Improve diagnostics when trying to extend existential type

Author: cbjeukendrup

include/swift/AST/DiagnosticsSema.def

@@ -1933,6 +1933,10 @@ NOTE(objc_generic_extension_using_type_parameter_here,none,
      "generic parameter used here", ())
 NOTE(objc_generic_extension_using_type_parameter_try_objc,none,
      "add '@objc' to allow uses of 'self' within the function body", ())
+ERROR(unsupported_existential_extension,none,
+      "extension of type %0 is not supported", (Type))
+NOTE(unsupported_existential_extension_rewrite,none,
+     "did you mean to extend %0 instead?", (Type))
 ERROR(invalid_nominal_extension,none,
       "extension of type %0 must be declared as an extension of %1",
       (Type, Type))

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -3042,15 +3042,24 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       const bool wasAlreadyInvalid = ED->isInvalid();
       ED->setInvalid();
       if (!extType->hasError() && extType->getAnyNominal()) {
+        auto canExtType = extType->getCanonicalType();
+        if (auto existential = canExtType->getAs<ExistentialType>()) {
+          ED->diagnose(diag::unsupported_existential_extension, existential)
+            .highlight(ED->getExtendedTypeRepr()->getSourceRange());
+          ED->diagnose(diag::unsupported_existential_extension_rewrite, existential->getConstraintType())
+            .fixItReplace(ED->getExtendedTypeRepr()->getSourceRange(),
+                          canExtType->getString());
+          return;
+        }
+          
         // If we've got here, then we have some kind of extension of a prima
-        // fascie non-nominal type.  This can come up when we're projecting
+        // facie non-nominal type.  This can come up when we're projecting
         // typealiases out of bound generic types.
         //
         // struct Array<T> { typealias Indices = Range<Int> }
         // extension Array.Indices.Bound {}
         //
         // Offer to rewrite it to the underlying nominal type.
-        auto canExtType = extType->getCanonicalType();
         if (canExtType.getPointer() != extType.getPointer()) {
           ED->diagnose(diag::invalid_nominal_extension, extType, canExtType)
             .highlight(ED->getExtendedTypeRepr()->getSourceRange());

test/decl/ext/extensions.swift

@@ -104,24 +104,51 @@ var c = C()
 var x = c.p1
 c.p1 = 1
 
+// Reject extension of existential type
+
+protocol P3 {}
+
+extension any P3 {
+// expected-error@-1 {{extension of type 'any P3' is not supported}}
+// expected-note@-2 {{did you mean to extend 'P3' instead?}}
+}
+
 // Reject extension of nominal type via inferred associated type
-protocol P3 {
+protocol P4 {
   associatedtype Assoc
   func foo() -> Assoc
 }
 
-struct X3 : P3 {
+struct X4 : P4 {
 }
 
-extension X3.Assoc {
-// expected-error@-1 {{extension of type 'X3.Assoc' (aka 'Int') must be declared as an extension of 'Int'}}
+extension X4.Assoc {
+// expected-error@-1 {{extension of type 'X4.Assoc' (aka 'Int') must be declared as an extension of 'Int'}}
 // expected-note@-2 {{did you mean to extend 'Int' instead?}}
 }
 
-extension X3 {
+extension X4 {
   func foo() -> Int { return 0 }
 }
 
+// Reject extension of nominal type via typealias with dependent underlying type
+
+struct Nest<Egg> { typealias Contents = Egg }
+struct Tree { 
+  typealias LimbContent = Nest<Int> 
+  typealias BoughPayload = Nest<Nest<Int>>
+}
+
+extension Tree.LimbContent.Contents {
+  // expected-error@-1 {{extension of type 'Tree.LimbContent.Contents' (aka 'Int') must be declared as an extension of 'Int'}}
+  // expected-note@-2 {{did you mean to extend 'Int' instead?}} {{11-36=Int}}
+}
+
+extension Tree.BoughPayload.Contents {
+  // expected-error@-1 {{extension of type 'Tree.BoughPayload.Contents' (aka 'Nest<Int>') must be declared as an extension of 'Nest<Int>'}}
+  // expected-note@-2 {{did you mean to extend 'Nest<Int>' instead?}}  {{11-37=Nest<Int>}}
+}
+
 // Make sure the test case from https://bugs.swift.org/browse/SR-3847 doesn't
 // cause problems when the later extension is incorrectly nested inside another
 // declaration.
@@ -334,24 +361,6 @@ extension ImposeClassReq2 {
   }
 }
 
-// Reject extension of nominal type via typealias with dependent underlying type
-
-struct Nest<Egg> { typealias Contents = Egg }
-struct Tree { 
-  typealias LimbContent = Nest<Int> 
-  typealias BoughPayload = Nest<Nest<Int>>
-}
-
-extension Tree.LimbContent.Contents {
-  // expected-error@-1 {{extension of type 'Tree.LimbContent.Contents' (aka 'Int') must be declared as an extension of 'Int'}}
-  // expected-note@-2 {{did you mean to extend 'Int' instead?}} {{11-36=Int}}
-}
-
-extension Tree.BoughPayload.Contents {
-  // expected-error@-1 {{extension of type 'Tree.BoughPayload.Contents' (aka 'Nest<Int>') must be declared as an extension of 'Nest<Int>'}}
-  // expected-note@-2 {{did you mean to extend 'Nest<Int>' instead?}}
-}
-
 // SR-10466 Check 'where' clause when referencing type defined inside extension
 struct SR_10466<T> {
   var a : A // expected-error {{'SR_10466<T>.A' (aka 'Int') requires the types 'T' and 'Never' be equivalent}}