[LLVMbugs] [Bug 20350] New: wrong initialization after template instantiation for aggregate initialization that implicitly default-constructs

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20350

            Bug ID: 20350
           Summary: wrong initialization after template instantiation for
                    aggregate initialization that implicitly
                    default-constructs
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: richard-llvm at metafoo.co.uk
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Clang rejects-valid on this:

namespace NoExtraCopy {
  struct S { S(); S(const S&) = delete; };
  struct T { S s; };
  template<typename> void f() {
    T t = {};
  }
  template void f<int>();
}

... because the template instantiation of the initializer initializes the S
subobject from the existing CXXConstructExpr, resulting in an unnecessary move
construction.

We should either make TreeTransform::TransformInitializer recurse through
InitListExprs (and throw away things that were implied), or (better) not redo
initialization sequence formation during template instantiation if we succeeded
in building the initialization sequence in the initial parse.

(In the latter case, we'd probably benefit from an AST node representing a
dependent initialization sequence, so we can get TreeTransform to rerun the
initialization step where needed.)

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