PR5803: clang++: Treat untyped 'C' inputs as C++.

 - Patch by Andrzej K. Haczewski, with a tweak by me to emit a 'deprecated'
   diagnostic when we do this. We'll see what zee users say.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96511 91177308-0d34-0410-b5e6-96231b3b80d8

5 files changed, 40 insertions, 1 deletions

diff --git a/include/clang/Basic/DiagnosticDriverKinds.td b/include/clang/Basic/DiagnosticDriverKinds.td
index dc5ccfdf52..9175bef60f 100644
--- a/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/include/clang/Basic/DiagnosticDriverKinds.td
@@ -88,6 +88,8 @@ def warn_ignoring_ftabstop_value : Warning<
 def warn_drv_missing_resource_library : Warning<
   "missing resource library '%0', link may fail">;
 def warn_drv_conflicting_deployment_targets : Warning<
-    "conflicting deployment targets, both MACOSX_DEPLOYMENT_TARGET '%0' and IPHONEOS_DEPLOYMENT_TARGET '%1' are present in environment">;
+  "conflicting deployment targets, both MACOSX_DEPLOYMENT_TARGET '%0' and IPHONEOS_DEPLOYMENT_TARGET '%1' are present in environment">;
+def warn_drv_treating_input_as_cxx : Warning<
+  "treating '%0' input as '%1' when in C++ mode, this behavior is deprecated">;
 
 }
diff --git a/include/clang/Driver/Types.h b/include/clang/Driver/Types.h
index 3a343b385e..d93323016f 100644
--- a/include/clang/Driver/Types.h
+++ b/include/clang/Driver/Types.h
@@ -80,6 +80,10 @@ namespace types {
   /// getCompilationPhase - Return the \args N th compilation phase to
   /// be done for this type.
   phases::ID getCompilationPhase(ID Id, unsigned N);
+  
+  /// lookupCXXTypeForCType - Lookup CXX input type that corresponds to given
+  /// C type (used for clang++ emulation of g++ behaviour)
+  ID lookupCXXTypeForCType(ID Id);
 
 } // end namespace types
 } // end namespace driver
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index 15df767d97..e729f0fb54 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -558,6 +558,17 @@ void Driver::BuildActions(const ArgList &Args, ActionList &Actions) const {
 
           if (Ty == types::TY_INVALID)
             Ty = types::TY_Object;
+
+          // If the driver is invoked as C++ compiler (like clang++ or c++) it
+          // should autodetect some input files as C++ for g++ compatibility.
+          if (CCCIsCXX) {
+            types::ID OldTy = Ty;
+            Ty = types::lookupCXXTypeForCType(Ty);
+
+            if (Ty != OldTy)
+              Diag(clang::diag::warn_drv_treating_input_as_cxx)
+                << getTypeName(OldTy) << getTypeName(Ty);
+          }
         }
 
         // -ObjC and -ObjC++ override the default language, but only for "source
diff --git a/lib/Driver/Types.cpp b/lib/Driver/Types.cpp
index 60d86a62a3..8857fb16a3 100644
--- a/lib/Driver/Types.cpp
+++ b/lib/Driver/Types.cpp
@@ -213,3 +213,19 @@ phases::ID types::getCompilationPhase(ID Id, unsigned N) {
 
   return phases::Link;
 }
+
+ID types::lookupCXXTypeForCType(ID Id) {
+  switch (Id) {
+  default:
+    return Id;
+    
+  case types::TY_C:
+    return types::TY_CXX;
+  case types::TY_PP_C:
+    return types::TY_PP_CXX;
+  case types::TY_CHeader:
+    return types::TY_CXXHeader;
+  case types::TY_PP_CHeader:
+    return types::TY_PP_CXXHeader;
+  }
+}
diff --git a/test/Driver/clang-c-as-cxx.c b/test/Driver/clang-c-as-cxx.c
new file mode 100644
index 0000000000..0e2817888d
--- /dev/null
+++ b/test/Driver/clang-c-as-cxx.c
@@ -0,0 +1,6 @@
+// RUN: %clangxx -### %s 2>&1 | FileCheck %s
+//
+// PR5803
+//
+// CHECK: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated
+// CHECK: "-cc1" {{.*}} "-x" "c++"