core: Fix detection of empty list/map defaults

This commit changes config parsing from weak decoding lists and maps
into []string and map[string]string respectively to decode into
[]interface{} and map[string]interface{} respectively. This is in order
to take advantage of the work integrated in #7082 to defeat the backward
compatibility features of the mapstructure library.

Test coverage of loading empty variables and validating their default
types against expectation.

config/config.go

@@ -891,7 +891,8 @@ func (v *Variable) ValidateTypeAndDefault() error {
 	}
 
 	if v.inferTypeFromDefault() != v.Type() {
-		return fmt.Errorf("'%s' has a default value which is not of type '%s'", v.Name, v.DeclaredType)
+		return fmt.Errorf("'%s' has a default value which is not of type '%s' (got '%s')",
+			v.Name, v.DeclaredType, v.inferTypeFromDefault().Printable())
 	}
 
 	return nil
@@ -924,13 +925,13 @@ func (v *Variable) inferTypeFromDefault() VariableType {
 		return VariableTypeString
 	}
 
-	var m map[string]string
+	var m map[string]interface{}
 	if err := hilmapstructure.WeakDecode(v.Default, &m); err == nil {
 		v.Default = m
 		return VariableTypeMap
 	}
 
-	var l []string
+	var l []interface{}
 	if err := hilmapstructure.WeakDecode(v.Default, &l); err == nil {
 		v.Default = l
 		return VariableTypeList

config/config_test.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"path/filepath"
+	"reflect"
 	"strings"
 	"testing"
 )
@@ -85,6 +86,31 @@ func TestConfigCount_var(t *testing.T) {
 	}
 }
 
+func TestConfig_emptyCollections(t *testing.T) {
+	c := testConfig(t, "empty-collections")
+	if len(c.Variables) != 3 {
+		t.Fatalf("bad: expected 3 variables, got %d", len(c.Variables))
+	}
+	for _, variable := range c.Variables {
+		switch variable.Name {
+		case "empty_string":
+			if variable.Default != "" {
+				t.Fatalf("bad: wrong default %q for variable empty_string", variable.Default)
+			}
+		case "empty_map":
+			if !reflect.DeepEqual(variable.Default, map[string]interface{}{}) {
+				t.Fatalf("bad: wrong default %#v for variable empty_map", variable.Default)
+			}
+		case "empty_list":
+			if !reflect.DeepEqual(variable.Default, []interface{}{}) {
+				t.Fatalf("bad: wrong default %#v for variable empty_list", variable.Default)
+			}
+		default:
+			t.Fatalf("Unexpected variable: %s", variable.Name)
+		}
+	}
+}
+
 func TestConfigValidate(t *testing.T) {
 	c := testConfig(t, "validate-good")
 	if err := c.Validate(); err != nil {

config/test-fixtures/empty-collections/main.tf

@@ -0,0 +1,11 @@
+variable "empty_string" {
+  default = ""
+}
+
+variable "empty_list" {
+  default = []
+}
+
+variable "empty_map" {
+  default = {}
+}