SILGen: Fix if #available for unavailable custom domains in zippered modules.

When generating SIL for an `if #available(SomeDomain)` query in code being
compiled for a zippered target, the generated code was mis-compiled if
`SomeDomain` were disabled at compile time. Empty version ranges need to be
handled explicitly by `SILGenFunction::emitZipperedOSVersionRangeCheck()`.

SILGen still miscompiles `if #unavailable` queries generally in code compiled
for a zippered target (rdar://147929876).

Resolves rdar://150888941.

Author: tshortli

lib/SILGen/SILGenDecl.cpp

@@ -1851,6 +1851,13 @@ SILValue SILGenFunction::emitZipperedOSVersionRangeCheck(
     return B.createIntegerLiteral(loc, i1, true);
   }
 
+  // If either version is "never" then the check is trivially false because it
+  // can never succeed.
+  if (OSVersion.isEmpty() || VariantOSVersion.isEmpty()) {
+    SILType i1 = SILType::getBuiltinIntegerType(1, getASTContext());
+    return B.createIntegerLiteral(loc, i1, false);
+  }
+
   // The variant-only availability-checking entrypoint is not part
   // of the Swift 5.0 ABI. It is only available in macOS 10.15 and above.
   bool isVariantEntrypointAvailable = !TargetTriple.isMacOSXVersionLT(10, 15);

test/IRGen/availability_custom_domains_maccatalyst_zippered.swift

@@ -0,0 +1,59 @@
+// RUN: %target-swift-emit-irgen -module-name Test %s -verify \
+// RUN:   -enable-experimental-feature CustomAvailability \
+// RUN:   -define-enabled-availability-domain EnabledDomain \
+// RUN:   -define-disabled-availability-domain DisabledDomain \
+// RUN:   -target %target-cpu-apple-macosx13 \
+// RUN:   -target-variant %target-cpu-apple-ios16-macabi \
+// RUN:   -Onone | %FileCheck %s --check-prefixes=CHECK
+
+// RUN: %target-swift-emit-irgen -module-name Test %s -verify \
+// RUN:   -enable-experimental-feature CustomAvailability \
+// RUN:   -define-enabled-availability-domain EnabledDomain \
+// RUN:   -define-disabled-availability-domain DisabledDomain \
+// RUN:   -target %target-cpu-apple-macosx13 \
+// RUN:   -target-variant %target-cpu-apple-ios16-macabi \
+// RUN:   -O | %FileCheck %s --check-prefixes=CHECK
+
+// REQUIRES: OS=macosx || OS=maccatalyst
+// REQUIRES: swift_feature_CustomAvailability
+
+@_silgen_name("always")
+public func always()
+
+@_silgen_name("never")
+public func never()
+
+// CHECK-NOT: call swiftcc void @never()
+
+// CHECK: call swiftcc void @always()
+// CHECK-NOT: call swiftcc void @never()
+if #available(EnabledDomain) {
+  always()
+} else {
+  never()
+}
+
+// CHECK: call swiftcc void @always()
+// CHECK-NOT: call swiftcc void @never()
+if #available(DisabledDomain) {
+  never()
+} else {
+  always()
+}
+
+// FIXME: [availability] These CHECK lines for if #unavailable are inverted (rdar://147929876)
+// CHECK-NOT: call swiftcc void @always()
+// CHECK: call swiftcc void @never()
+if #unavailable(EnabledDomain) {
+  never()
+} else {
+  always()
+}
+
+// CHECK-NOT: call swiftcc void @always()
+// CHECK: call swiftcc void @never()
+if #unavailable(DisabledDomain) {
+  always()
+} else {
+  never()
+}