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Interop support for initialization via std::initializer_list. (#6672)

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Add a new builtin function `cpp.std.initializer_list.make` that takes an
array and returns a `std::initializer_list`, initialized to refer to
that array. When C++ initialization wants to perform a
`std::initializer_list`-from-array construction, synthesize a
declaration of a matching builtin function and use that to perform the
initialization.

Ideally we would specify this conversion as an impl of `ImplicitAs` in
the prelude instead of hardcoding it in the interop layer, but
unfortunately that's not currently possible, for various reasons -- we
can't make the conversion form-generic, we can't deduce the array length
from the initializer, and we can't deduce against the arguments of
imported C++ class templates yet -- so for now synthesizing a builtin
function on demand is the best we can do.

Assisted-by: Gemini 3 Pro via Antigravity
This commit is contained in:
Richard Smith
2026-01-30 22:24:18 +00:00
committed by GitHub
parent 20a5c43e95
commit c0b24047dd
27 changed files with 2137 additions and 525 deletions
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toolchain/check/testdata/interop/cpp/impls/implicit_as.carbon
Vendored
+35 -32
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@@ -304,23 +304,26 @@ import Cpp library "aggregate.h";
fn InitFromTuple() {
//@dump-sem-ir-begin
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+6]]:26: error: semantics TODO: `Unsupported initialization sequence:
// CHECK:STDERR: Normal sequence: list aggregate initialization [struct Aggregate]
// CHECK:STDERR: ` [SemanticsTodo]
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+7]]:26: error: cannot implicitly convert expression of type `()` to `Cpp.Aggregate` [ConversionFailure]
// CHECK:STDERR: let _: Cpp.Aggregate = ();
// CHECK:STDERR: ^~
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+4]]:26: note: type `()` does not implement interface `Core.ImplicitAs(Cpp.Aggregate)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: let _: Cpp.Aggregate = ();
// CHECK:STDERR: ^~
// CHECK:STDERR:
let _: Cpp.Aggregate = ();
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+6]]:26: error: semantics TODO: `Unsupported initialization sequence:
// CHECK:STDERR: Normal sequence: list aggregate initialization [struct Aggregate]
// CHECK:STDERR: ` [SemanticsTodo]
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+7]]:26: error: cannot implicitly convert expression of type `(Core.IntLiteral,)` to `Cpp.Aggregate` [ConversionFailure]
// CHECK:STDERR: let _: Cpp.Aggregate = (1,);
// CHECK:STDERR: ^~~~
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+4]]:26: note: type `(Core.IntLiteral,)` does not implement interface `Core.ImplicitAs(Cpp.Aggregate)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: let _: Cpp.Aggregate = (1,);
// CHECK:STDERR: ^~~~
// CHECK:STDERR:
let _: Cpp.Aggregate = (1,);
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+6]]:26: error: semantics TODO: `Unsupported initialization sequence:
// CHECK:STDERR: Normal sequence: list aggregate initialization [struct Aggregate]
// CHECK:STDERR: ` [SemanticsTodo]
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+7]]:26: error: cannot implicitly convert expression of type `(Core.IntLiteral, Core.IntLiteral)` to `Cpp.Aggregate` [ConversionFailure]
// CHECK:STDERR: let _: Cpp.Aggregate = (1, 2);
// CHECK:STDERR: ^~~~~~
// CHECK:STDERR: fail_todo_aggregate_from_tuple.carbon:[[@LINE+4]]:26: note: type `(Core.IntLiteral, Core.IntLiteral)` does not implement interface `Core.ImplicitAs(Cpp.Aggregate)` [MissingImplInMemberAccessNote]
// CHECK:STDERR: let _: Cpp.Aggregate = (1, 2);
// CHECK:STDERR: ^~~~~~
// CHECK:STDERR:
@@ -1263,38 +1266,38 @@ fn InitFromStruct() {
// CHECK:STDOUT: fn @InitFromTuple() {
// CHECK:STDOUT: !entry:
// CHECK:STDOUT: name_binding_decl {
// CHECK:STDOUT: %_.patt.loc14: %pattern_type.235 = value_binding_pattern _ [concrete]
// CHECK:STDOUT: %_.patt.loc15: %pattern_type.235 = value_binding_pattern _ [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT: %.loc14_27.1: %empty_tuple.type = tuple_literal () [concrete = constants.%empty_tuple]
// CHECK:STDOUT: %.loc14_13: type = splice_block %Aggregate.ref.loc14 [concrete = constants.%Aggregate] {
// CHECK:STDOUT: %Cpp.ref.loc14: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
