ext/typeexpr: Convert defaults for optional attrs

When parsing optional object attribute defaults, we previously verified
that the default value was convertible to the attribute type. However,
we did not keep this converted value.

This commit uses the converted default value, rather than delaying
conversion until later. In turn this prevents crashes when transforming
collections which contain objects with optional attributes, caused by
incompatible object types at the time of defaults application.

ext/typeexpr/get_type.go

@@ -249,7 +249,7 @@ func getType(expr hcl.Expression, constraint, withDefaults bool) (cty.Type, *Def
 			// If a default is set for an optional attribute, verify that it is
 			// convertible to the attribute type.
 			if defaultVal, ok := defaultValues[attrName]; ok {
-				_, err := convert.Convert(defaultVal, aty)
+				convertedDefaultVal, err := convert.Convert(defaultVal, aty)
 				if err != nil {
 					diags = append(diags, &hcl.Diagnostic{
 						Severity: hcl.DiagError,
@@ -258,6 +258,8 @@ func getType(expr hcl.Expression, constraint, withDefaults bool) (cty.Type, *Def
 						Subject:  defaultExpr.Range().Ptr(),
 					})
 					delete(defaultValues, attrName)
+				} else {
+					defaultValues[attrName] = convertedDefaultVal
 				}
 			}
 

ext/typeexpr/get_type_test.go

@@ -504,7 +504,9 @@ func TestGetTypeDefaults(t *testing.T) {
 					}, []string{"b"}),
 				}, []string{"a"}),
 				DefaultValues: map[string]cty.Value{
-					"a": cty.EmptyObjectVal,
+					"a": cty.ObjectVal(map[string]cty.Value{
+						"b": cty.NullVal(cty.Number),
+					}),
 				},
 				Children: map[string]*Defaults{
 					"a": {