[Sema] Suggest for unwrap fix-it instead of coercion if a subtype relation exists with from and to types

Author: LucianoPAlmeida

lib/Sema/CSDiagnostics.cpp

@@ -2953,6 +2953,20 @@ bool ContextualFailure::tryTypeCoercionFixIt(
   if (!toType->hasTypeRepr())
     return false;
 
+  // If optional unwrapped type is a subtype of the specified contextual type,
+  // let's suggest a force unwrap "!". Otherwise fallback to potential coercion
+  // or force cast.
+  if (!bothOptional && fromType->getOptionalObjectType()) {
+    if (TypeChecker::isSubtypeOf(fromType->lookThroughAllOptionalTypes(),
+                                 toType, getDC())) {
+      diagnostic.fixItInsert(
+          Lexer::getLocForEndOfToken(getASTContext().SourceMgr,
+                                     getSourceRange().End),
+          "!");
+      return true;
+    }
+  }
+
   CheckedCastKind Kind =
       TypeChecker::typeCheckCheckedCast(fromType, toType,
                                         CheckedCastContextKind::None, getDC(),

test/Constraints/casts.swift

@@ -614,22 +614,40 @@ func decodeStringOrIntDictionary<T: FixedWidthInteger>() -> [Int: T] {
   }
 }
 
+
 // SR-15281
 struct SR15281_A { }
-
 struct SR15281_B {
   init(a: SR15281_A) { }
 }
 
 struct SR15281_S {
   var a: SR15281_A? = SR15281_A()
 
+  var b: SR15281_B {
+    a.flatMap(SR15281_B.init(a:)) // expected-error{{cannot convert return expression of type 'SR15281_B?' to return type 'SR15281_B'}} {{34-34=!}}
+  }
+
   var b1: SR15281_B {
     a.flatMap(SR15281_B.init(a:)) as! SR15281_B 
     // expected-warning@-1 {{forced cast from 'SR15281_B?' to 'SR15281_B' only unwraps optionals; did you mean to use '!'?}} {{34-34=!}} {{34-48=}}
   }
+}
+
+class SR15281_AC {}
+class SR15281_BC {
+  init(a: SR15281_AC) { }
+}
+class SR15281_CC: SR15281_BC {}
+
+struct SR15281_SC {
+  var a: SR15281_AC? = SR15281_AC()
+
+  var b: SR15281_BC {
+    a.flatMap(SR15281_BC.init(a:)) // expected-error{{cannot convert return expression of type 'SR15281_BC?' to return type 'SR15281_BC'}} {{35-35=!}}
+  }
 
-  var b: SR15281_B {
-    a.flatMap(SR15281_B.init(a:)) // expected-error{{cannot convert return expression of type 'SR15281_B?' to return type 'SR15281_B'}} {{34-34= as! SR15281_B}}
+  var c: SR15281_BC {
+    a.flatMap(SR15281_CC.init(a:)) // expected-error{{cannot convert return expression of type 'SR15281_CC?' to return type 'SR15281_BC'}} {{35-35=!}}
   }
 }