[ty] Provide completions on TypeVars (#20943)

## Summary

closes astral-sh/ty#1370

## Test Plan

New snapshot tests

Author: sharkdp

crates/ty_ide/src/completion.rs

@@ -3896,6 +3896,55 @@ print(t'''{Foo} and Foo.zqzq<CURSOR>
         assert_snapshot!(test.completions_without_builtins(), @"<No completions found>");
     }
 
+    #[test]
+    fn typevar_with_upper_bound() {
+        let test = cursor_test(
+            "\
+def f[T: str](msg: T):
+    msg.<CURSOR>
+",
+        );
+        test.assert_completions_include("upper");
+        test.assert_completions_include("capitalize");
+    }
+
+    #[test]
+    fn typevar_with_constraints() {
+        // Test TypeVar with constraints
+        let test = cursor_test(
+            "\
+from typing import TypeVar
+
+class A:
+    only_on_a: int
+    on_a_and_b: str
+
+class B:
+    only_on_b: float
+    on_a_and_b: str
+
+T = TypeVar('T', A, B)
+
+def f(x: T):
+    x.<CURSOR>
+",
+        );
+        test.assert_completions_include("on_a_and_b");
+        test.assert_completions_do_not_include("only_on_a");
+        test.assert_completions_do_not_include("only_on_b");
+    }
+
+    #[test]
+    fn typevar_without_bounds_or_constraints() {
+        let test = cursor_test(
+            "\
+def f[T](x: T):
+    x.<CURSOR>
+",
+        );
+        test.assert_completions_include("__repr__");
+    }
+
     // NOTE: The methods below are getting somewhat ridiculous.
     // We should refactor this by converting to using a builder
     // to set different modes. ---AG

crates/ty_python_semantic/src/types/ide_support.rs

@@ -14,7 +14,7 @@ use crate::types::call::{CallArguments, MatchedArgument};
 use crate::types::signatures::Signature;
 use crate::types::{
     ClassBase, ClassLiteral, DynamicType, KnownClass, KnownInstanceType, Type,
-    class::CodeGeneratorKind,
+    TypeVarBoundOrConstraints, class::CodeGeneratorKind,
 };
 use crate::{Db, HasType, NameKind, SemanticModel};
 use ruff_db::files::{File, FileRange};
@@ -177,6 +177,29 @@ impl<'db> AllMembers<'db> {
 
             Type::TypeAlias(alias) => self.extend_with_type(db, alias.value_type(db)),
 
+            Type::TypeVar(bound_typevar) => {
+                match bound_typevar.typevar(db).bound_or_constraints(db) {
+                    None => {
+                        self.extend_with_type(db, Type::object());
+                    }
+                    Some(TypeVarBoundOrConstraints::UpperBound(bound)) => {
+                        self.extend_with_type(db, bound);
+                    }
+                    Some(TypeVarBoundOrConstraints::Constraints(constraints)) => {
+                        self.members.extend(
+                            constraints
+                                .elements(db)
+                                .iter()
+                                .map(|ty| AllMembers::of(db, *ty).members)
+                                .reduce(|acc, members| {
+                                    acc.intersection(&members).cloned().collect()
+                                })
+                                .unwrap_or_default(),
+                        );
+                    }
+                }
+            }
+
             Type::IntLiteral(_)
             | Type::BooleanLiteral(_)
             | Type::StringLiteral(_)
@@ -194,7 +217,6 @@ impl<'db> AllMembers<'db> {
             | Type::ProtocolInstance(_)
             | Type::SpecialForm(_)
             | Type::KnownInstance(_)
-            | Type::TypeVar(_)
             | Type::BoundSuper(_)
             | Type::TypeIs(_) => match ty.to_meta_type(db) {
                 Type::ClassLiteral(class_literal) => {