SILGen: Reabstract subexpr lvalue before ABISafeConversion.

Fixes rdar://130016855. When preconcurrency compatibility introduces
implicit `@Sendable` conversions, the `ABISafeConversionExpr` representing
that conversion indicates an ABI-neutral conversion from the substituted
type of the original expression, so we need to reabstract in cases
where the original property is more generic than the type we're working
with.

lib/SILGen/SILGenLValue.cpp

@@ -4564,10 +4564,19 @@ LValue SILGenLValue::visitABISafeConversionExpr(ABISafeConversionExpr *e,
                                     LValueOptions options) {
   LValue lval = visitRec(e->getSubExpr(), accessKind, options);
   auto typeData = getValueTypeData(SGF, accessKind, e);
+  auto subExprType = e->getSubExpr()->getType()->getRValueType();
+  auto loweredSubExprType = SGF.getLoweredType(subExprType);
+  
+  // Ensure the lvalue is re-abstracted to the substituted type, since that's
+  // the type with which we have ABI compatibility.
+  if (lval.getTypeOfRValue().getASTType() != loweredSubExprType.getASTType()) {
+    // Logical components always re-abstract back to the substituted
+    // type.
+    assert(lval.isLastComponentPhysical());
+    lval.addOrigToSubstComponent(loweredSubExprType);
+  }
 
-  auto OrigType = e->getSubExpr()->getType();
-
-  lval.add<UncheckedConversionComponent>(typeData, OrigType);
+  lval.add<UncheckedConversionComponent>(typeData, subExprType);
 
   return lval;
 }

test/SILGen/preconcurrency-abi-safe-conversions.swift

@@ -0,0 +1,25 @@
+// RUN: %target-swift-emit-silgen -swift-version 5 -verify %s
+// RUN: %target-swift-emit-silgen -swift-version 6 -verify %s
+
+struct Text {
+    init<S>(_: S) where S: StringProtocol {}
+}
+
+// In Swift 5, we introduce an implicit @Sendable on the closures here.
+// Make sure that doing so doesn't disrupt SILGen's lvalue emission.
+// rdar://130016855
+public struct Header<TitleContent> {
+  @preconcurrency
+  private let titleContent:  @MainActor () -> TitleContent
+
+  init(title: String) where TitleContent == Text {
+    self.titleContent = {
+      Text(title)
+    }
+  }
+
+  func testGet() -> @MainActor () -> Text
+      where TitleContent == Text {
+    return titleContent // expected-warning * {{}}
+  }
+}