[Macros] Don't expand macros inside macro expession arguments

Macro expressions nested in macro arguments should be type checked, but
should not be expanded.

rdar://108622244

Author: rintaro

lib/Sema/CSApply.cpp

@@ -5389,7 +5389,15 @@ namespace {
       E->setMacroRef(macroRef);
       E->setType(expandedType);
 
-      if (!cs.Options.contains(ConstraintSystemFlags::DisableMacroExpansions)) {
+      // FIXME: Expansion should be lazy.
+      // i.e. 'ExpandMacroExpansionExprRequest' should be sinked into
+      // 'getRewritten()', and performed on-demand.
+      if (!cs.Options.contains(ConstraintSystemFlags::DisableMacroExpansions) &&
+          // Do not expand macros inside macro arguments. For example for
+          // '#stringify(#assert(foo))' when typechecking `#assert(foo)`,
+          // we don't want to expand it.
+          llvm::none_of(makeArrayRef(ExprStack).drop_back(1),
+                       [](Expr *E) { return isa<MacroExpansionExpr>(E); })) {
         (void)evaluateOrDefault(cs.getASTContext().evaluator,
                                 ExpandMacroExpansionExprRequest{E}, None);
       }

test/Macros/macro_expand.swift

@@ -179,6 +179,20 @@ func testDiscardableStringify(x: Int) {
 }
 #endif
 
+#if TEST_DIAGNOSTICS
+// This causes an error when non-'Bool' value is passed.
+@freestanding(expression) macro assertAny<T>(_ value: T) = #externalMacro(module: "MacroDefinition", type: "AssertMacro")
+
+func testNested() {
+  struct Nested { }
+  _ = #stringify(#assertAny(Nested()))
+  // expected-note@-1 2 {{in expansion of macro 'stringify' here}}
+// CHECK-DIAGS-NOT: error: cannot convert value of type 'Nested' to expected argument type 'Bool'
+// CHECK-DIAGS: @__swiftmacro_9MacroUser10testNestedyyF9stringifyfMf_9assertAnyfMf_.swift:1:8: error: cannot convert value of type 'Nested' to expected argument type 'Bool'
+// CHECK-DIAGS-NOT: error: cannot convert value of type 'Nested' to expected argument type 'Bool'
+}
+#endif
+
 func testStringifyWithThrows() throws {
   // Okay, we can put the try inside or outside
   _ = try #stringify(maybeThrowing())