[SR-12309] Cannot force unwrap 'nil'

Author: HassanElDesouky

include/swift/AST/DiagnosticsSema.def

@@ -3197,6 +3197,8 @@ 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

@@ -3207,6 +3207,15 @@ namespace {
     }
 
     Type visitForceValueExpr(ForceValueExpr *expr) {
+      // Diagnose force-unwrapping a 'nil' literal
+      auto &DE = CS.getASTContext().Diags;
+      auto subExpr = expr->getSubExpr();
+      if (isa<NilLiteralExpr>(subExpr)) {
+          DE.diagnose(subExpr->getLoc(), diag::cannot_force_unwrap_nil_literal);
+          return Type();
+      }
+
+
       // Force-unwrap an optional of type T? to produce a T.
       auto locator = CS.getConstraintLocator(expr);
 

test/Constraints/sr12309.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Foo {
+  var bar: Int
+}
+
+let f = Foo(bar: nil!) // expected-error {{'nil' literal cannot be force unwrapped}}