Handle 'import struct Foo' where Foo is a non-nominal typealias (#14797)

Previously this just crashed. Now it suggests `import typealias Foo`,
a syntax that works back to Swift 1.

rdar://problem/36756349

Author: jrose-apple

include/swift/AST/DiagnosticsSema.def

@@ -688,9 +688,13 @@ ERROR(decl_does_not_exist_in_module,none,
       "%1 does not exist in module %2",
       (/*ImportKind*/ unsigned, Identifier, Identifier))
 ERROR(imported_decl_is_wrong_kind,none,
-      "%0 was imported as '%1', but is a "
-      "%select{%error|type|struct|class|enum|protocol|variable|function}2",
+      "%0 was imported as '%1', but is "
+      "%select{%error|a type|a struct|a class|an enum|a protocol|a variable|"
+      "a function}2",
       (Identifier, StringRef, /*ImportKind*/ unsigned))
+ERROR(imported_decl_is_wrong_kind_typealias,none,
+      "%0 %1 cannot be imported as '%2'",
+      (DescriptiveDeclKind, Type, StringRef))
 ERROR(ambiguous_decl_in_module,none,
       "ambiguous name %0 in module %1", (Identifier, Identifier))
 

lib/AST/Decl.cpp

@@ -804,7 +804,10 @@ ImportKind ImportDecl::getBestImportKind(const ValueDecl *VD) {
 
   case DeclKind::TypeAlias: {
     Type type = cast<TypeAliasDecl>(VD)->getDeclaredInterfaceType();
-    return getBestImportKind(type->getAnyNominal());
+    auto *nominal = type->getAnyNominal();
+    if (!nominal)
+      return ImportKind::Type;
+    return getBestImportKind(nominal);
   }
 
   case DeclKind::Accessor:

lib/Sema/NameBinding.cpp

@@ -116,6 +116,22 @@ static bool isCompatibleImportKind(ImportKind expected, ImportKind actual) {
   llvm_unreachable("Unhandled ImportKind in switch.");
 }
 
+static bool isNominalImportKind(ImportKind kind) {
+  switch (kind) {
+  case ImportKind::Module:
+    llvm_unreachable("module imports do not bring in decls");
+  case ImportKind::Struct:
+  case ImportKind::Class:
+  case ImportKind::Enum:
+  case ImportKind::Protocol:
+    return true;
+  case ImportKind::Type:
+  case ImportKind::Var:
+  case ImportKind::Func:
+    return false;
+  }
+}
+
 static const char *getImportKindString(ImportKind kind) {
   switch (kind) {
   case ImportKind::Module:
@@ -258,12 +274,30 @@ void NameBinder::addImport(
         diagnose(next, diag::found_candidate);
 
     } else if (!isCompatibleImportKind(ID->getImportKind(), *actualKind)) {
-      diagnose(ID, diag::imported_decl_is_wrong_kind,
-               declPath.front().first,
-               getImportKindString(ID->getImportKind()),
-               static_cast<unsigned>(*actualKind))
-        .fixItReplace(SourceRange(ID->getKindLoc()),
-                      getImportKindString(*actualKind));
+      Optional<InFlightDiagnostic> emittedDiag;
+      if (*actualKind == ImportKind::Type &&
+          isNominalImportKind(ID->getImportKind())) {
+        assert(decls.size() == 1 &&
+               "if we start suggesting ImportKind::Type for, e.g., a mix of "
+               "structs and classes, we'll need a different message here");
+        assert(isa<TypeAliasDecl>(decls.front()) &&
+               "ImportKind::Type is only the best choice for a typealias");
+        auto *typealias = cast<TypeAliasDecl>(decls.front());
+        emittedDiag.emplace(diagnose(ID,
+            diag::imported_decl_is_wrong_kind_typealias,
+            typealias->getDescriptiveKind(),
+            typealias->getDeclaredInterfaceType(),
+            getImportKindString(ID->getImportKind())));
+      } else {
+        emittedDiag.emplace(diagnose(ID, diag::imported_decl_is_wrong_kind,
+            declPath.front().first,
+            getImportKindString(ID->getImportKind()),
+            static_cast<unsigned>(*actualKind)));
+      }
+
+      emittedDiag->fixItReplace(SourceRange(ID->getKindLoc()),
+                                getImportKindString(*actualKind));
+      emittedDiag->flush();
 
       if (decls.size() == 1)
         diagnose(decls.front(), diag::decl_declared_here,

test/NameBinding/Inputs/DeclsUsedWrongly.swift

@@ -0,0 +1,9 @@
+public var x: Int = 0
+
+public enum Choice {
+  case yes, no, maybeSo
+}
+
+public typealias Callback = () -> Void
+
+public typealias Pair<T> = (T, T)

test/NameBinding/import-specific-fixits.swift

@@ -3,8 +3,8 @@
 // RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/ambiguous_left.swift
 // RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/ambiguous_right.swift
 // RUN: %target-swift-frontend -emit-module -o %t -I %t %S/Inputs/ambiguous.swift
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/DeclsUsedWrongly.swift
 
-// RUN: echo "public var x = Int()" | %target-swift-frontend -module-name FooBar -emit-module -o %t -
 // RUN: %target-swift-frontend -typecheck -I %t -serialize-diagnostics-path %t.dia %s -verify
 // RUN: c-index-test -read-diagnostics %t.dia > %t.deserialized_diagnostics.txt 2>&1
 // RUN: %FileCheck --input-file=%t.deserialized_diagnostics.txt %s
@@ -29,7 +29,15 @@ import class Swift.Int64 // expected-error {{'Int64' was imported as 'class', bu
 
 import class Swift.Bool // expected-error {{'Bool' was imported as 'class', but is a struct}} {{8-13=struct}}
 
-import struct FooBar.x // expected-error {{'x' was imported as 'struct', but is a variable}} {{8-14=var}}
+import struct DeclsUsedWrongly.x // expected-error {{'x' was imported as 'struct', but is a variable}} {{8-14=var}}
+
+import struct DeclsUsedWrongly.Choice // expected-error {{'Choice' was imported as 'struct', but is an enum}} {{8-14=enum}}
+
+import struct DeclsUsedWrongly.Callback // expected-error {{type alias 'Callback' (aka '() -> ()') cannot be imported as 'struct'}} {{8-14=typealias}}
+import var DeclsUsedWrongly.Callback // expected-error {{'Callback' was imported as 'var', but is a type}} {{8-11=typealias}}
+
+import struct DeclsUsedWrongly.Pair // expected-error {{type alias 'Pair' cannot be imported as 'struct'}} {{8-14=typealias}}
+import var DeclsUsedWrongly.Pair // expected-error {{'Pair' was imported as 'var', but is a type}} {{8-11=typealias}}
 
 import struct Swift.print // expected-error {{'print' was imported as 'struct', but is a function}} {{8-14=func}}