[BuilderTransform] Case: Save result of buildBlock into a variable before using in buildEither

Fixes implementation issue where build block is type-checked as part
of `buildEither(...)`. This is incorrect according to the original
proposal (SE-0289), `buildBlock` should be type-checked independently
and `buildEither` should have no effect on what overload of `buildBlock`
gets selected.

Author: xedin

lib/Sema/BuilderTransform.cpp

@@ -261,7 +261,8 @@ class ResultBuilderTransform
   }
 
   std::pair<NullablePtr<Expr>, std::optional<UnsupportedElt>>
-  transform(BraceStmt *braceStmt, SmallVectorImpl<ASTNode> &newBody) {
+  transform(BraceStmt *braceStmt, SmallVectorImpl<ASTNode> &newBody,
+            bool isolateBuildBlock = false) {
     SmallVector<Expr *, 4> buildBlockArguments;
 
     auto failTransform = [&](UnsupportedElt unsupported) {
@@ -326,6 +327,14 @@ class ResultBuilderTransform
       auto *buildBlock = builder.buildCall(
           braceStmt->getStartLoc(), ctx.Id_buildBlock, buildBlockArguments,
           /*argLabels=*/{});
+
+      if (isolateBuildBlock) {
+        auto *buildBlockVar = captureExpr(buildBlock, newBody);
+        return std::make_pair(
+            builder.buildVarRef(buildBlockVar, braceStmt->getStartLoc()),
+            std::nullopt);
+      }
+
       return std::make_pair(buildBlock, std::nullopt);
     }
   }
@@ -634,7 +643,8 @@ class ResultBuilderTransform
     std::optional<UnsupportedElt> unsupported;
     SmallVector<ASTNode, 4> newBody;
 
-    std::tie(caseVarRef, unsupported) = transform(body, newBody);
+    std::tie(caseVarRef, unsupported) =
+        transform(body, newBody, /*isolateBuildBlock=*/true);
 
     if (unsupported) {
       recordUnsupported(*unsupported);

test/Constraints/result_builder_switch_with_vars.swift

@@ -90,3 +90,33 @@ tuplify {
     value.a
   }
 }
+
+tuplify { _ in
+  switch true {
+  // CHECK: (case_stmt {{.*}}
+  // CHECK:   (pattern_named implicit type="Int" "$__builder2")
+  // CHECK:   (declref_expr implicit type="(TupleBuilder.Type) -> (Int) -> Int" location={{.*}} range={{.*}} decl="{{.*}}buildBlock@{{.*}}" function_ref=single apply)
+
+  // CHECK: (call_expr implicit type="Either<Int, Int>" {{.*}}
+  // CHECK-NEXT: (dot_syntax_call_expr implicit type="(Int) -> Either<Int, Int>" {{.*}}
+  // CHECK-NEXT: (declref_expr implicit type="(TupleBuilder.Type) -> (Int) -> Either<Int, Int>" location={{.*}} range={{.*}} decl="{{.*}}buildEither(first:)@{{.*}}" function_ref=single apply)
+  // CHECK:   (argument_list implicit labels="first:"
+  // CHECK-NEXT: (argument label="first"
+  // CHECK-NEXT:   (load_expr implicit type="Int" {{.*}}
+  // CHECK-NEXT:     (declref_expr implicit type="@lvalue Int" location={{.*}} range={{.*}} decl="{{.*}}.$__builder2@{{.*}}" function_ref=unapplied))))))))
+  case true: 0
+
+  // CHECK: (case_stmt {{.*}}
+  // CHECK:   (pattern_named implicit type="Int" "$__builder4")
+  // CHECK:   (declref_expr implicit type="(TupleBuilder.Type) -> (Int) -> Int" location={{.*}} range={{.*}} decl="{{.*}}buildBlock@{{.*}}" function_ref=single apply)
+
+  // CHECK: (call_expr implicit type="Either<Int, Int>" {{.*}}
+  // CHECK-NEXT: (dot_syntax_call_expr implicit type="(Int) -> Either<Int, Int>" {{.*}}
+  // CHECK-NEXT: (declref_expr implicit type="(TupleBuilder.Type) -> (Int) -> Either<Int, Int>" location={{.*}} range={{.*}} decl="{{.*}}buildEither(second:)@{{.*}}" function_ref=single apply)
+  // CHECK:   (argument_list implicit labels="second:"
+  // CHECK-NEXT: (argument label="second"
+  // CHECK-NEXT:   (load_expr implicit type="Int" {{.*}}
+  // CHECK-NEXT:     (declref_expr implicit type="@lvalue Int" location={{.*}} range={{.*}} decl="{{.*}}.$__builder4@{{.*}}" function_ref=unapplied))))))))
+  case false: 1
+  }
+}