Implement typed throws support in _openExistential.

lib/Sema/ConstraintSystem.cpp

@@ -3145,11 +3145,14 @@ static DeclReferenceType getTypeOfReferenceWithSpecialTypeCheckingSemantics(
     auto result = CS.createTypeVariable(
         CS.getConstraintLocator(locator, ConstraintLocator::FunctionResult),
         TVO_CanBindToNoEscape);
+    auto thrownError = CS.createTypeVariable(
+        CS.getConstraintLocator(locator, ConstraintLocator::ThrownErrorType),
+        0);
     FunctionType::Param bodyArgs[] = {FunctionType::Param(openedTy)};
     auto bodyClosure = FunctionType::get(bodyArgs, result,
                                          FunctionType::ExtInfoBuilder()
                                              .withNoEscape(true)
-                                             .withThrows(true, /*FIXME:*/Type())
+                                             .withThrows(true, thrownError)
                                              .withAsync(true)
                                              .build());
     FunctionType::Param args[] = {
@@ -3159,7 +3162,7 @@ static DeclReferenceType getTypeOfReferenceWithSpecialTypeCheckingSemantics(
     auto refType = FunctionType::get(args, result,
                                      FunctionType::ExtInfoBuilder()
                                          .withNoEscape(false)
-                                         .withThrows(true, /*FIXME:*/Type())
+                                         .withThrows(true, thrownError)
                                          .withAsync(true)
                                          .build());
     return {refType, refType, refType, refType, Type()};

stdlib/public/core/Builtin.swift

@@ -1053,12 +1053,28 @@ func __abi_withoutActuallyEscaping<ClosureType, ResultType>(
   Builtin.unreachable()
 }
 
+@_alwaysEmitIntoClient
 @_transparent
 @_semantics("typechecker._openExistential(_:do:)")
-public func _openExistential<ExistentialType, ContainedType, ResultType>(
+public func _openExistential<ExistentialType, ContainedType, ResultType, Failure>(
+  _ existential: ExistentialType,
+  do body: (_ escapingClosure: ContainedType) throws(Failure) -> ResultType
+) throws(Failure) -> ResultType {
+  // This implementation is never used, since calls to
+  // `Swift._openExistential(_:do:)` are resolved as a special case by
+  // the type checker.
+  Builtin.staticReport(_trueAfterDiagnostics(), true._value,
+    ("internal consistency error: '_openExistential(_:do:)' operation failed to resolve"
+     as StaticString).utf8Start._rawValue)
+  Builtin.unreachable()
+}
+
+@usableFromInline
+@_silgen_name("$ss16_openExistential_2doq0_x_q0_q_KXEtKr1_lF")
+func __abi_openExistential<ExistentialType, ContainedType, ResultType>(
   _ existential: ExistentialType,
   do body: (_ escapingClosure: ContainedType) throws -> ResultType
-) rethrows -> ResultType {
+) throws -> ResultType {
   // This implementation is never used, since calls to
   // `Swift._openExistential(_:do:)` are resolved as a special case by
   // the type checker.

test/Constraints/openExistential.swift

@@ -11,3 +11,13 @@ func open(existential: P, mutExistential: inout P) {
   _openExistential(mutExistential, do: foo)
   _openExistential(type(of: mutExistential), do: bar)
 }
+
+enum HomeworkError: Error {
+case dogAteIt
+}
+
+func fooThrowing<T: P>(_: T) throws(HomeworkError) {}
+
+func openMaybeThrow(existential: P) throws(HomeworkError) {
+  try _openExistential(existential, do: fooThrowing)
+}

test/api-digester/stability-stdlib-abi-without-asserts.test

@@ -102,7 +102,9 @@ Func Result.get() has mangled name changing from 'Swift.Result.get() throws -> A
 Func withoutActuallyEscaping(_:do:) has been renamed to Func __abi_withoutActuallyEscaping(_:do:)
 Func withoutActuallyEscaping(_:do:) has mangled name changing from 'Swift.withoutActuallyEscaping<A, B>(_: A, do: (A) throws -> B) throws -> B' to 'Swift.__abi_withoutActuallyEscaping<A, B>(_: A, do: (A) throws -> B) throws -> B'
 Func withoutActuallyEscaping(_:do:) is now without @rethrows
-
+Func _openExistential(_:do:) has been renamed to Func __abi_openExistential(_:do:)
+Func _openExistential(_:do:) has mangled name changing from 'Swift._openExistential<A, B, C>(_: A, do: (B) throws -> C) throws -> C' to 'Swift.__abi_openExistential<A, B, C>(_: A, do: (B) throws -> C) throws -> C'
+Func _openExistential(_:do:) is now without @rethrows
 
 // These haven't actually been removed; they are simply marked unavailable.
 // This seems to be a false positive in the ABI checker. This is not an ABI