[LLVMbugs] [Bug 14908] New: Optional diagnostic for template functions with unused template arguments?

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=14908

             Bug #: 14908
           Summary: Optional diagnostic for template functions with unused
                    template arguments?
           Product: clang
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: arthur.j.odwyer at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


I see how functions such as
    template <template<int> class T, int n1, template<int> class U, int n2>
    void foo(T<n1>, T<n2>);
could be useful in some cases... but for one particular application it would be
useful to me to have a compiler diagnostic that would statically diagnose
template functions with unused template arguments ("U" in the above example).

If I try to *call*

    foo(myT, myU);

I'll get a verbose diagnostic that includes the note

    test.cc:3:6: note: candidate template ignored: deduced conflicting
templates for parameter ''S1,3
    void foo(T<n1> tn1, T<n2> un2);
         ^
or

    test.cc:3:6: note: candidate template ignored: couldn't infer template
argument 'U'
    void foo(T<n1> tn1, T<n2> un2);
         ^
but it would be useful to me to have a compiler diagnostic that would
statically diagnose this kind of undeducible-argument mistake right away,
regardless of whether the problematic template is actually used in the program.

Again, I understand that such templates can theoretically be useful, so this
wouldn't be a warning by default, and maybe the whole idea of "optional"
warnings doesn't fit with Clang's philosophy. But I figured I'd file the
feature request anyway.

(The way to use "foo" in the example above is to explicitly specify the
template arguments:
    foo<T,42,U,27>(myT, myU);
)

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