Merge pull request swiftlang#28073 from hborla/nil-conditional-cast

[CSGen] Add a tailored diagnostic for conditional casting from a nil literal.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -243,6 +243,8 @@ ERROR(cannot_subscript_ambiguous_base,none,
 
 ERROR(cannot_subscript_nil_literal,none,
       "cannot subscript a nil literal value", ())
+ERROR(conditional_cast_from_nil,none,
+      "nil literal cannot be the source of a conditional cast", ())
 
 ERROR(cannot_pass_rvalue_inout_subelement,none,
       "cannot pass immutable value as inout argument: %0",

lib/Sema/CSGen.cpp

@@ -2674,6 +2674,25 @@ namespace {
       if (!fromExpr) // Either wasn't constructed correctly or wasn't folded.
         return nullptr;
 
+      std::function<Expr *(Expr *)> nilLiteralExpr = [&](Expr *expr) -> Expr * {
+        expr = expr->getSemanticsProvidingExpr();
+        if (expr->getKind() == ExprKind::NilLiteral)
+          return expr;
+
+        if (auto *optionalEvalExpr = dyn_cast<OptionalEvaluationExpr>(expr))
+          return nilLiteralExpr(optionalEvalExpr->getSubExpr());
+
+        if (auto *bindOptionalExpr = dyn_cast<BindOptionalExpr>(expr))
+          return nilLiteralExpr(bindOptionalExpr->getSubExpr());
+
+        return nullptr;
+      };
+
+      if (auto nilLiteral = nilLiteralExpr(fromExpr)) {
+        tc.diagnose(nilLiteral->getLoc(), diag::conditional_cast_from_nil);
+        return nullptr;
+      }
+
       // Validate the resulting type.
       TypeResolutionOptions options(TypeResolverContext::ExplicitCastExpr);
       options |= TypeResolutionFlags::AllowUnboundGenerics;

test/Constraints/casts.swift

@@ -220,3 +220,5 @@ func process(p: Any?) {
 
 func compare<T>(_: T, _: T) {} // expected-note {{'compare' declared here}}
 func compare<T>(_: T?, _: T?) {}
+
+_ = nil? as? Int?? // expected-error {{nil literal cannot be the source of a conditional cast}}