diff --git a/lib/Sema/ConstraintSystem.h b/lib/Sema/ConstraintSystem.h
index b6fb85133b90c..06cba96996f96 100644
--- a/lib/Sema/ConstraintSystem.h
+++ b/lib/Sema/ConstraintSystem.h
@@ -1206,6 +1206,7 @@ struct DynamicCallableMethods {
 /// Describes the target to which a constraint system's solution can be
 /// applied.
 class SolutionApplicationTarget {
+public:
   enum class Kind {
     expression,
     function,
@@ -1213,6 +1214,7 @@ class SolutionApplicationTarget {
     caseLabelItem,
   } kind;
 
+private:
   union {
     struct {
       /// The expression being type-checked.
@@ -1418,6 +1420,12 @@ class SolutionApplicationTarget {
   /// For a pattern initialization target, retrieve the contextual pattern.
   ContextualPattern getContextualPattern() const;
 
+  /// Whether this target is for a for-in statement.
+  bool isForEachStmt() const {
+    return kind == Kind::expression &&
+           getExprContextualTypePurpose() == CTP_ForEachStmt;
+  }
+
   /// Whether this is an initialization for an Optional.Some pattern.
   bool isOptionalSomePatternInit() const {
     return kind == Kind::expression &&
@@ -1466,14 +1474,12 @@ class SolutionApplicationTarget {
   }
 
   const ForEachStmtInfo &getForEachStmtInfo() const {
-    assert(kind == Kind::expression);
-    assert(expression.contextualPurpose == CTP_ForEachStmt);
+    assert(isForEachStmt());
     return expression.forEachStmt;
   }
 
   ForEachStmtInfo &getForEachStmtInfo() {
-    assert(kind == Kind::expression);
-    assert(expression.contextualPurpose == CTP_ForEachStmt);
+    assert(isForEachStmt());
     return expression.forEachStmt;
   }
 
@@ -5284,6 +5290,11 @@ bool isKnownKeyPathDecl(ASTContext &ctx, ValueDecl *decl);
 /// statements that could produce non-void result.
 bool hasExplicitResult(ClosureExpr *closure);
 
+/// Emit diagnostics for syntactic restrictions within a given solution
+/// application target.
+void performSyntacticDiagnosticsForTarget(
+    const SolutionApplicationTarget &target, bool isExprStmt);
+
 } // end namespace constraints
 
 template<typename ...Args>
diff --git a/lib/Sema/TypeCheckConstraints.cpp b/lib/Sema/TypeCheckConstraints.cpp
index 47e0795f5ac4d..5787e0c0b485d 100644
--- a/lib/Sema/TypeCheckConstraints.cpp
+++ b/lib/Sema/TypeCheckConstraints.cpp
@@ -2008,6 +2008,31 @@ bool GenericRequirementsCheckListener::diagnoseUnsatisfiedRequirement(
   return false;
 }
 
+void constraints::performSyntacticDiagnosticsForTarget(
+    const SolutionApplicationTarget &target, bool isExprStmt) {
+  auto *dc = target.getDeclContext();
+  switch (target.kind) {
+  case SolutionApplicationTarget::Kind::expression: {
+    // First emit diagnostics for the main expression.
+    performSyntacticExprDiagnostics(target.getAsExpr(), dc, isExprStmt);
+
+    // If this is a for-in statement, we also need to check the where clause if
+    // present.
+    if (target.isForEachStmt()) {
+      if (auto *whereExpr = target.getForEachStmtInfo().whereExpr)
+        performSyntacticExprDiagnostics(whereExpr, dc, /*isExprStmt*/ false);
+    }
+    return;
+  }
+  case SolutionApplicationTarget::Kind::function:
+  case SolutionApplicationTarget::Kind::stmtCondition:
+  case SolutionApplicationTarget::Kind::caseLabelItem:
+    // Nothing to do for these.
+    return;
+  }
+  llvm_unreachable("Unhandled case in switch!");
+}
+
 #pragma mark High-level entry points
 Type TypeChecker::typeCheckExpression(Expr *&expr, DeclContext *dc,
                                       TypeLoc convertType,
@@ -2122,7 +2147,7 @@ TypeChecker::typeCheckExpression(
   // expression now.
   if (!cs.shouldSuppressDiagnostics()) {
     bool isExprStmt = options.contains(TypeCheckExprFlags::IsExprStmt);
-    performSyntacticExprDiagnostics(result, dc, isExprStmt);
+    performSyntacticDiagnosticsForTarget(*resultTarget, isExprStmt);
   }
 
   resultTarget->setExpr(result);
diff --git a/test/Constraints/availability.swift b/test/Constraints/availability.swift
index 5968441c4bc07..2fe93b9f20286 100644
--- a/test/Constraints/availability.swift
+++ b/test/Constraints/availability.swift
@@ -34,3 +34,12 @@ func test_contextual_member_with_availability() {
 
   _ = Test(.foo) // Ok
 }
+
+@available(*, unavailable)
+func unavailableFunction(_ x: Int) -> Bool { true } // expected-note {{'unavailableFunction' has been explicitly marked unavailable here}}
+
+// SR-13260: Availability checking not working in the where clause of a for
+// loop.
+func sr13260(_ arr: [Int]) {
+  for x in arr where unavailableFunction(x) {} // expected-error {{'unavailableFunction' is unavailable}}
+}