Only instantiate a default argument once.

By storing the instantiated expression back in the ParmVarDecl,
we remove the last need for separately storing the sub-expression
of a CXXDefaultArgExpr.  This makes PCH/Modules merging quite
simple: CXXDefaultArgExpr records are serialized as references
to the ParmVarDecl, and we ignore redundant attempts to overwrite
the instantiated expression.

This has some extremely marginal impact on user-facing semantics.
However, the major effect is that it avoids IRGen errors about
conflicting definitions due to lambdas in the argument being
instantiated multiple times while sharing the same mangling.
It should also slightly improve memory usage and module file size.

rdar://23810407

llvm-svn: 256983

1 files changed, 19 insertions, 0 deletions

diff --git a/clang/test/SemaTemplate/default-arguments-cxx0x.cpp b/clang/test/SemaTemplate/default-arguments-cxx0x.cpp
index 4cfd7a5843f..0c97c2056b7 100644
--- a/clang/test/SemaTemplate/default-arguments-cxx0x.cpp
+++ b/clang/test/SemaTemplate/default-arguments-cxx0x.cpp
@@ -56,3 +56,22 @@ namespace PR16975 {
 
   baz data{0};
 }
+
+// rdar://23810407
+// An IRGen failure due to a symbol collision due to a default argument
+// being instantiated twice.  Credit goes to Richard Smith for this
+// reduction to a -fsyntax-only failure.
+namespace rdar23810407 {
+  // Instantiating the default argument multiple times will produce two
+  // different lambda types and thus instantiate this function multiple
+  // times, which will produce conflicting extern variable declarations.
+  template<typename T> int f(T t) {
+    extern T rdar23810407_variable;
+    return 0;
+  }
+  template<typename T> int g(int a = f([] {}));
+  void test() {
+    g<int>();
+    g<int>();
+  }
+}