Allow escaped interpolation-like sequences in variable defaults

command/test-fixtures/validate-valid/main.tf

@@ -1,3 +1,8 @@
+variable "var_with_escaped_interp" {
+  # This is here because in the past it failed. See Github #13001
+  default = "foo-$${bar.baz}"
+}
+
 resource "test_instance" "foo" {
     ami = "bar"
 

config/config.go

@@ -285,8 +285,15 @@ func (c *Config) Validate() error {
 		}
 
 		interp := false
-		fn := func(ast.Node) (interface{}, error) {
-			interp = true
+		fn := func(n ast.Node) (interface{}, error) {
+			// LiteralNode is a literal string (outside of a ${ ... } sequence).
+			// interpolationWalker skips most of these. but in particular it
+			// visits those that have escaped sequences (like $${foo}) as a
+			// signal that *some* processing is required on this string. For
+			// our purposes here though, this is fine and not an interpolation.
+			if _, ok := n.(*ast.LiteralNode); !ok {
+				interp = true
+			}
 			return "", nil
 		}
 

config/config_test.go

@@ -572,6 +572,13 @@ func TestConfigValidate_varDefaultInterpolate(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_varDefaultInterpolateEscaped(t *testing.T) {
+	c := testConfig(t, "validate-var-default-interpolate-escaped")
+	if err := c.Validate(); err != nil {
+		t.Fatalf("should be valid, but got err: %s", err)
+	}
+}
+
 func TestConfigValidate_varDup(t *testing.T) {
 	c := testConfig(t, "validate-var-dup")
 	if err := c.Validate(); err == nil {

config/test-fixtures/validate-var-default-interpolate-escaped/main.tf

@@ -0,0 +1,5 @@
+variable "foo" {
+  # This should be considered valid since the sequence is escaped and is
+  # thus not actually an interpolation.
+  default = "foo bar $${aws_instance.foo.bar}"
+}