clear using_trail when calling resolveUse

fixes #1231

src/analysis.zig

@@ -2390,6 +2390,7 @@ pub fn innermostContainer(handle: *const DocumentStore.Handle, source_index: usi
 }
 
 fn resolveUse(analyser: *Analyser, uses: []const Ast.Node.Index, symbol: []const u8, handle: *const DocumentStore.Handle) error{OutOfMemory}!?DeclWithHandle {
+    analyser.using_trail.clearRetainingCapacity();
     for (uses) |index| {
         const gop = try analyser.using_trail.getOrPut(analyser.gpa, index);
         if (gop.found_existing) continue;

tests/lsp_features/completion.zig

@@ -483,6 +483,22 @@ test "completion - usingnamespace" {
     , &.{
         .{ .label = "inner", .kind = .Function, .detail = "fn inner() void" },
     });
+    try testCompletion(
+        \\fn Bar(comptime Self: type) type {
+        \\    return struct {
+        \\        fn inner(self: Self) void { _ = self; }
+        \\    };
+        \\}
+        \\const Foo = struct {
+        \\    pub usingnamespace Bar(Foo);
+        \\    fn deinit(self: Foo) void { _ = self; }
+        \\};
+        \\const foo: Foo = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        .{ .label = "inner", .kind = .Function, .detail = "fn inner(self: Self) void" },
+        .{ .label = "deinit", .kind = .Function, .detail = "fn deinit(self: Foo) void" },
+    });
 }
 
 test "completion - block" {