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Fix mangling collision for C++ class template specializations (#7269)

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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>
This commit is contained in:
Chandler Carruth
2026-05-30 08:13:00 +00:00
committed by GitHub
co-authored by Christopher Di Bella
parent 3ef128ac91
commit 8a59f2a76b
64 changed files with 4762 additions and 4330 deletions
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toolchain/check/testdata/interop/cpp/template/template_template_param.carbon
Vendored
+7 -7
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@@ -84,7 +84,7 @@ var x: Cpp.TwoTemplates(Cpp.A, true);
// CHECK:STDOUT:
// CHECK:STDOUT: constants {
// CHECK:STDOUT: %TwoTemplates: type = class_type @TwoTemplates [concrete]
// CHECK:STDOUT: %pattern_type.4dd: type = pattern_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]
@@ -93,9 +93,9 @@ var x: Cpp.TwoTemplates(Cpp.A, true);
// 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.335: %Default.type = facet_value %TwoTemplates, (%custom_witness.213) [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.impl_witness.d13: <witness> = impl_witness imports.%DefaultOrUnformed.impl_witness_table.b72, @T.as_type.as.DefaultOrUnformed.impl(%Default.facet.335) [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.facet: %DefaultOrUnformed.type = facet_value %TwoTemplates, (%DefaultOrUnformed.impl_witness.d13) [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 {
@@ -105,8 +105,8 @@ var x: Cpp.TwoTemplates(Cpp.A, true);
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: name_binding_decl {
// CHECK:STDOUT: %x.patt: %pattern_type.4dd = ref_binding_pattern x [concrete]
// CHECK:STDOUT: %x.var_patt: %pattern_type.4dd = var_pattern %x.patt [concrete]
// 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]
@@ -114,7 +114,7 @@ var x: Cpp.TwoTemplates(Cpp.A, true);
// CHECK:STDOUT:
// CHECK:STDOUT: fn @__global_init() {
// CHECK:STDOUT: !entry:
// CHECK:STDOUT: %DefaultOrUnformed.facet: %DefaultOrUnformed.type = facet_value constants.%TwoTemplates, (constants.%DefaultOrUnformed.impl_witness.d13) [concrete = constants.%DefaultOrUnformed.facet]
// 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]