Support self parameter detection on branching types

src/analysis.zig

@@ -415,7 +415,17 @@ pub fn firstParamIs(
         else => expected_type,
     };
 
-    return deref_type.eql(deref_expected_type);
+    return switch (deref_type.data) {
+        .either => |entries| {
+            for (entries) |entry| {
+                if (entry.type_data.eql(deref_expected_type.data)) {
+                    return true;
+                }
+            }
+            return false;
+        },
+        else => deref_type.eql(deref_expected_type),
+    };
 }
 
 pub fn getVariableSignature(

tests/lsp_features/completion.zig

@@ -4338,6 +4338,34 @@ test "generic function with @This() as self param" {
     });
 }
 
+test "methods of branching type" {
+    try testCompletion(
+        \\const Reader = switch (undefined) {
+        \\    .windows => struct {
+        \\        fn foo(_: *Reader) bool {}
+        \\    },
+        \\    else => struct {
+        \\        fn bar(_: *Reader) bool {}
+        \\    },
+        \\};
+        \\test {
+        \\    var reader: Reader = undefined;
+        \\    reader.<cursor>
+        \\}
+    , &.{
+        .{
+            .label = "foo",
+            .kind = .Function,
+            .detail = "fn (_: *either type) bool",
+        },
+        .{
+            .label = "bar",
+            .kind = .Function,
+            .detail = "fn (_: *either type) bool",
+        },
+    });
+}
+
 fn testCompletion(source: []const u8, expected_completions: []const Completion) !void {
     try testCompletionWithOptions(source, expected_completions, .{});
 }