[Type Checker] Improve diagnostic when referencing a member of an optional base.

Improve diagnostics when referencing a member of an optional base, where the
Optional type does not have the member but the wrapped type does. Specifically,
suggest both the chaining ‘?’ and the force-unwrapping ‘!’ Fix-Its via explanatory
notes, e.g.:

  error: value of optional type '[Int]?' must be unwrapped to refer to member 'subscript' of wrapped base type '[Int]'
    return foo.array[0]
               ^
  note: chain the optional using '?' to access member 'subscript' only for non-'nil' base values
    return foo.array[0]
               ^
                    ?
  note: force-unwrap using '!' to abort execution if the optional value contains 'nil'
    return foo.array[0]
               ^
                    !

More of rdar://problem/42081852.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -915,6 +915,12 @@ NOTE(unwrap_with_default_value,none,
 NOTE(unwrap_with_force_value,none,
      "force-unwrap using '!' to abort execution if the optional value contains "
      "'nil'", ())
+ERROR(optional_base_not_unwrapped,none,
+      "value of optional type %0 must be unwrapped to refer to member %1 of "
+      "wrapped base type %2", (Type, DeclName, Type))
+NOTE(optional_base_chain,none,
+     "chain the optional using '?' to access member %0 only for non-'nil' "
+     "base values", (DeclName))
 ERROR(missing_unwrap_optional_try,none,
       "value of optional type %0 not unwrapped; did you mean to use 'try!' "
       "or chain with '?'?",

lib/Sema/CSDiag.cpp

@@ -8067,14 +8067,14 @@ bool FailureDiagnosis::diagnoseMemberFailures(
       }
 
       if (!optionalResult.ViableCandidates.empty()) {
-        // By default we assume that the LHS type is not optional.
-        StringRef fixIt = "!";
-        auto contextualType = CS.getContextualType();
-        if (contextualType && isa<OptionalType>(contextualType.getPointer()))
-          fixIt = "?";
-
-        diagnose(BaseLoc, diag::missing_unwrap_optional, baseObjTy)
-            .fixItInsertAfter(baseExpr->getEndLoc(), fixIt);
+        diagnose(BaseLoc, diag::optional_base_not_unwrapped,
+                 baseObjTy, memberName, OT->getOptionalObjectType())
+          .highlight(memberRange);
+
+        diagnose(BaseLoc, diag::optional_base_chain, memberName)
+          .fixItInsertAfter(baseExpr->getEndLoc(), "?");
+        diagnose(BaseLoc, diag::unwrap_with_force_value)
+          .fixItInsertAfter(baseExpr->getEndLoc(), "!");
         return true;
       }
     }

test/Constraints/diagnostics.swift

@@ -221,7 +221,9 @@ struct StructWithOptionalArray {
 }
 
 func testStructWithOptionalArray(_ foo: StructWithOptionalArray) -> Int {
-  return foo.array[0]  // expected-error {{value of optional type '[Int]?' not unwrapped; did you mean to use '!' or '?'?}} {{19-19=!}}
+  return foo.array[0]  // expected-error {{value of optional type '[Int]?' must be unwrapped to refer to member 'subscript' of wrapped base type '[Int]'}}
+  // expected-note@-1{{chain the optional using '?' to access member 'subscript' only for non-'nil' base values}}{{19-19=?}}
+  // expected-note@-2{{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}{{19-19=!}}
 }
 
 

test/Constraints/fixes.swift

@@ -148,9 +148,15 @@ struct Q {
   let s: String?
 }
 let q = Q(s: nil)
-let a: Int? = q.s.utf8 // expected-error{{value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?}} {{18-18=?}}
-let b: Int = q.s.utf8 // expected-error{{value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?}} {{17-17=!}}
-let d: Int! = q.s.utf8 // expected-error{{value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?}} {{18-18=?}}
+let a: Int? = q.s.utf8 // expected-error{{value of optional type 'String?' must be unwrapped to refer to member 'utf8' of wrapped base type 'String'}}
+// expected-note@-1{{chain the optional using '?'}}{{18-18=?}}
+// expected-note@-2{{force-unwrap using '!'}}{{18-18=!}}
+let b: Int = q.s.utf8 // expected-error{{value of optional type 'String?' must be unwrapped to refer to member 'utf8' of wrapped base type 'String'}}
+// expected-note@-1{{chain the optional using '?'}}{{17-17=?}}
+// expected-note@-2{{force-unwrap using '!'}}{{17-17=!}}
+let d: Int! = q.s.utf8 // expected-error{{value of optional type 'String?' must be unwrapped to refer to member 'utf8' of wrapped base type 'String'}}
+// expected-note@-1{{chain the optional using '?'}}{{18-18=?}}
+// expected-note@-2{{force-unwrap using '!'}}{{18-18=!}}
 let c = q.s.utf8 // expected-error{{value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?}} {{12-12=?}}
 
 // SR-1116