[5.4] Fix a Family of Crashers in Availability Checking

lib/Sema/TypeCheckAvailability.cpp

@@ -147,7 +147,11 @@ static void computeExportContextBits(ASTContext &Ctx, Decl *D,
   if (D->isSPI())
     *spi = true;
 
-  if (D->isImplicit())
+  // Defer bodies are desugared to an implicit closure expression. We need to
+  // dilute the meaning of "implicit" to make sure we're still checking
+  // availability inside of defer statements.
+  const auto isDeferBody = isa<FuncDecl>(D) && cast<FuncDecl>(D)->isDeferBody();
+  if (D->isImplicit() && !isDeferBody)
     *implicit = true;
 
   if (D->getAttrs().getDeprecated(Ctx))

test/Sema/availability.swift

@@ -195,3 +195,32 @@ struct VarToFunc {
   }
 }
 
+struct DeferBody {
+  func foo() {
+    enum No: Error {
+      case no
+    }
+
+    defer {
+      do {
+        throw No.no
+      } catch No.no {
+      } catch {
+      }
+    }
+    _ = ()
+  }
+
+  func bar() {
+    @available(*, unavailable)
+    enum No: Error { // expected-note 2 {{'No' has been explicitly marked unavailable here}}
+      case no
+    }
+    do {
+      throw No.no
+      // expected-error@-1 {{'No' is unavailable}}
+    } catch No.no {} catch _ {}
+    // expected-error@-1 {{'No' is unavailable}}
+  }
+}
+

test/attr/attr_inlinable.swift

@@ -357,3 +357,12 @@ public struct PrivateInlinableCrash {
     // expected-error@-2 {{initializer 'init()' is private and cannot be referenced from an '@inlinable' function}}
   }
 }
+
+// Just make sure we don't crash.
+private func deferBodyTestCall() {} // expected-note {{global function 'deferBodyTestCall()' is not '@usableFromInline' or public}}
+@inlinable public func deferBodyTest() {
+  defer {
+    deferBodyTestCall() // expected-error {{global function 'deferBodyTestCall()' is private and cannot be referenced from an '@inlinable' function}}
+  }
+  _ = ()
+}