[Sema] Don't suggest fixits that try to force unwrap implicit $match

Check if a variable is actually spelled out in code before suggesting a
fixit that adds a force unwrap to it

This fixes SR-1827

lib/Sema/CSSimplify.cpp

@@ -2009,8 +2009,21 @@ commit_to_conversions:
     // If we have an optional type, try to force-unwrap it.
     // FIXME: Should we also try '?'?
     if (objectType1->getOptionalObjectType()) {
+      bool forceUnwrapPossible = true;
+      if (auto declRefExpr =
+            dyn_cast_or_null<DeclRefExpr>(locator.trySimplifyToExpr())) {
+        if (declRefExpr->getDecl()->isImplicit()) {
+          // The expression that provides the first type is implicit and never
+          // spelled out in source code, e.g. $match in an expression pattern.
+          // Thus we cannot force unwrap the first type
+          forceUnwrapPossible = false;
+        }
+      }
+
+      if (forceUnwrapPossible) {
         conversionsOrFixes.push_back(FixKind::ForceOptional);
       }
+    }
 
     // If we have a value of type AnyObject that we're trying to convert to
     // a class, force a downcast.

lib/Sema/TypeCheckConstraints.cpp

@@ -2150,6 +2150,8 @@ bool TypeChecker::typeCheckExprPattern(ExprPattern *EP, DeclContext *DC,
                                          Context.getIdentifier("$match"),
                                          rhsType,
                                          DC);
+
+  matchVar->setImplicit();
   EP->setMatchVar(matchVar);
   matchVar->setHasNonPatternBindingInit();
   

test/expr/unary/selector/selector.swift

@@ -120,3 +120,18 @@ switch optionalSel {
 case #selector(C1.method1)?:
   break
 }
+
+@objc class SR1827 {
+  func bar() {}
+}
+
+switch optionalSel {
+case #selector(SR1827.bar): // expected-error{{expression pattern of type 'Selector' cannot match values of type 'Selector?'}}
+  break
+case #selector(SR1827.bar)!: // expected-error{{cannot force unwrap value of non-optional type 'Selector'}}
+  break
+case #selector(SR1827.bar)?:
+  break
+default:
+  break
+}