Add test coverage for `ImplOfUndefinedInterface` diagnostic (#4484)

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/testdata/impl/no_prelude/fail_undefined_interface.carbon

@@ -0,0 +1,102 @@
+// 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
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/impl/no_prelude/fail_undefined_interface.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/impl/no_prelude/fail_undefined_interface.carbon
+
+// --- fail_empty_struct.carbon
+
+library "[[@TEST_NAME]]";
+
+interface I;
+// CHECK:STDERR: fail_empty_struct.carbon:[[@LINE+7]]:1: error: implementation of undefined interface I [ImplOfUndefinedInterface]
+// CHECK:STDERR: impl {} as I {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR: fail_empty_struct.carbon:[[@LINE-4]]:1: note: interface was forward declared here [InterfaceForwardDeclaredHere]
+// CHECK:STDERR: interface I;
+// CHECK:STDERR: ^~~~~~~~~~~~
+// CHECK:STDERR:
+impl {} as I {}
+
+// --- fail_class.carbon
+
+library "[[@TEST_NAME]]";
+
+interface J;
+class C {}
+// CHECK:STDERR: fail_class.carbon:[[@LINE+6]]:1: error: implementation of undefined interface J [ImplOfUndefinedInterface]
+// CHECK:STDERR: impl C as J {}
+// CHECK:STDERR: ^~~~~~~~~~~~~
+// CHECK:STDERR: fail_class.carbon:[[@LINE-5]]:1: note: interface was forward declared here [InterfaceForwardDeclaredHere]
+// CHECK:STDERR: interface J;
+// CHECK:STDERR: ^~~~~~~~~~~~
+impl C as J {}
+
+// CHECK:STDOUT: --- fail_empty_struct.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %I.type: type = interface_type @I [template]
+// CHECK:STDOUT:   %.1: type = struct_type {} [template]
+// CHECK:STDOUT:   %.2: type = tuple_type () [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .I = %I.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %I.decl: type = interface_decl @I [template = constants.%I.type] {} {}
+// CHECK:STDOUT:   impl_decl @impl [template] {} {
+// CHECK:STDOUT:     %.loc12_7.1: %.1 = struct_literal ()
+// CHECK:STDOUT:     %.loc12_7.2: type = converted %.loc12_7.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:     %I.ref: type = name_ref I, file.%I.decl [template = constants.%I.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @I;
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl: %.loc12_7.2 as %I.ref {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_class.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %J.type: type = interface_type @J [template]
+// CHECK:STDOUT:   %C: type = class_type @C [template]
+// CHECK:STDOUT:   %.1: type = struct_type {} [template]
+// CHECK:STDOUT:   %.2: <witness> = complete_type_witness %.1 [template]
+// CHECK:STDOUT:   %.3: type = tuple_type () [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .J = %J.decl
+// CHECK:STDOUT:     .C = %C.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %J.decl: type = interface_decl @J [template = constants.%J.type] {} {}
+// CHECK:STDOUT:   %C.decl: type = class_decl @C [template = constants.%C] {} {}
+// CHECK:STDOUT:   impl_decl @impl [template] {} {
+// CHECK:STDOUT:     %C.ref: type = name_ref C, file.%C.decl [template = constants.%C]
+// CHECK:STDOUT:     %J.ref: type = name_ref J, file.%J.decl [template = constants.%J.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @J;
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl: %C.ref as %J.ref {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @C {
+// CHECK:STDOUT:   %.loc5: <witness> = complete_type_witness %.1 [template = constants.%.2]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%C
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/diagnostics/coverage_test.cpp

@@ -43,7 +43,6 @@ constexpr DiagnosticKind UntestedDiagnosticKinds[] = {
     DiagnosticKind::ExternLibraryInImporter,
     DiagnosticKind::ExternLibraryOnDefinition,
     DiagnosticKind::HexadecimalEscapeMissingDigits,
-    DiagnosticKind::ImplOfUndefinedInterface,
     DiagnosticKind::IncompleteTypeInFunctionParam,
     DiagnosticKind::InvalidDigit,
     DiagnosticKind::InvalidDigitSeparator,