[MLIR] Fix missing return statement warning in PatternMatch.h

[bondhugula]

Summary

Fix missing return statement warning in PatternMatch.h. NFC.

mlir/include/mlir/IR/PatternMatch.h:983:3: warning: no return statement in
function returning non-void [-Wreturn-type]

[rriddle]

Wouldn't have expected this given the static assert, but weird.

mlir/include/mlir/IR/PatternMatch.h

983

This technically returns a std::string, can we use {} or something instead?

[bondhugula]

Switch to {}.

[bondhugula]

In D123756#3450844, @rriddle wrote:

Wouldn't have expected this given the static assert, but weird.

I do get that warning with (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10).

mlir/include/mlir/IR/PatternMatch.h

 template <>
 struct ProcessPDLValue<std::string>
     : public ProcessPDLValueBasedOn<std::string, StringAttr> {
   template <typename T>
   static std::string processAsArg(T value) {
     static_assert(always_false<T>,
                   "`std::string` arguments require a string copy, use "
                   "`StringRef` for string-like arguments instead");
+    return {};
   }
   static void processAsResult(PatternRewriter &rewriter, PDLResultList &results,
                               StringRef value) {
     results.push_back(rewriter.getStringAttr(value));
   }
 };
 
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