Fix crash lowering an imported impl method.

While we don't need a lookup table for an imported impl from a different
library, we do still need to import the name scope so we can compute the
parent scope for mangling purposes.

toolchain/check/import_ref.cpp

@@ -1803,23 +1803,27 @@ class ImportRefResolver {
 
     new_impl.witness_id = witness_id;
 
-    // Import the definition scope, if we might need it. Name lookup is never
-    // performed into this scope by a user of the impl, so this is only
-    // necessary in the same library that defined the impl, in order to support
-    // defining members of the impl out of line in the impl file when the impl
-    // is defined in the API file.
-    if (import_ir_id_ == SemIR::ImportIRId::ApiForImpl) {
+    if (import_impl.scope_id.is_valid()) {
       new_impl.scope_id = context_.name_scopes().Add(
           new_impl.first_owning_decl_id, SemIR::NameId::Invalid,
           new_impl.parent_scope_id);
-      auto& new_scope = context_.name_scopes().Get(new_impl.scope_id);
-      const auto& import_scope =
-          import_ir_.name_scopes().Get(import_impl.scope_id);
-
-      // Push a block so that we can add scoped instructions to it.
-      context_.inst_block_stack().Push();
-      AddNameScopeImportRefs(import_scope, new_scope);
-      new_impl.body_block_id = context_.inst_block_stack().Pop();
+      // Import the contents of the definition scope, if we might need it. Name
+      // lookup is never performed into this scope by a user of the impl, so
+      // this is only necessary in the same library that defined the impl, in
+      // order to support defining members of the impl out of line in the impl
+      // file when the impl is defined in the API file.
+      // TODO: Check to see if this impl is owned by the API file, rather than
+      // merely being imported into it.
+      if (import_ir_id_ == SemIR::ImportIRId::ApiForImpl) {
+        auto& new_scope = context_.name_scopes().Get(new_impl.scope_id);
+        const auto& import_scope =
+            import_ir_.name_scopes().Get(import_impl.scope_id);
+
+        // Push a block so that we can add scoped instructions to it.
+        context_.inst_block_stack().Push();
+        AddNameScopeImportRefs(import_scope, new_scope);
+        new_impl.body_block_id = context_.inst_block_stack().Pop();
+      }
     }
   }
 

toolchain/lower/testdata/impl/import.carbon

@@ -0,0 +1,81 @@
+// 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/lower/testdata/impl/import.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/impl/import.carbon
+
+// --- impl_def.carbon
+
+library "[[@TEST_NAME]]";
+
+interface I {
+  fn F[self: Self]() -> i32;
+}
+
+class A {
+  var n: i32;
+}
+
+impl A as I {
+  fn F[self: A]() -> i32 {
+    return self.n;
+  }
+}
+
+// --- impl_use.carbon
+
+import library "impl_def";
+
+fn Call(a: A) -> i32 {
+  return a.(I.F)();
+}
+
+// CHECK:STDOUT: ; ModuleID = 'impl_def.carbon'
+// CHECK:STDOUT: source_filename = "impl_def.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define i32 @"_CF.A.Main:I.Main"(ptr %self) !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %.loc14_16.1.n = getelementptr inbounds nuw { i32 }, ptr %self, i32 0, i32 0, !dbg !7
+// CHECK:STDOUT:   %.loc14_16.2 = load i32, ptr %.loc14_16.1.n, align 4, !dbg !7
+// CHECK:STDOUT:   ret i32 %.loc14_16.2, !dbg !8
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!2}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !3 = !DIFile(filename: "impl_def.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.A.Main:I.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !7 = !DILocation(line: 14, column: 12, scope: !4)
+// CHECK:STDOUT: !8 = !DILocation(line: 14, column: 5, scope: !4)
+// CHECK:STDOUT: ; ModuleID = 'impl_use.carbon'
+// CHECK:STDOUT: source_filename = "impl_use.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define i32 @_CCall.Main(ptr %a) !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %F.call = call i32 @"_CF.A.Main:I.Main"(ptr %a), !dbg !7
+// CHECK:STDOUT:   ret i32 %F.call, !dbg !8
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare i32 @"_CF.A.Main:I.Main"(ptr)
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!2}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !3 = !DIFile(filename: "impl_use.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Call", linkageName: "_CCall.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !7 = !DILocation(line: 16, column: 10, scope: !4)
+// CHECK:STDOUT: !8 = !DILocation(line: 16, column: 3, scope: !4)