Merge pull request swiftlang#30409 from HassanElDesouky/SR-12309

[CSGen] Force unwrapping 'nil' compiles without warning

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -3204,6 +3204,8 @@ ERROR(cannot_infer_base_of_unresolved_member,none,
       "cannot infer contextual base in reference to member %0", (DeclNameRef))
 ERROR(unresolved_nil_literal,none,
       "'nil' requires a contextual type", ())
+ERROR(cannot_force_unwrap_nil_literal,none,
+      "'nil' literal cannot be force unwrapped", ())
 
 ERROR(type_of_expression_is_ambiguous,none,
       "type of expression is ambiguous without more context", ())

lib/Sema/CSGen.cpp

@@ -1152,6 +1152,22 @@ namespace {
       // `_ = nil`, let's diagnose it here because solver can't
       // attempt any types for it.
       auto *parentExpr = CS.getParentExpr(expr);
+      if (parentExpr && isa<ParenExpr>(parentExpr))
+        parentExpr = CS.getParentExpr(parentExpr);
+
+      if (parentExpr) {
+        // `_ = nil!`
+        if (isa<ForceValueExpr>(parentExpr)) {
+          DE.diagnose(expr->getLoc(), diag::cannot_force_unwrap_nil_literal);
+          return Type();
+        }
+
+        // `_ = nil?`
+        if (isa<OptionalEvaluationExpr>(parentExpr)) {
+          DE.diagnose(expr->getLoc(), diag::unresolved_nil_literal);
+          return Type();
+        }
+      }
 
       // `_ = nil`
       if (auto *assignment = dyn_cast_or_null<AssignExpr>(parentExpr)) {

test/Constraints/optional.swift

@@ -427,3 +427,18 @@ func invalidOptionalChaining(a: Any) {
   // expected-error@-1 {{value of protocol type 'Any' cannot conform to 'Equatable'; only struct/enum/class types can conform to protocols}}
   // expected-note@-2 {{requirement from conditional conformance of 'Any?' to 'Equatable'}}
 }
+
+// SR-12309 - Force unwrapping 'nil' compiles without warning
+func sr_12309() {
+  struct S {
+    var foo: Int
+  }
+
+  _ = S(foo: nil!) // expected-error {{'nil' literal cannot be force unwrapped}}
+  _ = nil! // expected-error {{'nil' literal cannot be force unwrapped}}
+  _ = nil? // expected-error {{value of optional type 'Optional<_>' must be unwrapped to a value of type '_'}}
+           // expected-note@-1 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
+           // expected-note@-2 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
+  _ = (nil) // expected-error {{'nil' requires a contextual type}}
+  _ = nil // expected-error {{'nil' requires a contextual type}}
+}