[Macros] Disallow expression macro as default argument

include/swift/AST/DiagnosticsSema.def

@@ -7493,6 +7493,9 @@ ERROR(extension_macro_invalid_conformance,none,
 ERROR(macro_attached_to_invalid_decl,none,
       "'%0' macro cannot be attached to %1 (%base2)",
       (StringRef, DescriptiveDeclKind, const Decl *))
+ERROR(macro_as_default_argument, none,
+      "non-built-in macro cannot be used as default argument",
+      ())
 ERROR(conformance_macro,none,
       "conformance macros are replaced by extension macros",
       ())

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1108,6 +1108,12 @@ Expr *DefaultArgumentExprRequest::evaluate(Evaluator &evaluator,
   auto *initExpr = param->getStructuralDefaultExpr();
   assert(initExpr);
 
+  // Prohibit default argument that is a non-built-in macro to avoid confusion.
+  if (isa<MacroExpansionExpr>(initExpr)) {
+    ctx.Diags.diagnose(initExpr->getLoc(), diag::macro_as_default_argument);
+    return new (ctx) ErrorExpr(initExpr->getSourceRange(), ErrorType::get(ctx));
+  }
+
   // If the param has an error type, there's no point type checking the default
   // expression, unless we are type checking for code completion, in which case
   // the default expression might contain the code completion token.

test/Macros/macro_default_argument.swift

@@ -0,0 +1,32 @@
+// REQUIRES: swift_swift_parser
+
+// RUN: %empty-directory(%t)
+// RUN: %host-build-swift -swift-version 5 -emit-library -o %t/%target-library-name(MacroDefinition) -module-name=MacroDefinition %S/Inputs/syntax_macro_definitions.swift -g -no-toolchain-stdlib-rpath
+
+// RUN: %target-typecheck-verify-swift -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) %s
+
+@freestanding(expression)
+macro MagicLine() -> Int = #externalMacro(module: "MacroDefinition", type: "MagicLineMacro")
+
+struct LineContainer {
+    let line: Int
+}
+
+func partOfDefaultArgumentOkay(container: LineContainer = .init(line: #MagicLine)) {
+    print(container.line)
+}
+
+func parenthesizedExpansionAtDeclOkay(line: Int = (#MagicLine)) {
+    print(line)
+}
+
+func builtInOkay(line: Int = #line) {
+    print(line)
+}
+
+// expected-error@+1{{non-built-in macro cannot be used as default argument}}
+func asDefaultArgument(line: Int = #MagicLine) {
+    print(line)
+}
+
+asDefaultArgument()

test/ModuleInterface/unbuildable.swift

@@ -22,7 +22,7 @@
 // RUN: not %target-swift-frontend -typecheck-module-from-interface %t/UnbuildableCurrent.swiftinterface 2>&1 | %FileCheck -check-prefixes=ALL,CURRENT-VERIFY %s
 // RUN: not %target-swift-frontend -typecheck-module-from-interface %t/UnbuildableFuture.swiftinterface 2>&1 | %FileCheck -check-prefixes=ALL,FUTURE-VERIFY %s
 
-// ALL: Unbuildable{{[^.]+}}.swiftinterface:{{[0-9]+}}:{{[0-9]+}}: error: no macro named 'somethingYouveNeverHeardOf'
+// ALL: Unbuildable{{[^.]+}}.swiftinterface:{{[0-9]+}}:{{[0-9]+}}: error: non-built-in macro cannot be used as default argument
 
 #if CURRENT
 import UnbuildableCurrent