[ResultBuilders] Allow pre-check to look into single-statement closures

While determining whether result builder body is correct, allow
pre-check phase to look into single-statement closures because
their bodies participate in type-check and would fail constraint
generation if they contain `ErrorExpr`s, so it's better to determine
that the body is invalid early and skip result builder application.

Resolves: rdar://79746785

Author: xedin

lib/Sema/BuilderTransform.cpp

@@ -1869,12 +1869,8 @@ class PreCheckResultBuilderApplication : public ASTWalker {
           E, DC, /*replaceInvalidRefsWithErrors=*/true);
       HasError |= transaction.hasErrors();
 
-      if (!HasError) {
-        E->forEachChildExpr([&](Expr *expr) {
-          HasError |= isa<ErrorExpr>(expr);
-          return HasError ? nullptr : expr;
-        });
-      }
+      if (!HasError)
+        HasError |= containsErrorExpr(E);
 
       if (SuppressDiagnostics)
         transaction.abort();
@@ -1896,6 +1892,29 @@ class PreCheckResultBuilderApplication : public ASTWalker {
     return std::make_pair(true, S);
   }
 
+  /// Check whether given expression (including single-statement
+  /// closures) contains `ErrorExpr` as one of its sub-expressions.
+  bool containsErrorExpr(Expr *expr) {
+    bool hasError = false;
+
+    expr->forEachChildExpr([&](Expr *expr) -> Expr * {
+      hasError |= isa<ErrorExpr>(expr);
+      if (hasError)
+        return nullptr;
+
+      if (auto *closure = dyn_cast<ClosureExpr>(expr)) {
+        if (shouldTypeCheckInEnclosingExpression(closure)) {
+          hasError |= containsErrorExpr(closure->getSingleExpressionBody());
+          return hasError ? nullptr : expr;
+        }
+      }
+
+      return expr;
+    });
+
+    return hasError;
+  }
+
   /// Ignore patterns.
   std::pair<bool, Pattern*> walkToPatternPre(Pattern *pat) override {
     return { false, pat };

test/Constraints/result_builder_diags.swift

@@ -727,4 +727,13 @@ struct TuplifiedStructWithInvalidClosure {
       42
     }
   }
+
+  @TupleBuilder var nestedErrorsDiagnosedByParser: some Any {
+    tuplify(true) { _ in
+      tuplify { _ in
+        self. // expected-error {{expected member name following '.'}}
+      }
+      42
+    }
+  }
 }