[CSDiagnostics] SE-0438: Add a tailored diagnostic for unsupported static member references

Libraries of modules built with older compilers (< 6.1) don't have
symbols required to enable staitc member support in key path context.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -666,6 +666,13 @@ ERROR(expr_keypath_mutating_getter,none,
 ERROR(expr_keypath_static_member,none,
       "%select{key path|dynamic key path member lookup}1 cannot refer to static member %0",
       (const ValueDecl *, bool))
+ERROR(keypath_static_member_access_from_unsupported_module,none,
+     "%select{key path|dynamic key path member lookup}3 cannot refer "
+     "to static member %1 of type %0 from module %2",
+      (Type, const ValueDecl *, const ModuleDecl *, bool))
+NOTE(keypath_static_member_access_from_unsupported_module_note,none,
+     "rebuild %0 to enable static member use in key paths",
+     (const ModuleDecl *))
 ERROR(expr_keypath_enum_case,none,
       "%select{key path|dynamic key path member lookup}1 cannot refer to enum case %0",
       (const ValueDecl *, bool))

lib/Sema/CSDiagnostics.cpp

@@ -6352,6 +6352,18 @@ bool InvalidStaticMemberRefInKeyPath::diagnoseAsError() {
   return true;
 }
 
+bool UnsupportedStaticMemberRefInKeyPath::diagnoseAsError() {
+  auto *member = getMember();
+  auto *module = member->getDeclContext()->getParentModule();
+
+  emitDiagnostic(diag::keypath_static_member_access_from_unsupported_module,
+                 BaseType->getMetatypeInstanceType(), member, module,
+                 getLocator()->isForKeyPathDynamicMemberLookup());
+  emitDiagnostic(
+      diag::keypath_static_member_access_from_unsupported_module_note, module);
+  return true;
+}
+
 bool InvalidEnumCaseRefInKeyPath::diagnoseAsError() {
   emitDiagnostic(diag::expr_keypath_enum_case, getMember(),
                  isForKeyPathDynamicMemberLookup());

lib/Sema/CSDiagnostics.h

@@ -1767,13 +1767,31 @@ class InvalidStaticMemberRefInKeyPath final : public InvalidMemberRefInKeyPath {
   InvalidStaticMemberRefInKeyPath(const Solution &solution, Type baseType,
                                   ValueDecl *member, ConstraintLocator *locator)
       : InvalidMemberRefInKeyPath(solution, member, locator),
-        BaseType(baseType->getRValueType()) {}
+        BaseType(resolveType(baseType)->getRValueType()) {}
 
   Type getBaseType() const { return BaseType; }
 
   bool diagnoseAsError() override;
 };
 
+/// Diagnose an attempt to reference a static member from an unsupported module.
+///
+/// Only modules built either from source with 6.1+ compiler or
+/// from .swiftinterface that was generated by 6.1+ compiler are supported.
+class UnsupportedStaticMemberRefInKeyPath final
+    : public InvalidMemberRefInKeyPath {
+  Type BaseType;
+
+public:
+  UnsupportedStaticMemberRefInKeyPath(const Solution &solution, Type baseType,
+                                      ValueDecl *member,
+                                      ConstraintLocator *locator)
+      : InvalidMemberRefInKeyPath(solution, member, locator),
+        BaseType(resolveType(baseType)->getRValueType()) {}
+
+  bool diagnoseAsError() override;
+};
+
 /// Diagnose an attempt to reference an enum case as a key path component
 /// e.g.
 ///

lib/Sema/CSFix.cpp

@@ -1237,7 +1237,9 @@ bool AllowInvalidRefInKeyPath::diagnose(const Solution &solution,
   }
 
   case RefKind::UnsupportedStaticMember: {
-    return false;
+    UnsupportedStaticMemberRefInKeyPath failure(solution, BaseType, Member,
+                                                getLocator());
+    return failure.diagnose(asNote);
   }
 
   case RefKind::EnumCase: {