[Type Resolution] Emit a tailored diagnostic for the incorrect 'any P?' syntax.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -4755,6 +4755,9 @@ ERROR(unchecked_not_existential,none,
 ERROR(any_not_existential,none,
        "'any' has no effect on %select{concrete type|type parameter}0 %1",
        (bool, Type))
+ERROR(incorrect_optional_any,none,
+      "optional 'any' type must be written %0",
+      (Type))
 ERROR(existential_requires_any,none,
       "use of %select{protocol |}2%0 as a type must be written %1",
       (Type, Type, bool))

lib/Sema/TypeCheckType.cpp

@@ -3957,9 +3957,24 @@ TypeResolver::resolveExistentialType(ExistentialTypeRepr *repr,
   if (constraintType->hasError())
     return ErrorType::get(getASTContext());
 
-  auto anyStart = repr->getAnyLoc();
-  auto anyEnd = Lexer::getLocForEndOfToken(getASTContext().SourceMgr, anyStart);
   if (!constraintType->isExistentialType()) {
+    // Emit a tailored diagnostic for the incorrect optional
+    // syntax 'any P?' with a fix-it to add parenthesis.
+    auto wrapped = constraintType->getOptionalObjectType();
+    if (wrapped && (wrapped->is<ExistentialType>() ||
+                    wrapped->is<ExistentialMetatypeType>())) {
+      std::string fix;
+      llvm::raw_string_ostream OS(fix);
+      constraintType->print(OS, PrintOptions::forDiagnosticArguments());
+      diagnose(repr->getLoc(), diag::incorrect_optional_any,
+               constraintType)
+        .fixItReplace(repr->getSourceRange(), fix);
+      return constraintType;
+    }
+
+    auto anyStart = repr->getAnyLoc();
+    auto anyEnd = Lexer::getLocForEndOfToken(getASTContext().SourceMgr,
+                                             anyStart);
     diagnose(repr->getLoc(), diag::any_not_existential,
              constraintType->isTypeParameter(),
              constraintType)

test/type/explicit_existential.swift

@@ -308,4 +308,9 @@ func testAnyFixIt() {
   let _: HasAssoc.Type? = ConformingType.self
   // expected-warning@+1 {{use of protocol 'HasAssoc' as a type must be written 'any HasAssoc'}}{{10-18=(any HasAssoc)}}
   let _: HasAssoc.Protocol? = (any HasAssoc).self
+
+  // expected-error@+1 {{optional 'any' type must be written '(any HasAssoc)?'}}{{10-23=(any HasAssoc)?}}
+  let _: any HasAssoc? = nil
+  // expected-error@+1 {{optional 'any' type must be written '(any HasAssoc.Type)?'}}{{10-28=(any HasAssoc.Type)?}}
+  let _: any HasAssoc.Type? = nil
 }