[Sema] Avoid resolving placeholders for protocol requirement params

We can't ever have default arguments in protocols anyway.

Author: hamishknight

lib/Sema/TypeCheckDecl.cpp

@@ -2317,9 +2317,17 @@ static Type validateParameterType(ParamDecl *decl) {
                      : TypeResolverContext::FunctionInput);
   options |= TypeResolutionFlags::Direct;
 
+  // We allow placeholders in parameter types to improve recovery since if a
+  // default argument is present we can suggest the inferred type. Avoid doing
+  // this for protocol requirements though since those can't ever have default
+  // arguments anyway.
+  HandlePlaceholderTypeReprFn placeholderOpener;
+  if (!isa<ProtocolDecl>(dc->getParent()))
+    placeholderOpener = PlaceholderType::get;
+
   const auto resolution =
       TypeResolution::forInterface(dc, options, unboundTyOpener,
-                                   PlaceholderType::get,
+                                   placeholderOpener,
                                    /*packElementOpener*/ nullptr);
 
   if (isa<VarargTypeRepr>(nestedRepr)) {

test/Sema/placeholder_type.swift

@@ -294,13 +294,12 @@ do {
 
 // Make sure we reject placeholders here.
 protocol TestPlaceholderRequirement {
-  func foo(_:_) // expected-error {{type placeholder may not appear in top-level parameter}}
+  func foo(_:_) // expected-error {{type placeholder not allowed here}}
   func bar() -> _ // expected-error {{type placeholder not allowed here}}
   func baz() -> [_] // expected-error {{type placeholder not allowed here}}
-  func qux(_: [_]) // expected-error {{type placeholder may not appear in top-level parameter}}
+  func qux(_: [_]) // expected-error {{type placeholder not allowed here}}
 
-  // FIXME: Shouldn't diagnose twice
-  subscript(_: _) -> Void { get } // expected-error 2{{type placeholder may not appear in top-level parameter}}
+  subscript(_: _) -> Void { get } // expected-error {{type placeholder not allowed here}}
   subscript() -> _ { get } // expected-error {{type placeholder not allowed here}}
 }
 
@@ -311,6 +310,7 @@ var testPlaceholderComputed1: _ { 0 } // expected-error {{type placeholder not a
 var testPlaceholderComputed2: [_] { [0] } // expected-error {{type placeholder not allowed here}}
 
 struct TestPlaceholderSubscript {
+  // FIXME: Shouldn't diagnose twice.
   subscript(_: _) -> Void { () } // expected-error 2{{type placeholder may not appear in top-level parameter}}
   subscript(_: [_]) -> Void { () } // expected-error 2{{type placeholder may not appear in top-level parameter}}
   subscript() -> _ { () } // expected-error {{type placeholder not allowed here}}

validation-test/compiler_crashers_2/e1f8b6a26663e9b5.swift renamed to validation-test/compiler_crashers_2_fixed/e1f8b6a26663e9b5.swift

@@ -1,4 +1,4 @@
 // {"signature":"swift::CanTypeVisitor<swift::TypeMatcher<(anonymous namespace)::AssociatedTypeInference::getPotentialTypeWitnessesByMatchingTypes(swift::ValueDecl*, swift::ValueDecl*)::MatchVisitor>::MatchVisitor, bool, swift::Type, swift::Type>::visit(swift::CanType, swift::Type, swift::Type)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 protocol a { associatedtype b func c(_ : _ d: b }
                          extension a { c(_ : _ d: b struct e : a