VS: Allow toolset version selection to specify default toolset

Teach the feature added by commit v3.12.0-rc1~38^2 (VS: Add option to
select the version of the toolset used by VS 2017, 2018-05-19) to accept
the default toolset version in addition to older versions.  If the
default toolset version is supplied, simply clear it so the default will
be used.

Fixes: #18107

Source/cmGlobalVisualStudio10Generator.cxx

@@ -263,25 +263,32 @@ bool cmGlobalVisualStudio10Generator::SetGeneratorToolset(
       this->GeneratorToolsetVersion.clear();
     }
 
-    std::string const toolsetPath = this->GetAuxiliaryToolset();
-    if (!toolsetPath.empty() && !cmSystemTools::FileExists(toolsetPath)) {
-
-      std::ostringstream e;
-      /* clang-format off */
-      e <<
-        "Generator\n"
-        "  " << this->GetName() << "\n"
-        "given toolset and version specification\n"
-        "  " << this->GetPlatformToolsetString() << ",version=" <<
-        this->GeneratorToolsetVersion << "\n"
-        "does not seem to be installed at\n" <<
-        "  " << toolsetPath;
-      ;
-      /* clang-format on */
-      mf->IssueMessage(cmake::FATAL_ERROR, e.str());
-
-      // Clear the configured tool-set
+    bool const isDefaultToolset =
+      this->IsDefaultToolset(this->GeneratorToolsetVersion);
+    if (isDefaultToolset) {
+      // If the given version is the default toolset, remove the setting
       this->GeneratorToolsetVersion.clear();
+    } else {
+      std::string const toolsetPath = this->GetAuxiliaryToolset();
+      if (!toolsetPath.empty() && !cmSystemTools::FileExists(toolsetPath)) {
+
+        std::ostringstream e;
+        /* clang-format off */
+        e <<
+          "Generator\n"
+          "  " << this->GetName() << "\n"
+          "given toolset and version specification\n"
+          "  " << this->GetPlatformToolsetString() << ",version=" <<
+          this->GeneratorToolsetVersion << "\n"
+          "does not seem to be installed at\n" <<
+          "  " << toolsetPath;
+        ;
+        /* clang-format on */
+        mf->IssueMessage(cmake::FATAL_ERROR, e.str());
+
+        // Clear the configured tool-set
+        this->GeneratorToolsetVersion.clear();
+      }
     }
   }
 
@@ -615,6 +622,12 @@ cmGlobalVisualStudio10Generator::GetPlatformToolsetCudaString() const
   return this->GeneratorToolsetCuda;
 }
 
+bool cmGlobalVisualStudio10Generator::IsDefaultToolset(
+  const std::string&) const
+{
+  return true;
+}
+
 std::string cmGlobalVisualStudio10Generator::GetAuxiliaryToolset() const
 {
   return {};

Source/cmGlobalVisualStudio10Generator.h

@@ -106,6 +106,7 @@ public:
   std::string Encoding() override;
   virtual const char* GetToolsVersion() { return "4.0"; }
 
+  virtual bool IsDefaultToolset(const std::string& version) const;
   virtual std::string GetAuxiliaryToolset() const;
 
   bool FindMakeProgram(cmMakefile* mf) override;

Source/cmGlobalVisualStudio15Generator.cxx

@@ -158,6 +158,27 @@ bool cmGlobalVisualStudio15Generator::GetVSInstance(std::string& dir) const
   return vsSetupAPIHelper.GetVSInstanceInfo(dir);
 }
 
+bool cmGlobalVisualStudio15Generator::IsDefaultToolset(
+  const std::string& version) const
+{
+  if (version.empty()) {
+    return true;
+  }
+
+  std::string vcToolsetVersion;
+  if (this->vsSetupAPIHelper.GetVCToolsetVersion(vcToolsetVersion)) {
+
+    cmsys::RegularExpression regex("[0-9][0-9]\\.[0-9]+");
+    if (regex.find(version) && regex.find(vcToolsetVersion)) {
+      const auto majorMinorEnd = vcToolsetVersion.find('.', 3);
+      const auto majorMinor = vcToolsetVersion.substr(0, majorMinorEnd);
+      return version == majorMinor;
+    }
+  }
+
+  return false;
+}
+
 std::string cmGlobalVisualStudio15Generator::GetAuxiliaryToolset() const
 {
   const char* version = this->GetPlatformToolsetVersion();

Source/cmGlobalVisualStudio15Generator.h

@@ -32,6 +32,7 @@ public:
 
   bool GetVSInstance(std::string& dir) const;
 
+  bool IsDefaultToolset(const std::string& version) const override;
   std::string GetAuxiliaryToolset() const override;
 
 protected: