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carbon-lang/toolchain/check/testdata/interop/cpp/template/template_template_param.carbon
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Chandler CarruthandChristopher Di Bella 8a59f2a76b Fix mangling collision for C++ class template specializations (#7269) 
Carbon-side thunks (for example the `Copy`/`Destroy` witness thunks
generated for imported C++ types) are mangled by Carbon, and their names
incorporate a fingerprint of the involved types. The instruction
fingerprinter identifies a class only by its name and parent scope,
which is sufficient for Carbon classes but not for imported C++ classes:
different specializations of one class template (and other cases such as
types in anonymous namespaces) share a Carbon name and parent scope. As
a result, the thunks for two distinct specializations could mangle to
the same name, producing a single LLVM function with two definitions and
failing `verifyModule` during lowering.

When fingerprinting a class imported from C++, also include the Clang
mangled name of its type.

Test: toolchain/lower/testdata/interop/cpp/thunks.carbon gains a split
with two specializations of one class template, each requiring a thunk;
their thunks now get distinct mangled names instead of colliding.

Assisted-by: Claude Code

---------

Co-authored-by: Christopher Di Bella <cjdb.ns@gmail.com>
2026-05-30 08:13:00 +00:00

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// 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/testdata/min_prelude/convert.carbon
//
// AUTOUPDATE
// TIP: To test this file alone, run:
// TIP: bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/template/template_template_param.carbon
// TIP: To dump output, run:
// TIP: bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/template/template_template_param.carbon
// --- templates.h
template<typename> struct A {};
template<typename> struct B {};
template<typename, typename> struct C {};
template<template<typename> typename, template<typename> typename>
struct TwoTemplates {};
// --- valid.carbon
library "[[@TEST_NAME]]";
import Cpp library "templates.h";
//@dump-sem-ir-begin
var x: Cpp.TwoTemplates(Cpp.A, Cpp.B);
//@dump-sem-ir-end
// --- fail_template_mismatch.carbon
library "[[@TEST_NAME]]";
import Cpp library "templates.h";
//@dump-sem-ir-begin
// CHECK:STDERR: fail_template_mismatch.carbon:[[@LINE+15]]:35: error: too few template arguments for class template 'C' [CppInteropParseError]
// CHECK:STDERR: 22 | var x: Cpp.TwoTemplates(Cpp.A, Cpp.C);
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_template_mismatch.carbon:[[@LINE+12]]:35: note: template template argument has different template parameters than its corresponding template template parameter [CppInteropParseNote]
// CHECK:STDERR: 22 | var x: Cpp.TwoTemplates(Cpp.A, Cpp.C);
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_template_mismatch.carbon:[[@LINE-9]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./templates.h:6:66: note: previous template template parameter is here [CppInteropParseNote]
// CHECK:STDERR: 6 | template<template<typename> typename, template<typename> typename>
// CHECK:STDERR: | ~~~~~~~~~~~~~~~~~~ ^
// CHECK:STDERR: fail_template_mismatch.carbon:[[@LINE-13]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./templates.h:4:37: note: template is declared here [CppInteropParseNote]
// CHECK:STDERR: 4 | template<typename, typename> struct C {};
// CHECK:STDERR: | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
// CHECK:STDERR:
var x: Cpp.TwoTemplates(Cpp.A, Cpp.C);
//@dump-sem-ir-end
// --- fail_type_argument.carbon
library "[[@TEST_NAME]]";
import Cpp library "templates.h";
class X {}
// CHECK:STDERR: fail_type_argument.carbon:[[@LINE+4]]:32: error: template argument for template template parameter must be a class template or type alias template [CppInteropParseError]
// CHECK:STDERR: 12 | var x: Cpp.TwoTemplates(Cpp.A, X);
// CHECK:STDERR: | ^
// CHECK:STDERR:
var x: Cpp.TwoTemplates(Cpp.A, X);
// --- fail_non_type_argument.carbon
library "[[@TEST_NAME]]";
import Cpp library "templates.h";
// CHECK:STDERR: fail_non_type_argument.carbon:[[@LINE+4]]:32: error: template argument for template template parameter must be a class template or type alias template [CppInteropParseError]
// CHECK:STDERR: 10 | var x: Cpp.TwoTemplates(Cpp.A, true);
// CHECK:STDERR: | ^
