diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 80a8ddb742bcf..089b22044b35f 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -5056,6 +5056,10 @@ NOTE(function_builder_infer_add_return, none,
 NOTE(function_builder_infer_pick_specific, none,
      "apply function builder %0 (inferred from %select{protocol|dynamic replacement of}1 %2)",
      (Type, unsigned, DeclName))
+WARNING(function_builder_missing_limited_availability, none,
+        "function builder %0 does not implement `buildLimitedAvailability`; "
+        "this code may crash on earlier versions of the OS",
+        (Type))
 
 //------------------------------------------------------------------------------
 // MARK: Tuple Shuffle Diagnostics
diff --git a/include/swift/AST/KnownIdentifiers.def b/include/swift/AST/KnownIdentifiers.def
index 70e1df66519db..f807e96211255 100644
--- a/include/swift/AST/KnownIdentifiers.def
+++ b/include/swift/AST/KnownIdentifiers.def
@@ -37,6 +37,7 @@ IDENTIFIER(buildEither)
 IDENTIFIER(buildExpression)
 IDENTIFIER(buildFinalResult)
 IDENTIFIER(buildIf)
+IDENTIFIER(buildLimitedAvailability)
 IDENTIFIER(buildOptional)
 IDENTIFIER(callAsFunction)
 IDENTIFIER(Change)
diff --git a/lib/Sema/BuilderTransform.cpp b/lib/Sema/BuilderTransform.cpp
index a02e65adc2bd3..3421fb3caa6ff 100644
--- a/lib/Sema/BuilderTransform.cpp
+++ b/lib/Sema/BuilderTransform.cpp
@@ -19,6 +19,7 @@
 #include "MiscDiagnostics.h"
 #include "SolutionResult.h"
 #include "TypeChecker.h"
+#include "TypeCheckAvailability.h"
 #include "swift/AST/ASTVisitor.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/AST/NameLookup.h"
@@ -38,6 +39,24 @@ using namespace constraints;
 
 namespace {
 
+/// Find the first #available condition within the statement condition,
+/// or return NULL if there isn't one.
+const StmtConditionElement *findAvailabilityCondition(StmtCondition stmtCond) {
+  for (const auto &cond : stmtCond) {
+    switch (cond.getKind()) {
+    case StmtConditionElement::CK_Boolean:
+    case StmtConditionElement::CK_PatternBinding:
+      continue;
+
+    case StmtConditionElement::CK_Availability:
+      return &cond;
+      break;
+    }
+  }
+
+  return nullptr;
+}
+
 /// Visitor to classify the contents of the given closure.
 class BuilderClosureVisitor
     : private StmtVisitor<BuilderClosureVisitor, VarDecl *> {
@@ -502,10 +521,20 @@ class BuilderClosureVisitor
     if (!cs || !thenVar || (elseChainVar && !*elseChainVar))
       return nullptr;
 
+    // If there is a #available in the condition, the 'then' will need to
+    // be wrapped in a call to buildAvailabilityErasure(_:), if available.
+    Expr *thenVarRefExpr = buildVarRef(
+        thenVar, ifStmt->getThenStmt()->getEndLoc());
+    if (findAvailabilityCondition(ifStmt->getCond()) &&
+        builderSupports(ctx.Id_buildLimitedAvailability)) {
+      thenVarRefExpr = buildCallIfWanted(
+          ifStmt->getThenStmt()->getEndLoc(), ctx.Id_buildLimitedAvailability,
+          { thenVarRefExpr }, { Identifier() });
+    }
+
     // Prepare the `then` operand by wrapping it to produce a chain result.
     Expr *thenExpr = buildWrappedChainPayload(
-        buildVarRef(thenVar, ifStmt->getThenStmt()->getEndLoc()),
-        payloadIndex, numPayloads, isOptional);
+        thenVarRefExpr, payloadIndex, numPayloads, isOptional);
 
     // Prepare the `else operand:
     Expr *elseExpr;
@@ -1054,6 +1083,35 @@ class BuilderClosureRewriter
             capturedThen.first, {capturedThen.second.front()}));
     ifStmt->setThenStmt(newThen);
 
+    // Look for a #available condition. If there is one, we need to check
+    // that the resulting type of the "then" does refer to any types that
+    // are unavailable in the enclosing context.
+    //
+    // Note that this is for staging in support for
+    if (auto availabilityCond = findAvailabilityCondition(ifStmt->getCond())) {
+      SourceLoc loc = availabilityCond->getStartLoc();
+      Type thenBodyType = solution.simplifyType(
+          solution.getType(target.captured.second[0]));
+      thenBodyType.findIf([&](Type type) {
+        auto nominal = type->getAnyNominal();
+        if (!nominal)
+          return false;
+
+        if (auto reason = TypeChecker::checkDeclarationAvailability(
+                              nominal, loc, dc)) {
+          // Note that the problem is with the function builder, not the body.
+          // This is for staging only. We want to disable #available in
+          // function builders that don't support this operation.
+          ctx.Diags.diagnose(
+              loc, diag::function_builder_missing_limited_availability,
+              builderTransform.builderType);
+          return true;
+        }
+
+        return false;
+      });
+    }
+
     if (auto elseBraceStmt =
             dyn_cast_or_null<BraceStmt>(ifStmt->getElseStmt())) {
       // Translate the "else" branch when it's a stmt-brace.
diff --git a/test/Constraints/function_builder_availability.swift b/test/Constraints/function_builder_availability.swift
index ac63adc4a7f7f..07b3187959b57 100644
--- a/test/Constraints/function_builder_availability.swift
+++ b/test/Constraints/function_builder_availability.swift
@@ -16,7 +16,7 @@ struct TupleBuilder {
   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)
@@ -55,14 +55,17 @@ func globalFuncAvailableOn10_9() -> Int { return 9 }
 func globalFuncAvailableOn10_51() -> Int { return 10 }
 
 @available(OSX, introduced: 10.52)
-func globalFuncAvailableOn10_52() -> Int { return 11 }
+struct Only10_52 { }
+
+@available(OSX, introduced: 10.52)
+func globalFuncAvailableOn10_52() -> Only10_52 { .init() }
 
 tuplify(true) { cond in
   globalFuncAvailableOn10_9()
   if #available(OSX 10.51, *) {
     globalFuncAvailableOn10_51()
     tuplify(false) { cond2 in
-      if cond, #available(OSX 10.52, *) {
+      if cond, #available(OSX 10.52, *) { // expected-warning{{function builder 'TupleBuilder' does not implement `buildLimitedAvailability`; this code may crash on earlier versions of the OS}}
         cond2
         globalFuncAvailableOn10_52()
       } else {
@@ -72,3 +75,55 @@ tuplify(true) { cond in
     }
   }
 }
+
+// Function builder that can perform type erasure for #available.
+@_functionBuilder
+struct TupleBuilderAvailability {
+  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)
+  }
+
+  static func buildLimitedAvailability<T>(_ value: T) -> Any {
+    return value
+  }
+}
+
+func tuplifyWithAvailabilityErasure<T>(_ cond: Bool, @TupleBuilderAvailability body: (Bool) -> T) {
+  print(body(cond))
+}
+
+tuplifyWithAvailabilityErasure(true) { cond in
+  if cond, #available(OSX 10.52, *) {
+    globalFuncAvailableOn10_52()
+  }
+}