[flang] Avoid forming a reference from null pointer (#84787)

kparzysz · web-flow · commit 1def98d9f2eb · 2024-03-11T14:35:31.000-05:00
Doing so is an undefined behavior.

This was detected by the undefined behavior sanitizer.
diff --git a/flang/lib/Parser/token-sequence.cpp b/flang/lib/Parser/token-sequence.cpp
@@ -136,7 +136,10 @@ void TokenSequence::Put(
 }
 
 void TokenSequence::Put(const CharBlock &t, Provenance provenance) {
-  Put(&t[0], t.size(), provenance);
+  // Avoid t[0] if t is empty: it would create a reference to nullptr,
+  // which is UB.
+  const char *addr{t.size() ? &t[0] : nullptr};
+  Put(addr, t.size(), provenance);
 }
 
 void TokenSequence::Put(const std::string &s, Provenance provenance) {

Original file line number	Diff line number	Diff line change
`@@ -136,7 +136,10 @@ void TokenSequence::Put(`
`136`	`136`	`}`
`137`	`137`
`138`	`138`	`void TokenSequence::Put(const CharBlock &t, Provenance provenance) {`
`139`		`- Put(&t[0], t.size(), provenance);`
	`139`	`+ // Avoid t[0] if t is empty: it would create a reference to nullptr,`
	`140`	`+ // which is UB.`
	`141`	`+ const char *addr{t.size() ? &t[0] : nullptr};`
	`142`	`+ Put(addr, t.size(), provenance);`
`140`	`143`	`}`
`141`	`144`
`142`	`145`	`void TokenSequence::Put(const std::string &s, Provenance provenance) {`