Add test for impl lookup on struct types with different field names and orders (#5416)

This test demonstrates the scenarios discussed in
#5413

Author: danakj

toolchain/check/testdata/impl/lookup/min_prelude/struct.carbon

@@ -0,0 +1,54 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/min_prelude/facet_types.carbon
+// EXTRA-ARGS: --custom-core --no-dump-sem-ir
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/impl/lookup/min_prelude/struct.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/impl/lookup/min_prelude/struct.carbon
+
+interface Z {
+  let X:! type;
+  fn ZZ() -> X;
+}
+
+class C { adapt (); }
+class D { adapt (); }
+
+impl {.a: (), .b: ()} as Z where .X = C {
+  fn ZZ() -> C { return () as C; }
+}
+
+// A struct with different field names. Structs with different field names are
+// different types and can be matched by an impl lookup with matching field
+// names.
+impl {.aa: (), .bb: ()} as Z where .X = D {
+  fn ZZ() -> D { return () as D; }
+}
+
+// TODO: Are structs with different field orders different types too, for impl
+// lookup? Or should this impl be diagnosed as overlapping with the impl on
+// `{.a, .b}`? Raised in:
+// https://github.com/carbon-language/carbon-lang/issues/5413
+impl {.b: (), .a: ()} as Z where .X = D {
+  fn ZZ() -> D { return () as D; }
+}
+
+fn F(T:! Z) -> T.X {
+  return T.ZZ();
+}
+
+fn G() {
+  // Check which impl is selected for struct literals with different field
+  // names.
+  let c1: C = F({.a: (), .b: ()});
+  let d1: D = F({.aa: (), .bb: ()});
+
+  // TODO: It is unclear if `F` should return `C` or `D`, since it has the same
+  // field names but different order as the impl that would give us `C` here.
+  let d2: D = F({.b: (), .a: ()});
+}