// CHECK:STDOUT: %Aggregate.ref.loc14: type = name_ref Aggregate, imports.%Aggregate.decl [concrete = constants.%Aggregate]
// CHECK:STDOUT: %.loc15_27.1: %empty_tuple.type = tuple_literal () [concrete = constants.%empty_tuple]
// CHECK:STDOUT: %.loc15_13: type = splice_block %Aggregate.ref.loc15 [concrete = constants.%Aggregate] {
// CHECK:STDOUT: %Cpp.ref.loc15: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
// CHECK:STDOUT: %Aggregate.ref.loc15: type = name_ref Aggregate, imports.%Aggregate.decl [concrete = constants.%Aggregate]
// CHECK:STDOUT: }
// CHECK:STDOUT: %.loc14_27.2: %Aggregate = converted %.loc14_27.1, <error> [concrete = <error>]
// CHECK:STDOUT: %_.loc14: %Aggregate = value_binding _, <error> [concrete = <error>]
// CHECK:STDOUT: %.loc15_27.2: %Aggregate = converted %.loc15_27.1, <error> [concrete = <error>]
// CHECK:STDOUT: %_.loc15: %Aggregate = value_binding _, <error> [concrete = <error>]
// CHECK:STDOUT: name_binding_decl {
// CHECK:STDOUT: %_.patt.loc21: %pattern_type.235 = value_binding_pattern _ [concrete]
// CHECK:STDOUT: %_.patt.loc23: %pattern_type.235 = value_binding_pattern _ [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT: %int_1.loc21: Core.IntLiteral = int_value 1 [concrete = constants.%int_1]
// CHECK:STDOUT: %.loc21_29.1: %tuple.type.985 = tuple_literal (%int_1.loc21) [concrete = constants.%tuple.378]
// CHECK:STDOUT: %.loc21_13: type = splice_block %Aggregate.ref.loc21 [concrete = constants.%Aggregate] {
// CHECK:STDOUT: %Cpp.ref.loc21: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
// CHECK:STDOUT: %Aggregate.ref.loc21: type = name_ref Aggregate, imports.%Aggregate.decl [concrete = constants.%Aggregate]
// CHECK:STDOUT: %int_1.loc23: Core.IntLiteral = int_value 1 [concrete = constants.%int_1]
// CHECK:STDOUT: %.loc23_29.1: %tuple.type.985 = tuple_literal (%int_1.loc23) [concrete = constants.%tuple.378]
// CHECK:STDOUT: %.loc23_13: type = splice_block %Aggregate.ref.loc23 [concrete = constants.%Aggregate] {
// CHECK:STDOUT: %Cpp.ref.loc23: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
// CHECK:STDOUT: %Aggregate.ref.loc23: type = name_ref Aggregate, imports.%Aggregate.decl [concrete = constants.%Aggregate]
// CHECK:STDOUT: }
// CHECK:STDOUT: %.loc21_29.2: %Aggregate = converted %.loc21_29.1, <error> [concrete = <error>]
// CHECK:STDOUT: %_.loc21: %Aggregate = value_binding _, <error> [concrete = <error>]
// CHECK:STDOUT: %.loc23_29.2: %Aggregate = converted %.loc23_29.1, <error> [concrete = <error>]
// CHECK:STDOUT: %_.loc23: %Aggregate = value_binding _, <error> [concrete = <error>]
// CHECK:STDOUT: name_binding_decl {
// CHECK:STDOUT: %_.patt.loc28: %pattern_type.235 = value_binding_pattern _ [concrete]
// CHECK:STDOUT: %_.patt.loc31: %pattern_type.235 = value_binding_pattern _ [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT: %int_1.loc28: Core.IntLiteral = int_value 1 [concrete = constants.%int_1]
// CHECK:STDOUT: %int_1.loc31: Core.IntLiteral = int_value 1 [concrete = constants.%int_1]
// CHECK:STDOUT: %int_2: Core.IntLiteral = int_value 2 [concrete = constants.%int_2]
// CHECK:STDOUT: %.loc28_31.1: %tuple.type.f94 = tuple_literal (%int_1.loc28, %int_2) [concrete = constants.%tuple.ad8]
// CHECK:STDOUT: %.loc28_13: type = splice_block %Aggregate.ref.loc28 [concrete = constants.%Aggregate] {
// CHECK:STDOUT: %Cpp.ref.loc28: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
// CHECK:STDOUT: %Aggregate.ref.loc28: type = name_ref Aggregate, imports.%Aggregate.decl [concrete = constants.%Aggregate]
// CHECK:STDOUT: %.loc31_31.1: %tuple.type.f94 = tuple_literal (%int_1.loc31, %int_2) [concrete = constants.%tuple.ad8]
// CHECK:STDOUT: %.loc31_13: type = splice_block %Aggregate.ref.loc31 [concrete = constants.%Aggregate] {
// CHECK:STDOUT: %Cpp.ref.loc31: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
// CHECK:STDOUT: %Aggregate.ref.loc31: type = name_ref Aggregate, imports.%Aggregate.decl [concrete = constants.%Aggregate]
// CHECK:STDOUT: }
// CHECK:STDOUT: %.loc28_31.2: %Aggregate = converted %.loc28_31.1, <error> [concrete = <error>]
// CHECK:STDOUT: %_.loc28: %Aggregate = value_binding _, <error> [concrete = <error>]
// CHECK:STDOUT: %.loc31_31.2: %Aggregate = converted %.loc31_31.1, <error> [concrete = <error>]
// CHECK:STDOUT: %_.loc31: %Aggregate = value_binding _, <error> [concrete = <error>]
// CHECK:STDOUT: <elided>
// CHECK:STDOUT: }
// CHECK:STDOUT: