into the map value
+			"single-map",
+			&ResourceConfig{
+				Config: map[string]interface{}{
+					"value": []map[string]interface{}{
+						map[string]interface{}{"a": "b"},
+					},
+				},
+			},
+			false,
+		},
+		{
+			// we can't apply a multi-valued map to a variable, so this should error
+			"multi-map",
+			&ResourceConfig{
+				Config: map[string]interface{}{
+					"value": []map[string]interface{}{
+						map[string]interface{}{"a": "b"},
+						map[string]interface{}{"c": "d"},
+					},
+				},
+			},
+			true,
+		},
+	}
+
+	for _, tc := range cases {
+		evalNode := &EvalWriteOutput{Name: tc.name}
+		ctx.InterpolateConfigResult = tc.cfg
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := evalNode.Eval(ctx)
+			if err != nil && !tc.err {
+				t.Fatal(err)
+			}
+		})
+	}
+}