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Improve interop for classes with multiple inheritance. (#6130)

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If there's a unique "preferred" base class, then treat that as "the"
base class for Carbon's purposes. In particular:

* If there's exactly one polymorphic base class, that's our preferred
base class.
* If there's exactly one non-empty base class, that's our perferred base
class.
* (Degenerate case) If there's exactly one base class, that's our
preferred base class.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
This commit is contained in:
Richard Smith
2025-09-25 20:25:48 +00:00
committed by GitHub
co-authored by Jon Ross-Perkins
parent f194acbf96
commit 949ec17da2
2 changed files with 147 additions and 9 deletions
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toolchain/check/testdata/interop/cpp/class/base.carbon
Vendored
+120
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@@ -125,6 +125,20 @@ struct A { int a; };
struct B { int b; };
struct C : A, B {};
struct Empty1 {};
struct Empty2 {};
struct OneNonEmptyBase : Empty1, A, Empty2 {};
struct TwoNonEmptyBases : Empty1, A, B, Empty2 {};
struct Polymorphic1 {
virtual void f();
};
struct Polymorphic2 {
virtual void g();
};
struct OnePolymorphicBase : A, B, Polymorphic1, Empty1 {};
struct TwoPolymorphicBases : Polymorphic1, Polymorphic2 {};
// --- fail_todo_use_multiple_inheritance.carbon
library "[[@TEST_NAME]]";
@@ -153,6 +167,112 @@ fn ConvertB(p: Cpp.C*) -> Cpp.B* {
return p;
}
fn ConvertOneNonEmptyBaseToEmpty1(p: Cpp.OneNonEmptyBase*) -> Cpp.Empty1* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.OneNonEmptyBase*` to `Cpp.Empty1*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.OneNonEmptyBase*` does not implement interface `Core.ImplicitAs(Cpp.Empty1*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertOneNonEmptyBaseToEmpty2(p: Cpp.OneNonEmptyBase*) -> Cpp.Empty2* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.OneNonEmptyBase*` to `Cpp.Empty2*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.OneNonEmptyBase*` does not implement interface `Core.ImplicitAs(Cpp.Empty2*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertTwoNonEmptyBasesToA(p: Cpp.TwoNonEmptyBases*) -> Cpp.A* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.TwoNonEmptyBases*` to `Cpp.A*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.TwoNonEmptyBases*` does not implement interface `Core.ImplicitAs(Cpp.A*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertTwoNonEmptyBasesToB(p: Cpp.TwoNonEmptyBases*) -> Cpp.B* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.TwoNonEmptyBases*` to `Cpp.B*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.TwoNonEmptyBases*` does not implement interface `Core.ImplicitAs(Cpp.B*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertOnePolymorphicBaseToA(p: Cpp.OnePolymorphicBase*) -> Cpp.A* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.OnePolymorphicBase*` to `Cpp.A*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.OnePolymorphicBase*` does not implement interface `Core.ImplicitAs(Cpp.A*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertOnePolymorphicBaseToB(p: Cpp.OnePolymorphicBase*) -> Cpp.B* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.OnePolymorphicBase*` to `Cpp.B*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.OnePolymorphicBase*` does not implement interface `Core.ImplicitAs(Cpp.B*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertTwoPolymorphicBasesToPolymorphic1(p: Cpp.TwoPolymorphicBases*) -> Cpp.Polymorphic1* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.TwoPolymorphicBases*` to `Cpp.Polymorphic1*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.TwoPolymorphicBases*` does not implement interface `Core.ImplicitAs(Cpp.Polymorphic1*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
fn ConvertTwoPolymorphicBasesToPolymorphic2(p: Cpp.TwoPolymorphicBases*) -> Cpp.Polymorphic2* {
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.TwoPolymorphicBases*` to `Cpp.Polymorphic2*` [ConversionFailure]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR: fail_todo_use_multiple_inheritance.carbon:[[@LINE+4]]:3: note: type `Cpp.TwoPolymorphicBases*` does not implement interface `Core.ImplicitAs(Cpp.Polymorphic2*)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: return p;
// CHECK:STDERR: ^~~~~~~~~
// CHECK:STDERR:
return p;
}
// --- multiple_inheritance_with_preferred_base.carbon
library "[[@TEST_NAME]]";
import Cpp library "multiple_inheritance.h";
// If there's only one non-empty base, that's our preferred base class. We can
// convert to that.
fn ConvertOneNonEmptyBaseToA(p: Cpp.OneNonEmptyBase*) -> Cpp.A* {
return p;
}
// If there's only one polymorphic base, that's our preferred base class. We can
// convert to that.
fn ConvertOnePolymorphicBaseToPolymorphic1(p: Cpp.OnePolymorphicBase*) -> Cpp.Polymorphic1* {
return p;
}
// --- virtual_inheritance.h
struct A { int a; };