Mangle impls even if they impl an interface with associated constants (#5348)

Fixes issue #5307

toolchain/lower/mangler.cpp

@@ -54,12 +54,12 @@ auto Mangler::MangleInverseQualifiedNameScope(llvm::raw_ostream& os,
             constant_values().GetConstantInstId(impl.constraint_id));
         const auto& facet_type_info =
             sem_ir().facet_types().Get(facet_type.facet_type_id);
-        auto interface_type = facet_type_info.TryAsSingleInterface();
-        CARBON_CHECK(interface_type,
+        CARBON_CHECK(facet_type_info.extend_constraints.size() == 1,
                      "Mangling of an impl of something other than a single "
                      "interface is not yet supported.");
+        auto interface_type = facet_type_info.extend_constraints.front();
         const auto& interface =
-            sem_ir().interfaces().Get(interface_type->interface_id);
+            sem_ir().interfaces().Get(interface_type.interface_id);
         names_to_render.push_back(
             {.name_scope_id = interface.scope_id, .prefix = ':'});
 

toolchain/lower/testdata/interface/where.carbon

@@ -0,0 +1,41 @@
+// 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/interface/where.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/interface/where.carbon
+
+interface I {
+  let T:! type;
+  fn F();
+}
+
+class C {}
+
+impl C as I where .T = bool {
+  fn F() {
+  }
+}
+
+// CHECK:STDOUT: ; ModuleID = 'where.carbon'
+// CHECK:STDOUT: source_filename = "where.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @"_CF.C.Main:I.Main"() !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   ret void, !dbg !7
+// 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: "where.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.C.Main:I.Main", scope: null, file: !3, line: 19, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !7 = !DILocation(line: 19, column: 3, scope: !4)