[CSDiagnostics] Don't suggest unwrapping a key path root without an explicit type

A key path without an explicit root cannot be unwrapped since the
optionality is inferred from context and we cannot suggest removing
that, adding explicit type is not going to change anything and
`!` and `?` after the leading dot is not supported.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -1493,10 +1493,10 @@ bool MemberAccessOnOptionalBaseFailure::diagnoseAsError() {
     } else {
       emitDiagnostic(diag::invalid_optional_inferred_keypath_root, baseType,
                      Member, unwrappedBaseType);
-      emitDiagnostic(diag::optional_key_path_root_base_chain, Member)
-          .fixItInsert(sourceRange.End, "?.");
-      emitDiagnostic(diag::optional_key_path_root_base_unwrap, Member)
-          .fixItInsert(sourceRange.End, "!.");
+      // Note that unwrapping fix-its cannot be suggested in this case
+      // because neither `.?` nor `.!` can be used to start a key path
+      // literal. Suggesting an explicit type here won't work either
+      // because contextual root is going to be optional still.
     }
   } else {
     // Check whether or not the base of this optional unwrap is implicit self

test/expr/unary/keypath/keypath.swift

@@ -1010,16 +1010,10 @@ func testMemberAccessOnOptionalKeyPathComponent() {
   func kp(_: KeyPath<String?, Int>) {}
 
   kp(\.count) // expected-error {{key path root inferred as optional type 'String?' must be unwrapped to refer to member 'count' of unwrapped type 'String'}}
-  // expected-note@-1 {{chain the optional using '?.' to access unwrapped type member 'count'}} {{8-8=?.}}
-  // expected-note@-2 {{unwrap the optional using '!.' to access unwrapped type member 'count'}} {{8-8=!.}}
   let _ : KeyPath<String?, Int> = \.count // expected-error {{key path root inferred as optional type 'String?' must be unwrapped to refer to member 'count' of unwrapped type 'String'}}
-  // expected-note@-1 {{chain the optional using '?.' to access unwrapped type member 'count'}} {{37-37=?.}}
-  // expected-note@-2 {{unwrap the optional using '!.' to access unwrapped type member 'count'}} {{37-37=!.}}
 
   let _ : KeyPath<String?, Int> = \.utf8.count 
   // expected-error@-1 {{key path root inferred as optional type 'String?' must be unwrapped to refer to member 'utf8' of unwrapped type 'String'}}
-  // expected-note@-2 {{chain the optional using '?.' to access unwrapped type member 'utf8'}} {{37-37=?.}}
-  // expected-note@-3 {{unwrap the optional using '!.' to access unwrapped type member 'utf8'}} {{37-37=!.}}
 }
 
 func testSyntaxErrors() {
@@ -1070,6 +1064,7 @@ func f_56996() {
 // Key-path missing optional crashes compiler: Inactive constraints left over?
 func f_55805() {
   let _: KeyPath<String?, Int?> = \.utf8.count
+  // expected-error@-1 {{key path root inferred as optional type 'String?' must be unwrapped to refer to member 'utf8' of unwrapped type 'String'}}
   // expected-error@-2 {{value of optional type 'String.UTF8View?' must be unwrapped to refer to member 'count' of wrapped base type 'String.UTF8View'}}
   // expected-note@-3 {{chain the optional using '?' to access member 'count' only for non-'nil' base values}}
 }