cmGlobalGenerator: compute target features before synthetic targets

For `import std;` support, CMake needs to know the standard library
involved in order to ensure that the right target is linked. Afterwards,
the created synthetic targets need their target compile features
computed.

Source/cmGeneratorTarget.cxx

@@ -8414,6 +8414,8 @@ void ComputeLinkImplTransitive(cmGeneratorTarget const* self,
 bool cmGeneratorTarget::DiscoverSyntheticTargets(cmSyntheticTargetCache& cache,
                                                  std::string const& config)
 {
+  std::vector<std::string> allConfigs =
+    this->Makefile->GetGeneratorConfigs(cmMakefile::IncludeEmptyConfig);
   cmOptionalLinkImplementation impl;
   this->ComputeLinkImplementationLibraries(config, impl, this,
                                            LinkInterfaceFor::Link);
@@ -8488,9 +8490,19 @@ bool cmGeneratorTarget::DiscoverSyntheticTargets(cmSyntheticTargetCache& cache,
 
         // Create the generator target and attach it to the local generator.
         auto gtp = cm::make_unique<cmGeneratorTarget>(tgt, lg);
+
         synthDep = gtp.get();
         cache.CxxModuleTargets[targetName] = synthDep;
+
+        // See `localGen->ComputeTargetCompileFeatures()` call in
+        // `cmGlobalGenerator::Compute` for where non-synthetic targets resolve
+        // this.
+        for (auto const& innerConfig : allConfigs) {
+          gtp->ComputeCompileFeatures(innerConfig);
+        }
+
         gtp->DiscoverSyntheticTargets(cache, config);
+
         lg->AddGeneratorTarget(std::move(gtp));
       } else {
         synthDep = cached->second;

Source/cmGlobalGenerator.cxx

@@ -1581,6 +1581,19 @@ bool cmGlobalGenerator::Compute()
   }
 #endif
 
+  // Perform up-front computation in order to handle errors (such as unknown
+  // features) at this point. While processing the compile features we also
+  // calculate and cache the language standard required by the compile
+  // features.
+  //
+  // Synthetic targets performed this inside of
+  // `cmLocalGenerator::DiscoverSyntheticTargets`
+  for (const auto& localGen : this->LocalGenerators) {
+    if (!localGen->ComputeTargetCompileFeatures()) {
+      return false;
+    }
+  }
+
   // Iterate through all targets and set up C++20 module targets.
   // Create target templates for each imported target with C++20 modules.
   // INTERFACE library with BMI-generating rules and a collation step?
@@ -1588,6 +1601,9 @@ bool cmGlobalGenerator::Compute()
   // Make `add_dependencies(imported_target
   // $<$<TARGET_NAME_IF_EXISTS:uses_imported>:synth1>
   // $<$<TARGET_NAME_IF_EXISTS:other_uses_imported>:synth2>)`
+  //
+  // Note that synthetic target creation performs the above marked
+  // steps on the created targets.
   if (!this->DiscoverSyntheticTargets()) {
     return false;
   }
@@ -1597,16 +1613,6 @@ bool cmGlobalGenerator::Compute()
     localGen->AddHelperCommands();
   }
 
-  // Perform up-front computation in order to handle errors (such as unknown
-  // features) at this point. While processing the compile features we also
-  // calculate and cache the language standard required by the compile
-  // features.
-  for (const auto& localGen : this->LocalGenerators) {
-    if (!localGen->ComputeTargetCompileFeatures()) {
-      return false;
-    }
-  }
-
   // Add automatically generated sources (e.g. unity build).
   // Add unity sources after computing compile features.  Unity sources do
   // not change the set of languages or features, but we need to know them