[ADT] Handle and document multiple matches in StringSwitch (llvm#166177)

Specify that the first match is returned and bail out early when
processing multiple case values.

Author: kuhar

llvm/include/llvm/ADT/StringSwitch.h

@@ -41,6 +41,8 @@ namespace llvm {
 ///   .Cases({"violet", "purple"}, Violet)
 ///   .Default(UnknownColor);
 /// \endcode
+///
+/// When multiple matches are found, the value of the first match is returned.
 template<typename T, typename R = T>
 class StringSwitch {
   /// The string we are matching.
@@ -213,23 +215,30 @@ class StringSwitch {
   [[nodiscard]] operator R() { return DefaultUnreachable(); }
 
 private:
-  // Returns true when `Str` matches the `S` argument, and stores the result.
+  // Returns true when a match is found. If `Str` matches the `S` argument,
+  // stores the result.
   bool CaseImpl(StringLiteral S, T &Value) {
-    if (!Result && Str == S) {
-      Result = std::move(Value);
+    if (Result)
       return true;
-    }
-    return false;
+
+    if (Str != S)
+      return false;
+
+    Result = std::move(Value);
+    return true;
   }
 
-  // Returns true when `Str` matches the `S` argument (case-insensitive), and
-  // stores the result.
+  // Returns true when a match is found. If `Str` matches the `S` argument
+  // (case-insensitive), stores the result.
   bool CaseLowerImpl(StringLiteral S, T &Value) {
-    if (!Result && Str.equals_insensitive(S)) {
-      Result = std::move(Value);
+    if (Result)
       return true;
-    }
-    return false;
+
+    if (!Str.equals_insensitive(S))
+      return false;
+
+    Result = std::move(Value);
+    return true;
   }
 
   StringSwitch &CasesImpl(std::initializer_list<StringLiteral> Cases,

llvm/unittests/ADT/StringSwitchTest.cpp

@@ -240,6 +240,23 @@ TEST(StringSwitchTest, CasesCopies) {
   EXPECT_EQ(NumCopies, 1u);
 }
 
+TEST(StringSwitchTest, StringSwitchMultipleMatches) {
+  auto Translate = [](StringRef S) {
+    return llvm::StringSwitch<int>(S)
+        .CaseLower("A", 0)
+        .Case("b", 1)
+        .Case("a", 2)
+        .CasesLower({"a", "b"}, 3)
+        .DefaultUnreachable();
+  };
+
+  // Check that the value of the first match is returned.
+  EXPECT_EQ(0, Translate("A"));
+  EXPECT_EQ(0, Translate("a"));
+  EXPECT_EQ(3, Translate("B"));
+  EXPECT_EQ(1, Translate("b"));
+}
+
 TEST(StringSwitchTest, DefaultUnreachable) {
   auto Translate = [](StringRef S) {
     return llvm::StringSwitch<int>(S)