Add exception to allow macro decls to have arbitrary specializations

Author: Greg Titus

lib/Sema/CSSimplify.cpp

@@ -14046,6 +14046,11 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
 
   auto genericParams = getGenericParams(decl);
   if (!decl->getAsGenericContext() || !genericParams) {
+    // Allow Macros to have specializations that we don't think they need.
+    if (isa<MacroDecl>(decl)) {
+      return SolutionKind::Solved;
+    }
+
     return recordFix(AllowConcreteTypeSpecialization::create(
                *this, type1, getConstraintLocator(locator)))
                ? SolutionKind::Error

test/Macros/attached_macros_diags.swift

@@ -69,4 +69,8 @@ struct TestMacroArgs {
   // expected-error@-1{{missing argument for parameter #1 in macro expansion}}
 
   @m3(message: #stringify(Nested.x).1) struct Args8 {}
+
+  // Allow macros to have arbitrary generic specialization lists
+  // https://github.com/swiftlang/swift/issues/75500
+  @m1<UInt> struct Args9 {}
 }