Allow :: as an alias for . in scoped imports

Author: beccadax

include/swift/AST/DiagnosticsParse.def

@@ -101,6 +101,8 @@ NOTE(extra_whitespace_module_selector,none,
      "remove extraneous whitespace after '::'", ())
 ERROR(module_selector_submodule_not_allowed,none,
       "module selector cannot specify a submodule", ())
+NOTE(replace_module_selector_with_member_lookup,none,
+     "replace '::' with '.'", ())
 
 //------------------------------------------------------------------------------
 // MARK: Lexer diagnostics

lib/Parse/ParseDecl.cpp

@@ -6681,7 +6681,13 @@ ParserResult<ImportDecl> Parser::parseDeclImport(ParseDeclOptions Flags,
           .fixItRemove(Tok.getLoc());
         return nullptr;
     }
-    HasNext = consumeIf(tok::period);
+    if (Tok.is(tok::colon_colon)
+          && (Kind == ImportKind::Module || importPath.size() > 1)) {
+      diagnose(Tok, diag::module_selector_submodule_not_allowed);
+      diagnose(Tok, diag::replace_module_selector_with_member_lookup)
+        .fixItReplace(Tok.getLoc(), ".");
+    }
+    HasNext = consumeIf(tok::period) || consumeIf(tok::colon_colon);
   } while (HasNext);
 
   if (Tok.is(tok::code_complete)) {

test/Parse/module_selector.swift

@@ -5,6 +5,26 @@
 
 // REQUIRES: swift_feature_ModuleSelector
 
+// ModuleSelectorImports
+import struct ModuleSelectorTestingKit::A
+
+import struct _::A
+// expected-error@-1 {{'_' cannot be used as an identifier here}}
+// expected-note@-2 {{if this name is unavoidable, use backticks to escape it}} {{15-16=`_`}}
+
+import struct ModuleSelectorTestingKit::Submodule::A
+// expected-error@-1 {{module selector cannot specify a submodule}}
+// expected-note@-2 {{replace '::' with '.'}} {{50-52=.}}
+
+import struct ModuleSelectorTestingKit.Submodule::A
+// expected-error@-1 {{module selector cannot specify a submodule}}
+// expected-note@-2 {{replace '::' with '.'}} {{49-51=.}}
+
+import ctypes::bits
+// expected-error@-1 {{module selector cannot specify a submodule}}
+// expected-note@-2 {{replace '::' with '.'}} {{14-16=.}}
+
+
 // ModuleSelectorCorrectCode
 extension ModuleSelectorTestingKit::A {}