GenEx: add bound-operand binding mechanism and $<_0>

Introduce "binding operations": generator expressions that evaluate a <body> once for each value they supply, with $<_0> expanding to that value. This is the foundation the $<LIST:TRANSFORM,...,APPLY> action and the predicate selectors build on, letting a <body> refer to the element being processed. Using $<_0> outside a binding operation is reported as an error rather than silently expanding to nothing. Issue: #27892
2026-09-25 04:09:36 +03:00 · 2026-06-20 12:32:30 -07:00
parent caa51f5689
commit c0a0b7fdd9
10 changed files with 97 additions and 0 deletions
@@ -1048,6 +1048,22 @@ List Ordering

    $<LIST:SORT,list,CASE:SENSITIVE,COMPARE:STRING,ORDER:DESCENDING>

+.. _GenEx Bound Operands:
+
+Bound Operands
+^^^^^^^^^^^^^^
+
+.. genex:: $<_0>
+
+  .. versionadded:: 4.5
+
+  ``$<_0>`` is the *bound operand* of the enclosing *binding operation*: a
+  generator expression that evaluates a ``<body>`` once for each value it
+  supplies, with ``$<_0>`` expanding to that value.
+
+  ``$<_0>`` is only valid inside the body of a binding operation.  Using it
+  anywhere else is an error.
+
 Path Expressions
 ----------------

@@ -3,6 +3,7 @@
 #pragma once

 #include <string>
+#include <utility>

 #include <cm/optional>

@@ -25,9 +26,26 @@ struct Context final
  void SetCMP0189(cmPolicies::PolicyStatus cmp0189);
  cmPolicies::PolicyStatus GetCMP0189() const;

+  void SetBoundOperand(std::string value);
+  bool HasBoundOperand() const;
+  std::string const& GetBoundOperand() const;
+
 private:
  cm::optional<cmPolicies::PolicyStatus> CMP0189;
+  cm::optional<std::string> BoundOperand;
 };

+inline void Context::SetBoundOperand(std::string value)
+{
+  this->BoundOperand = std::move(value);
+}
+inline bool Context::HasBoundOperand() const
+{
+  return this->BoundOperand.has_value();
+}
+inline std::string const& Context::GetBoundOperand() const
+{
+  return *this->BoundOperand;
+}
 }
 }
@@ -97,6 +97,12 @@ struct GeneratorExpressionContent : public cmGeneratorExpressionEvaluator

  std::string GetOriginalExpression() const;

+  std::vector<cmGeneratorExpressionEvaluatorVector> const& GetParamChildren()
+    const
+  {
+    return this->ParamChildren;
+  }
+
  ~GeneratorExpressionContent() override;

 private:
@@ -141,6 +141,27 @@ static const struct OneNode : public cmGeneratorExpressionNode
  }
 } oneNode;

+static const struct BoundOperandNode : public cmGeneratorExpressionNode
+{
+  BoundOperandNode() {} // NOLINT(modernize-use-equals-default)
+
+  int NumExpectedParameters() const override { return 0; }
+
+  std::string Evaluate(
+    std::vector<std::string> const& /*parameters*/,
+    cm::GenEx::Evaluation* eval, GeneratorExpressionContent const* content,
+    cmGeneratorExpressionDAGChecker* /*dagChecker*/) const override
+  {
+    if (!eval->Context.HasBoundOperand()) {
+      reportError(eval, content->GetOriginalExpression(),
+                  "$<_0> may only be used inside the body of a binding "
+                  "operation.");
+      return std::string();
+    }
+    return eval->Context.GetBoundOperand();
+  }
+} boundOperandNode;
+
 static const struct OneNode buildInterfaceNode;

 static const struct ZeroNode installInterfaceNode;
@@ -5932,6 +5953,7 @@ cmGeneratorExpressionNode const* cmGeneratorExpressionNode::GetNode(
    { "PATH_EQUAL", &pathEqualNode },
    { "MAKE_C_IDENTIFIER", &makeCIdentifierNode },
    { "BOOL", &boolNode },
+    { "_0", &boundOperandNode },
    { "IF", &ifNode },
    { "ANGLE-R", &angle_rNode },
    { "COMMA", &commaNode },
@@ -14,6 +14,7 @@ set(CMakeLib_TESTS
  testDocumentationFormatter.cxx
  testGccDepfileReader.cxx
  testGeneratedFileStream.cxx
+  testGenExBoundOperand.cxx
  testJSONHelpers.cxx
  testRST.cxx
  testRange.cxx
@@ -0,0 +1,30 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file LICENSE.rst or https://cmake.org/licensing for details.  */
+#include <iostream>
+#include <string>
+
+#include "cmGenExContext.h"
+
+static bool testContextBinding()
+{
+  cm::GenEx::Context ctx(nullptr, "Debug");
+  bool ok = true;
+  if (ctx.HasBoundOperand()) {
+    std::cerr << "binding should start unset\n";
+    ok = false;
+  }
+  ctx.SetBoundOperand("net");
+  if (!ctx.HasBoundOperand() || ctx.GetBoundOperand() != "net") {
+    std::cerr << "binding did not round-trip\n";
+    ok = false;
+  }
+  return ok;
+}
+
+int testGenExBoundOperand(int /*argc*/, char* /*argv*/[])
+{
+  if (!testContextBinding()) {
+    return 1;
+  }
+  return 0;
+}
@@ -0,0 +1 @@
+1
@@ -0,0 +1 @@
+\$<_0> may only be used inside the body of a binding operation
@@ -0,0 +1 @@
+file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/bad.txt" CONTENT "$<_0>")
@@ -57,6 +57,7 @@ run_cmake(FILTER-InvalidOperator)
 run_cmake(FILTER-Exclude)
 run_cmake(FILTER-Include)
 run_cmake(LIST-edgecases)
+run_cmake(BoundOperandOutsideBinding)

 function(run_cmake_build test)
  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/${test}-build)
				`@@ -0,0 +1 @@`
				`\$<_0> may only be used inside the body of a binding operation`
				`@@ -0,0 +1 @@`
				`file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/bad.txt" CONTENT "$<_0>")`