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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/lower/testdata/interop/cpp/class/import/dynamic.carbon
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+2 -2
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@@ -171,7 +171,7 @@ auto DoThing() -> void {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define weak_odr void @"_COp.9262bcc12ced4e1e:core.Destroy.Core"(ptr %self) #5 !dbg !33 {
// CHECK:STDOUT: define weak_odr void @"_COp.47e6e98bb48502be:core.Destroy.Core"(ptr %self) #5 !dbg !33 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: ret void, !dbg !36
// CHECK:STDOUT: }
@@ -297,7 +297,7 @@ auto DoThing() -> void {
// CHECK:STDOUT: !30 = !{!31}
// CHECK:STDOUT: !31 = !DILocalVariable(arg: 1, scope: !29, type: !21)
// CHECK:STDOUT: !32 = !DILocation(line: 22, column: 1, scope: !29)
// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "Op", linkageName: "_COp.9262bcc12ced4e1e:core.Destroy.Core", scope: null, file: !6, line: 22, type: !19, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !34)
// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "Op", linkageName: "_COp.47e6e98bb48502be:core.Destroy.Core", scope: null, file: !6, line: 22, type: !19, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !34)
// CHECK:STDOUT: !34 = !{!35}
// CHECK:STDOUT: !35 = !DILocalVariable(arg: 1, scope: !33, type: !21)
// CHECK:STDOUT: !36 = !DILocation(line: 22, column: 1, scope: !33)