AST: Fix runtime unavailability for universally unavailable decls.

The logic that computes whether a decl is unavailable at runtime was allowing
`@available` attributes for specific availability domains override
`@available(*, unavailable)`. Universally unavailable decls are always
unavailable, regardless of their other `@available` attributes.

lib/AST/Availability.cpp

@@ -623,6 +623,10 @@ computeDeclRuntimeAvailability(const Decl *decl) {
       continue;
 
     llvm::erase_if(unavailableDomains, [domain](auto unavailableDomain) {
+      // Unavailability in '*' cannot be superseded by an @available attribute
+      // for a more specific availability domain.
+      if (unavailableDomain.isUniversal())
+        return false;
       return unavailableDomain.contains(domain);
     });
   }

test/SILGen/unavailable_decl_custom_domain.swift

@@ -3,15 +3,7 @@
 // RUN:   -define-enabled-availability-domain EnabledDomain \
 // RUN:   -define-disabled-availability-domain DisabledDomain \
 // RUN:   -define-dynamic-availability-domain DynamicDomain \
-// RUN:   | %FileCheck %s
-
-// RUN: %target-swift-emit-silgen -module-name Test %s -verify \
-// RUN:   -enable-experimental-feature CustomAvailability \
-// RUN:   -define-enabled-availability-domain EnabledDomain \
-// RUN:   -define-disabled-availability-domain DisabledDomain \
-// RUN:   -define-dynamic-availability-domain DynamicDomain \
-// RUN:   -unavailable-decl-optimization=stub \
-// RUN:   | %FileCheck %s
+// RUN:   | %FileCheck %s --check-prefixes=CHECK,CHECK-NOOPT
 
 // RUN: %target-swift-emit-silgen -module-name Test %s -verify \
 // RUN:   -enable-experimental-feature CustomAvailability \
@@ -19,7 +11,7 @@
 // RUN:   -define-disabled-availability-domain DisabledDomain \
 // RUN:   -define-dynamic-availability-domain DynamicDomain \
 // RUN:   -unavailable-decl-optimization=complete \
-// RUN:   | %FileCheck %s
+// RUN:   | %FileCheck %s --check-prefixes=CHECK,CHECK-OPT
 
 // REQUIRES: swift_feature_CustomAvailability
 
@@ -84,3 +76,25 @@ public func availableInEnabledAndDisabledDomain() { }
 @available(DisabledDomain)
 @available(EnabledDomain)
 public func availableInDisabledAndEnabledDomain() { }
+
+// CHECK-NOOPT: s4Test49availableInEnabledDomainAndUnavailableUniversallyyyF
+// CHECK-OPT-NOT: s4Test49availableInEnabledDomainAndUnavailableUniversallyyyF
+@available(*, unavailable)
+@available(EnabledDomain)
+public func availableInEnabledDomainAndUnavailableUniversally() { }
+
+// CHECK-NOT: s4Test40unavailableInEnabledDomainAndUniversallyyyF
+@available(*, unavailable)
+@available(EnabledDomain, unavailable)
+public func unavailableInEnabledDomainAndUniversally() { }
+
+// CHECK-NOT: s4Test50availableInDisabledDomainAndUnavailableUniversallyyyF
+@available(*, unavailable)
+@available(DisabledDomain)
+public func availableInDisabledDomainAndUnavailableUniversally() { }
+
+// CHECK-NOOPT: s4Test41unavailableInDisabledDomainAndUniversallyyyF
+// CHECK-OPT-NOT: s4Test41unavailableInDisabledDomainAndUniversallyyyF
+@available(*, unavailable)
+@available(DisabledDomain, unavailable)
+public func unavailableInDisabledDomainAndUniversally() { }

test/SILGen/unavailable_decl_optimization_stub.swift

@@ -12,6 +12,14 @@ public func unavailableFunc() -> S {
   return S()
 }
 
+// CHECK-LABEL: sil{{.*}}@$s4Test33unavailableFuncIntroducedInSwift5yyF
+// CHECK:         [[FNREF:%.*]] = function_ref @$[[DIAGNOSEFN:(ss31_diagnoseUnavailableCodeReacheds5NeverOyF|ss31_diagnoseUnavailableCodeReacheds5NeverOyFTwb)]] : $@convention(thin) () -> Never
+// CHECK-NEXT:    [[APPLY:%.*]] = apply [[FNREF]]()
+// CHECK:       } // end sil function '$s4Test33unavailableFuncIntroducedInSwift5yyF'
+@available(*, unavailable)
+@available(swift 5)
+public func unavailableFuncIntroducedInSwift5() { }
+
 // CHECK-LABEL: sil{{.*}}@$s4Test025unavailableFuncWithNestedC0yyF
 // CHECK:         [[FNREF:%.*]] = function_ref @$[[DIAGNOSEFN:(ss31_diagnoseUnavailableCodeReacheds5NeverOyF|ss31_diagnoseUnavailableCodeReacheds5NeverOyFTwb)]] : $@convention(thin) () -> Never
 // CHECK-NEXT:    [[APPLY:%.*]] = apply [[FNREF]]()

test/SILGen/unavailable_decl_optimization_stub_macos.swift

@@ -24,6 +24,14 @@ public func unavailableFunc() {}
 @available(macOS, unavailable)
 public func unavailableOnMacOSFunc() {}
 
+// CHECK-LABEL:     sil{{.*}}@$s4Test36unavailableFuncWithMacOSIntroductionyyF
+// CHECK:             [[FNREF:%.*]] = function_ref @$ss31_diagnoseUnavailableCodeReacheds5NeverOyFTwb : $@convention(thin) () -> Never
+// CHECK-NEXT:        [[APPLY:%.*]] = apply [[FNREF]]()
+// CHECK:           } // end sil function '$s4Test36unavailableFuncWithMacOSIntroductionyyF'
+@available(*, unavailable)
+@available(macOS 12.0, *)
+public func unavailableFuncWithMacOSIntroduction() {}
+
 // CHECK-LABEL:     sil{{.*}}@$s4Test31unavailableOnMacOSExtensionFuncyyF
 // CHECK-NOT:         _diagnoseUnavailableCodeReached
 // CHECK:           } // end sil function '$s4Test31unavailableOnMacOSExtensionFuncyyF'