Handle extracting case clause expression as constant

Author: amcasey

src/harness/unittests/extractConstants.ts

@@ -273,6 +273,13 @@ let i: I = [#|{ a: 1 }|];
 const myObj: { member(x: number, y: string): void } = {
     member: [#|(x, y) => x + y|],
 }
+`);
+
+        testExtractConstant("extractConstant_CaseClauseExpression", `
+switch (1) {
+    case [#|1|]:
+        break;
+}
 `);
     });
 

src/services/refactors/extractSymbol.ts

@@ -1321,6 +1321,13 @@ namespace ts.refactor.extractSymbol {
                     }
                     prevStatement = statement;
                 }
+
+                if (!prevStatement && isCaseClause(curr)) {
+                    // We must have been in the expression of the case clause.
+                    Debug.assert(isSwitchStatement(curr.parent.parent));
+                    return curr.parent.parent;
+                }
+
                 // There must be at least one statement since we started in one.
                 Debug.assert(prevStatement !== undefined);
                 return prevStatement;

tests/baselines/reference/extractConstant/extractConstant_CaseClauseExpression.js

@@ -0,0 +1,13 @@
+// ==ORIGINAL==
+
+switch (1) {
+    case /*[#|*/1/*|]*/:
+        break;
+}
+
+// ==SCOPE::Extract to constant in enclosing scope==
+const newLocal = 1;
+switch (1) {
+    case /*RENAME*/newLocal:
+        break;
+}

tests/baselines/reference/extractConstant/extractConstant_CaseClauseExpression.ts

@@ -0,0 +1,13 @@
+// ==ORIGINAL==
+
+switch (1) {
+    case /*[#|*/1/*|]*/:
+        break;
+}
+
+// ==SCOPE::Extract to constant in enclosing scope==
+const newLocal = 1;
+switch (1) {
+    case /*RENAME*/newLocal:
+        break;
+}