[Frontend] Allow aliasing the stdlib Swift module

Author: xymus

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -755,15 +755,12 @@ bool ModuleAliasesConverter::computeModuleAliases(std::vector<std::string> args,
       if (!allowModuleName) {
         if (value == options.ModuleName ||
             value == options.ModuleABIName ||
-            value == options.ModuleLinkName) {
+            value == options.ModuleLinkName ||
+            value == STDLIB_NAME) {
           diags.diagnose(SourceLoc(), diag::error_module_alias_forbidden_name, value);
           return false;
         }
       }
-      if (value == STDLIB_NAME) {
-        diags.diagnose(SourceLoc(), diag::error_module_alias_forbidden_name, value);
-        return false;
-      }
       if (!Lexer::isIdentifier(value)) {
         diags.diagnose(SourceLoc(), diag::error_bad_module_name, value, false);
         return false;

test/Frontend/module-alias-invalid-input.swift

@@ -5,6 +5,7 @@
 
 // RUN: not %target-swift-frontend -emit-silgen -parse-as-library %s -module-name foo -module-alias Swift=Bar 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS1 %s
 // INVALID_MODULE_ALIAS1: error: invalid module alias "Swift"; make sure the alias differs from the module name, module ABI name, module link name, and a standard library name
+// RUN: %target-swift-frontend -emit-silgen -parse-as-library %s -module-name foo -module-alias Bar=Swift -verify
 
 // RUN: not %target-swift-frontend -emit-silgen -parse-as-library %s -module-name foo -module-alias bar=bar 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS2 %s
 // INVALID_MODULE_ALIAS2: error: duplicate module alias; the name "bar" is already used for a module alias or an underlying name