[Macros] Don't assert when a macro parameter has a default argument

Author: DougGregor

lib/Parse/ParseDecl.cpp

@@ -9502,9 +9502,8 @@ ParserResult<MacroDecl> Parser::parseDeclMacro(DeclAttributes &attributes) {
   } else {
     // Parameter list.
     SmallVector<Identifier, 2> namePieces;
-    DefaultArgumentInfo defaultArgs;
     auto parameterResult = parseSingleParameterClause(
-        ParameterContextKind::Macro, &namePieces, &defaultArgs);
+        ParameterContextKind::Macro, &namePieces, nullptr);
     status |= parameterResult;
     parameterList = parameterResult.getPtrOrNull();
 

test/Macros/macros_diagnostics.swift

@@ -134,3 +134,8 @@ func testExternalMacroOutOfPlace() {
   let _: Int = #externalMacro(module: "A", type: "B")
   // expected-error@-1{{macro 'externalMacro' can only be used to define another macro}}
 }
+
+@freestanding(expression)
+public macro macroWithDefaults(_: Int = 17) = #externalMacro(module: "A", type: "B")
+// expected-error@-1{{default arguments are not allowed in macros}}
+// expected-warning@-2{{external macro implementation type 'A.B' could not be found for macro 'macroWithDefaults'}}