[Function builders] Add support for "if #available".

Availability checks in if statements don't need any actual semantic
checking, so enable them within function builders.

Author: DougGregor

lib/Sema/BuilderTransform.cpp

@@ -831,6 +831,9 @@ class BuilderClosureRewriter
     auto condition = ifStmt->getCond();
     for (auto &condElement : condition) {
       switch (condElement.getKind()) {
+      case StmtConditionElement::CK_Availability:
+        continue;
+
       case StmtConditionElement::CK_Boolean: {
         auto condExpr = condElement.getBoolean();
         auto finalCondExpr = rewriteExpr(condExpr);
@@ -846,7 +849,6 @@ class BuilderClosureRewriter
       }
 
       case StmtConditionElement::CK_PatternBinding:
-      case StmtConditionElement::CK_Availability:
         llvm_unreachable("unhandled statement condition");
       }
     }

lib/Sema/CSGen.cpp

@@ -3801,11 +3801,11 @@ Type ConstraintSystem::generateConstraints(Pattern *pattern,
 bool ConstraintSystem::canGenerateConstraints(StmtCondition condition) {
   for (const auto &element : condition) {
     switch (element.getKind()) {
+    case StmtConditionElement::CK_Availability:
     case StmtConditionElement::CK_Boolean:
       continue;
 
     case StmtConditionElement::CK_PatternBinding:
-    case StmtConditionElement::CK_Availability:
       return false;
     }
   }
@@ -3823,6 +3823,10 @@ bool ConstraintSystem::generateConstraints(StmtCondition condition,
 
   for (const auto &condElement : condition) {
     switch (condElement.getKind()) {
+    case StmtConditionElement::CK_Availability:
+      // Nothing to do here.
+      continue;
+
     case StmtConditionElement::CK_Boolean: {
       Expr *condExpr = condElement.getBoolean();
       condExpr = generateConstraints(condExpr, dc);
@@ -3838,7 +3842,6 @@ bool ConstraintSystem::generateConstraints(StmtCondition condition,
     }
 
     case StmtConditionElement::CK_PatternBinding:
-    case StmtConditionElement::CK_Availability:
       llvm_unreachable("unhandled statement condition");
     }
   }

test/Constraints/function_builder_availability.swift

@@ -0,0 +1,74 @@
+// RUN: %target-typecheck-verify-swift -target x86_64-apple-macosx10.50
+
+// REQUIRES: OS=macosx
+
+enum Either<T,U> {
+  case first(T)
+  case second(U)
+}
+
+@_functionBuilder
+struct TupleBuilder {
+  static func buildBlock<T1>(_ t1: T1) -> (T1) {
+    return (t1)
+  }
+
+  static func buildBlock<T1, T2>(_ t1: T1, _ t2: T2) -> (T1, T2) {
+    return (t1, t2)
+  }
+  
+  static func buildBlock<T1, T2, T3>(_ t1: T1, _ t2: T2, _ t3: T3)
+      -> (T1, T2, T3) {
+    return (t1, t2, t3)
+  }
+
+  static func buildBlock<T1, T2, T3, T4>(_ t1: T1, _ t2: T2, _ t3: T3, _ t4: T4)
+      -> (T1, T2, T3, T4) {
+    return (t1, t2, t3, t4)
+  }
+
+  static func buildBlock<T1, T2, T3, T4, T5>(
+    _ t1: T1, _ t2: T2, _ t3: T3, _ t4: T4, _ t5: T5
+  ) -> (T1, T2, T3, T4, T5) {
+    return (t1, t2, t3, t4, t5)
+  }
+
+  static func buildDo<T>(_ value: T) -> T { return value }
+  static func buildIf<T>(_ value: T?) -> T? { return value }
+
+  static func buildEither<T,U>(first value: T) -> Either<T,U> {
+    return .first(value)
+  }
+  static func buildEither<T,U>(second value: U) -> Either<T,U> {
+    return .second(value)
+  }
+}
+
+func tuplify<T>(_ cond: Bool, @TupleBuilder body: (Bool) -> T) {
+  print(body(cond))
+}
+
+@available(OSX, introduced: 10.9)
+func globalFuncAvailableOn10_9() -> Int { return 9 }
+
+@available(OSX, introduced: 10.51)
+func globalFuncAvailableOn10_51() -> Int { return 10 }
+
+@available(OSX, introduced: 10.52)
+func globalFuncAvailableOn10_52() -> Int { return 11 }
+
+tuplify(true) { cond in
+  globalFuncAvailableOn10_9()
+  if #available(OSX 10.51, *) {
+    globalFuncAvailableOn10_51()
+    tuplify(false) { cond2 in
+      if cond, #available(OSX 10.52, *) {
+        cond2
+        globalFuncAvailableOn10_52()
+      } else {
+        globalFuncAvailableOn10_52() // expected-error{{'globalFuncAvailableOn10_52()' is only available in macOS 10.52 or newer}}
+        // expected-note@-1{{add 'if #available' version check}}
+      }
+    }
+  }
+}