// CHECK:STDERR:
var x: Cpp.TwoTemplates(Cpp.A, true);
// CHECK:STDOUT: --- valid.carbon
// CHECK:STDOUT:
// CHECK:STDOUT: constants {
// CHECK:STDOUT: %TwoTemplates: type = class_type @TwoTemplates [concrete]
// CHECK:STDOUT: %pattern_type.681: type = pattern_type %TwoTemplates [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.type: type = facet_type <@DefaultOrUnformed> [concrete]
// CHECK:STDOUT: %Default.type: type = facet_type <@Default> [concrete]
// CHECK:STDOUT: %T.4ca: %Default.type = symbolic_binding T, 0 [symbolic]
// CHECK:STDOUT: %T.as_type.as.DefaultOrUnformed.impl.Op.type.47c: type = fn_type @T.as_type.as.DefaultOrUnformed.impl.Op, @T.as_type.as.DefaultOrUnformed.impl(%T.4ca) [symbolic]
// CHECK:STDOUT: %T.as_type.as.DefaultOrUnformed.impl.Op.25a: %T.as_type.as.DefaultOrUnformed.impl.Op.type.47c = struct_value () [symbolic]
// CHECK:STDOUT: %TwoTemplates.Op.type: type = fn_type @TwoTemplates.Op [concrete]
// CHECK:STDOUT: %TwoTemplates.Op: %TwoTemplates.Op.type = struct_value () [concrete]
// CHECK:STDOUT: %custom_witness.213: <witness> = custom_witness (%TwoTemplates.Op), @Default [concrete]
// CHECK:STDOUT: %Default.facet.1b5: %Default.type = facet_value %TwoTemplates, (%custom_witness.213) [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.impl_witness.36d: <witness> = impl_witness imports.%DefaultOrUnformed.impl_witness_table.b72, @T.as_type.as.DefaultOrUnformed.impl(%Default.facet.1b5) [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.facet: %DefaultOrUnformed.type = facet_value %TwoTemplates, (%DefaultOrUnformed.impl_witness.36d) [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: imports {
// CHECK:STDOUT: %Core.import_ref.dfa: @T.as_type.as.DefaultOrUnformed.impl.%T.as_type.as.DefaultOrUnformed.impl.Op.type (%T.as_type.as.DefaultOrUnformed.impl.Op.type.47c) = import_ref Core//prelude/parts/default, loc{{\d+_\d+}}, loaded [symbolic = @T.as_type.as.DefaultOrUnformed.impl.%T.as_type.as.DefaultOrUnformed.impl.Op (constants.%T.as_type.as.DefaultOrUnformed.impl.Op.25a)]
// CHECK:STDOUT: %DefaultOrUnformed.impl_witness_table.b72 = impl_witness_table (%Core.import_ref.dfa), @T.as_type.as.DefaultOrUnformed.impl [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: name_binding_decl {
// CHECK:STDOUT: %x.patt: %pattern_type.681 = ref_binding_pattern x [concrete]
// CHECK:STDOUT: %x.var_patt: %pattern_type.681 = var_pattern %x.patt [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT: %x.var: ref %TwoTemplates = var %x.var_patt [concrete]
// CHECK:STDOUT: %x: ref %TwoTemplates = ref_binding x, %x.var [concrete = %x.var]
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: fn @__global_init() {
// CHECK:STDOUT: !entry:
// CHECK:STDOUT: %DefaultOrUnformed.facet: %DefaultOrUnformed.type = facet_value constants.%TwoTemplates, (constants.%DefaultOrUnformed.impl_witness.36d) [concrete = constants.%DefaultOrUnformed.facet]
// CHECK:STDOUT: %.loc7_38.1: %DefaultOrUnformed.type = converted constants.%TwoTemplates, %DefaultOrUnformed.facet [concrete = constants.%DefaultOrUnformed.facet]
// CHECK:STDOUT: %as_type: type = facet_access_type %.loc7_38.1 [concrete = constants.%TwoTemplates]
// CHECK:STDOUT: %.loc7_38.2: type = converted %.loc7_38.1, %as_type [concrete = constants.%TwoTemplates]
// CHECK:STDOUT: <elided>
// CHECK:STDOUT: %.loc7_1: ref %TwoTemplates = splice_block file.%x.var [concrete = file.%x.var] {}
// CHECK:STDOUT: %T.as_type.as.DefaultOrUnformed.impl.Op.call: init %TwoTemplates to %.loc7_1 = call %T.as_type.as.DefaultOrUnformed.impl.Op.specific_fn()
// CHECK:STDOUT: assign file.%x.var, %T.as_type.as.DefaultOrUnformed.impl.Op.call
// CHECK:STDOUT: <elided>
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: --- fail_template_mismatch.carbon
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: name_binding_decl {
// CHECK:STDOUT: %x.patt: <error> = ref_binding_pattern x [concrete]
// CHECK:STDOUT: %x.var_patt: <error> = var_pattern %x.patt [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT: %x.var: ref <error> = var %x.var_patt [concrete = <error>]
// CHECK:STDOUT: %x: ref <error> = ref_binding x, <error> [concrete = <error>]
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: fn @__global_init() {
// CHECK:STDOUT: !entry:
// CHECK:STDOUT: assign file.%x.var, <error>
// CHECK:STDOUT: <elided>
// CHECK:STDOUT: }
// CHECK:STDOUT:
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