file(GENERATE): Add policy CMP0070 to define relative path behavior

Previously `file(GENERATE)` did not define any behavior for relative
paths given to the `OUTPUT` or `INPUT` arguments.  Define behavior
consistent with CMake conventions and add a policy to provide
compatibility for projects that relied on the old accidental behavior.

Fixes: #16786

Source/cmGeneratorExpressionEvaluationFile.cxx

@@ -20,11 +20,13 @@
 cmGeneratorExpressionEvaluationFile::cmGeneratorExpressionEvaluationFile(
   const std::string& input,
   CM_AUTO_PTR<cmCompiledGeneratorExpression> outputFileExpr,
-  CM_AUTO_PTR<cmCompiledGeneratorExpression> condition, bool inputIsContent)
+  CM_AUTO_PTR<cmCompiledGeneratorExpression> condition, bool inputIsContent,
+  cmPolicies::PolicyStatus policyStatusCMP0070)
   : Input(input)
   , OutputFileExpr(outputFileExpr)
   , Condition(condition)
   , InputIsContent(inputIsContent)
+  , PolicyStatusCMP0070(policyStatusCMP0070)
 {
 }
 
@@ -58,6 +60,8 @@ void cmGeneratorExpressionEvaluationFile::Generate(
 
   if (cmSystemTools::FileIsFullPath(outputFileName)) {
     outputFileName = cmSystemTools::CollapseFullPath(outputFileName);
+  } else {
+    outputFileName = this->FixRelativePath(outputFileName, PathForOutput, lg);
   }
 
   std::map<std::string, std::string>::iterator it =
@@ -118,6 +122,8 @@ void cmGeneratorExpressionEvaluationFile::Generate(cmLocalGenerator* lg)
     std::string inputFileName = this->Input;
     if (cmSystemTools::FileIsFullPath(inputFileName)) {
       inputFileName = cmSystemTools::CollapseFullPath(inputFileName);
+    } else {
+      inputFileName = this->FixRelativePath(inputFileName, PathForInput, lg);
     }
     lg->GetMakefile()->AddCMakeDependFile(inputFileName);
     cmSystemTools::GetPermissions(inputFileName.c_str(), perm);
@@ -167,3 +173,57 @@ void cmGeneratorExpressionEvaluationFile::Generate(cmLocalGenerator* lg)
     }
   }
 }
+
+std::string cmGeneratorExpressionEvaluationFile::FixRelativePath(
+  std::string const& relativePath, PathRole role, cmLocalGenerator* lg)
+{
+  std::string resultPath;
+  switch (this->PolicyStatusCMP0070) {
+    case cmPolicies::WARN: {
+      std::string arg;
+      switch (role) {
+        case PathForInput:
+          arg = "INPUT";
+          break;
+        case PathForOutput:
+          arg = "OUTPUT";
+          break;
+      }
+      std::ostringstream w;
+      /* clang-format off */
+      w <<
+        cmPolicies::GetPolicyWarning(cmPolicies::CMP0070) << "\n"
+        "file(GENERATE) given relative " << arg << " path:\n"
+        "  " << relativePath << "\n"
+        "This is not defined behavior unless CMP0070 is set to NEW.  "
+        "For compatibility with older versions of CMake, the previous "
+        "undefined behavior will be used."
+        ;
+      /* clang-format on */
+      lg->IssueMessage(cmake::AUTHOR_WARNING, w.str());
+    }
+      CM_FALLTHROUGH;
+    case cmPolicies::OLD:
+      // OLD behavior is to use the relative path unchanged,
+      // which ends up being used relative to the working dir.
+      resultPath = relativePath;
+      break;
+    case cmPolicies::REQUIRED_IF_USED:
+    case cmPolicies::REQUIRED_ALWAYS:
+    case cmPolicies::NEW:
+      // NEW behavior is to interpret the relative path with respect
+      // to the current source or binary directory.
+      switch (role) {
+        case PathForInput:
+          resultPath = cmSystemTools::CollapseFullPath(
+            relativePath, lg->GetCurrentSourceDirectory());
+          break;
+        case PathForOutput:
+          resultPath = cmSystemTools::CollapseFullPath(
+            relativePath, lg->GetCurrentBinaryDirectory());
+          break;
+      }
+      break;
+  }
+  return resultPath;
+}

Source/cmGeneratorExpressionEvaluationFile.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "cmGeneratorExpression.h"
+#include "cmPolicies.h"
 #include "cm_auto_ptr.hxx"
 #include "cm_sys_stat.h"
 
@@ -21,7 +22,8 @@ public:
   cmGeneratorExpressionEvaluationFile(
     const std::string& input,
     CM_AUTO_PTR<cmCompiledGeneratorExpression> outputFileExpr,
-    CM_AUTO_PTR<cmCompiledGeneratorExpression> condition, bool inputIsContent);
+    CM_AUTO_PTR<cmCompiledGeneratorExpression> condition, bool inputIsContent,
+    cmPolicies::PolicyStatus policyStatusCMP0070);
 
   void Generate(cmLocalGenerator* lg);
 
@@ -35,12 +37,21 @@ private:
                 cmCompiledGeneratorExpression* inputExpression,
                 std::map<std::string, std::string>& outputFiles, mode_t perm);
 
+  enum PathRole
+  {
+    PathForInput,
+    PathForOutput
+  };
+  std::string FixRelativePath(std::string const& filePath, PathRole role,
+                              cmLocalGenerator* lg);
+
 private:
   const std::string Input;
   const CM_AUTO_PTR<cmCompiledGeneratorExpression> OutputFileExpr;
   const CM_AUTO_PTR<cmCompiledGeneratorExpression> Condition;
   std::vector<std::string> Files;
   const bool InputIsContent;
+  cmPolicies::PolicyStatus PolicyStatusCMP0070;
 };
 
 #endif

Source/cmMakefile.cxx

@@ -592,7 +592,8 @@ void cmMakefile::AddEvaluationFile(
   CM_AUTO_PTR<cmCompiledGeneratorExpression> condition, bool inputIsContent)
 {
   this->EvaluationFiles.push_back(new cmGeneratorExpressionEvaluationFile(
-    inputFile, outputName, condition, inputIsContent));
+    inputFile, outputName, condition, inputIsContent,
+    this->GetPolicyStatus(cmPolicies::CMP0070)));
 }
 
 std::vector<cmGeneratorExpressionEvaluationFile*>

Source/cmPolicies.h

@@ -206,6 +206,9 @@ class cmMakefile;
          cmPolicies::WARN)                                                    \
   SELECT(POLICY, CMP0069,                                                     \
          "INTERPROCEDURAL_OPTIMIZATION is enforced when enabled.", 3, 9, 0,   \
+         cmPolicies::WARN)                                                    \
+  SELECT(POLICY, CMP0070,                                                     \
+         "Define file(GENERATE) behavior for relative paths.", 3, 10, 0,      \
          cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)