[CS] Introduce performSyntacticDiagnosticsForTarget

Add a function that deals with invoking syntactic
diagnostics for all the expressions involved in
a SolutionApplicationTarget.

Resolves SR-13260
Resolves rdar://65903005

Author: hamishknight

lib/Sema/ConstraintSystem.h

@@ -1206,13 +1206,15 @@ struct DynamicCallableMethods {
 /// Describes the target to which a constraint system's solution can be
 /// applied.
 class SolutionApplicationTarget {
+public:
   enum class Kind {
     expression,
     function,
     stmtCondition,
     caseLabelItem,
   } kind;
 
+private:
   union {
     struct {
       /// The expression being type-checked.
@@ -5288,6 +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>

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);

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}}
+}