[ty] Check typeshed VERSIONS for parent modules when reporting failed stdlib imports

crates/ty_python_semantic/resources/mdtest/import/basic.md

@@ -192,6 +192,26 @@ from string.templatelib import Template  # error: [unresolved-import]
 from importlib.resources import abc  # error: [unresolved-import]
 ```
 
+## Attempting to import a stdlib submodule when both parts haven't yet been added
+
+`compression` and `compression.zstd` were both added in 3.14 so there is a typeshed `VERSIONS` entry
+for `compression` but not `compression.zstd`. We can't be confident `compression.zstd` exists but we
+do know `compression` does and can still give good diagnostics about it.
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+import compression.zstd  # error: [unresolved-import]
+from compression import zstd  # error: [unresolved-import]
+import compression.fakebutwhocansay  # error: [unresolved-import]
+from compression import fakebutwhocansay  # error: [unresolved-import]
+```
+
 ## Attempting to import a stdlib module that was previously removed
 
 <!-- snapshot-diagnostics -->

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Attempting_to_import…_(dba22bd97137ee38).snap

@@ -0,0 +1,83 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Attempting to import a stdlib submodule when both parts haven't yet been added
+mdtest path: crates/ty_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import compression.zstd  # error: [unresolved-import]
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+```
+
+# Diagnostics
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression.zstd`
+ --> src/mdtest_snippet.py:1:8
+  |
+1 | import compression.zstd  # error: [unresolved-import]
+  |        ^^^^^^^^^^^^^^^^
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression`
+ --> src/mdtest_snippet.py:2:6
+  |
+1 | import compression.zstd  # error: [unresolved-import]
+2 | from compression import zstd  # error: [unresolved-import]
+  |      ^^^^^^^^^^^
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression.fakebutwhocansay`
+ --> src/mdtest_snippet.py:3:8
+  |
+1 | import compression.zstd  # error: [unresolved-import]
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression`
+ --> src/mdtest_snippet.py:4:6
+  |
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+  |      ^^^^^^^^^^^
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -4790,21 +4790,26 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 let program = Program::get(self.db());
                 let typeshed_versions = program.search_paths(self.db()).typeshed_versions();
 
-                if let Some(version_range) = typeshed_versions.exact(&module_name) {
-                    // We know it is a stdlib module on *some* Python versions...
-                    let python_version = program.python_version(self.db());
-                    if !version_range.contains(python_version) {
-                        // ...But not on *this* Python version.
-                        diagnostic.info(format_args!(
-                            "The stdlib module `{module_name}` is only available on Python {version_range}",
-                            version_range = version_range.diagnostic_display(),
-                        ));
-                        add_inferred_python_version_hint_to_diagnostic(
-                            self.db(),
-                            &mut diagnostic,
-                            "resolving modules",
-                        );
-                        return;
+                // Loop over ancestors in case we have info on the parent module but not submodule
+                for module_name in module_name.ancestors() {
+                    if let Some(version_range) = typeshed_versions.exact(&module_name) {
+                        // We know it is a stdlib module on *some* Python versions...
+                        let python_version = program.python_version(self.db());
+                        if !version_range.contains(python_version) {
+                            // ...But not on *this* Python version.
+                            diagnostic.info(format_args!(
+                                "The stdlib module `{module_name}` is only available on Python {version_range}",
+                                version_range = version_range.diagnostic_display(),
+                            ));
+                            add_inferred_python_version_hint_to_diagnostic(
+                                self.db(),
+                                &mut diagnostic,
+                                "resolving modules",
+                            );
+                            return;
+                        }
+                        // We found the most precise answer we could, stop searching
+                        break;
                     }
                 }
             }