configs: Include context when variable default has nested problem

Previously this error message was including only the main error message
and ignoring any context about what part of a potential nested data
structure it arose from.

Now we'll use our usual path formatting utility to introduce additional
context whenever a problem arises with a nested value.

internal/configs/named_values.go

@@ -14,6 +14,7 @@ import (
 	"github.com/zclconf/go-cty/cty/convert"
 
 	"github.com/hashicorp/terraform/internal/addrs"
+	"github.com/hashicorp/terraform/internal/tfdiags"
 )
 
 // A consistent detail message for all "not a valid identifier" diagnostics.
@@ -163,8 +164,11 @@ func decodeVariableBlock(block *hcl.Block, override bool) (*Variable, hcl.Diagno
 				diags = append(diags, &hcl.Diagnostic{
 					Severity: hcl.DiagError,
 					Summary:  "Invalid default value for variable",
-					Detail:   fmt.Sprintf("This default value is not compatible with the variable's type constraint: %s.", err),
-					Subject:  attr.Expr.Range().Ptr(),
+					Detail: fmt.Sprintf(
+						"This default value is not compatible with the variable's type constraint: %s.",
+						tfdiags.FormatError(err),
+					),
+					Subject: attr.Expr.Range().Ptr(),
 				})
 				val = cty.DynamicVal
 			}

internal/configs/named_values_test.go

@@ -0,0 +1,47 @@
+package configs
+
+import (
+	"testing"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+)
+
+func TestVariableInvalidDefault(t *testing.T) {
+	src := `
+		variable foo {
+			type = map(object({
+				foo = bool
+			}))
+
+			default = {
+				"thingy" = {
+					foo = "string where bool is expected"
+				}
+			}
+		}
+	`
+
+	hclF, diags := hclsyntax.ParseConfig([]byte(src), "test.tf", hcl.InitialPos)
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
+
+	_, diags = parseConfigFile(hclF.Body, nil, false, false)
+	if !diags.HasErrors() {
+		t.Fatal("unexpected success; want error")
+	}
+
+	for _, diag := range diags {
+		if diag.Severity != hcl.DiagError {
+			continue
+		}
+		if diag.Summary != "Invalid default value for variable" {
+			t.Errorf("unexpected diagnostic summary: %q", diag.Summary)
+			continue
+		}
+		if got, want := diag.Detail, `This default value is not compatible with the variable's type constraint: ["thingy"].foo: a bool is required.`; got != want {
+			t.Errorf("wrong diagnostic detault\ngot:  %s\nwant: %s", got, want)
+		}
+	}
+}