Allow '.' in workspace.default-members in non-virtual workspaces.

src/cargo/core/workspace.rs

@@ -690,7 +690,9 @@ impl<'cfg> Workspace<'cfg> {
             for path in default {
                 let normalized_path = paths::normalize_path(&path);
                 let manifest_path = normalized_path.join("Cargo.toml");
-                if !self.members.contains(&manifest_path) {
+                if !self.members.contains(&manifest_path)
+                    && (self.is_virtual() || manifest_path != root_manifest_path)
+                {
                     // default-members are allowed to be excluded, but they
                     // still must be referred to by the original (unfiltered)
                     // members list. Note that we aren't testing against the

tests/testsuite/workspaces.rs

@@ -119,6 +119,35 @@ fn non_virtual_default_members_build_other_member() {
         .run();
 }
 
+#[cargo_test]
+fn non_virtual_default_members_build_root_project() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+                authors = []
+
+                [workspace]
+                members = ["bar"]
+                default-members = ["."]
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .file("bar/Cargo.toml", &basic_manifest("bar", "0.1.0"))
+        .file("bar/src/lib.rs", "pub fn bar() {}")
+        .build();
+
+    p.cargo("build")
+        .with_stderr(
+            "[..] Compiling foo v0.1.0 ([..])\n\
+             [..] Finished dev [unoptimized + debuginfo] target(s) in [..]\n",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn inferred_root() {
     let p = project()