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`clang-tidy -std=c++20` with `modernize-use-nullptr` mistakenly inserts
`nullptr` in place of the comparison operator if the comparison internally
expands in the AST to a rewritten spaceship operator. This can be reproduced by
running the new `modernize-use-nullptr-cxx20.cpp` test without applying the
supplied patch to `UseNullptrCheck.cpp`; the current clang-tidy will mistakenly
replace:
result = (a1 < a2);
with
result = (a1 nullptr a2);
Oops!
The supplied patch fixes this by adding just one comment and one line of code
to `UseNullptrCheck.cpp`:
// Skip implicit casts to 0 generated by operator<=> rewrites.
unless(hasAncestor(cxxRewrittenBinaryOperator())),
I looked for a `hasGrandparent` matcher which would be more precise, but none
exists. This fix slightly overly-aggressively eliminates matches, so even with
this patch `modernize-use-nullptr` will not convert
result = (a1 > (ptr == 0 ? a1 : a2));
to
result = (a1 > (ptr == nullptr ? a1 : a2));
because `ptr == 0` has an ancestor that is a rewritten spaceship operator.
Changing the proposed one-line fix to:
unless(hasGrandparent(cxxRewrittenBinaryOperator())),
resolves this, but requires another 57 lines of code across AstMatchers.h,
AstMatchersInternal.h, and AstMatchFinder.cpp to create `hasGrandparent`. I'd
be happy to supply that patch instead if folks think it worthwhile? Maybe
others in the future can benefit from `hasGrandparent`?
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D95714
Files:
clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
clang-tools-extra/test/clang-tidy/checkers/modernize-use-nullptr-cxx20.cpp
Index:
clang-tools-extra/test/clang-tidy/checkers/modernize-use-nullptr-cxx20.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/test/clang-tidy/checkers/modernize-use-nullptr-cxx20.cpp
@@ -0,0 +1,19 @@
+// RUN: %check_clang_tidy -std=c++20 %s modernize-use-nullptr %t
+
+#include <compare>
+
+class A {
+public:
+ auto operator<=>(const A &other) const = default;
+};
+
+void test_cxx_rewritten_binary_ops() {
+ A a1, a2;
+ bool result;
+ // should not change next line to (a1 nullptr a2)
+ result = (a1 < a2);
+ // CHECK-FIXES: result = (a1 < a2);
+ // should not change next line to (a1 nullptr a2)
+ result = (a1 >= a2);
+ // CHECK-FIXES: result = (a1 >= a2);
+}
Index: clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
+++ clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
@@ -45,6 +45,8 @@
TK_AsIs,
castExpr(anyOf(ImplicitCastToNull,
explicitCastExpr(hasDescendant(ImplicitCastToNull))),
+ // Skip implicit casts to 0 generated by operator<=> rewrites.
+ unless(hasAncestor(cxxRewrittenBinaryOperator())),
unless(hasAncestor(explicitCastExpr())))
.bind(CastSequence));
}
Index: clang-tools-extra/test/clang-tidy/checkers/modernize-use-nullptr-cxx20.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/test/clang-tidy/checkers/modernize-use-nullptr-cxx20.cpp
@@ -0,0 +1,19 @@
+// RUN: %check_clang_tidy -std=c++20 %s modernize-use-nullptr %t
+
+#include <compare>
+
+class A {
+public:
+ auto operator<=>(const A &other) const = default;
+};
+
+void test_cxx_rewritten_binary_ops() {
+ A a1, a2;
+ bool result;
+ // should not change next line to (a1 nullptr a2)
+ result = (a1 < a2);
+ // CHECK-FIXES: result = (a1 < a2);
+ // should not change next line to (a1 nullptr a2)
+ result = (a1 >= a2);
+ // CHECK-FIXES: result = (a1 >= a2);
+}
Index: clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
+++ clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
@@ -45,6 +45,8 @@
TK_AsIs,
castExpr(anyOf(ImplicitCastToNull,
explicitCastExpr(hasDescendant(ImplicitCastToNull))),
+ // Skip implicit casts to 0 generated by operator<=> rewrites.
+ unless(hasAncestor(cxxRewrittenBinaryOperator())),
unless(hasAncestor(explicitCastExpr())))
.bind(CastSequence));
}
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