Detect usage of block-scoped variables from earlier case blocks

src/compiler/checker.ts

@@ -25248,6 +25248,28 @@ namespace ts {
             if (isCaptured) {
                 getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.CapturedBlockScopedBinding;
             }
+
+            // Check for usage of a variable from a case clause prior to the referencing one
+            // switch(n) {
+            //     case 0:
+            //         let x = 0;
+            //     case 1:
+            //         x; // <- bad
+            //         if (true) x; // <- bad (note: need to walk to parent)
+            // }
+            if (container.kind === SyntaxKind.CaseBlock) {
+                let usageCaseClause = getAncestor(node, SyntaxKind.CaseClause);
+                if (usageCaseClause) {
+                    // Walk up until we're at the same level as the declaring block
+                    while (usageCaseClause.parent !== container) {
+                        usageCaseClause = usageCaseClause.parent;
+                    }
+
+                    if (usageCaseClause.pos > symbol.valueDeclaration.pos) {
+                        error(node, Diagnostics.Variable_0_is_declared_in_a_prior_case_block, symbolToString(symbol));
+                    }
+                }
+            }
         }
 
         function isBindingCapturedByNode(node: Node, decl: VariableDeclaration | BindingElement) {
@@ -26107,6 +26129,7 @@ namespace ts {
         // Don't do this for assignment declarations unless there is a type tag on the assignment, to avoid circularity from checking the right operand.
         function getContextualTypeForAssignmentDeclaration(binaryExpression: BinaryExpression): Type | undefined {
             const kind = getAssignmentDeclarationKind(binaryExpression);
+            let valueDeclaration;
             switch (kind) {
                 case AssignmentDeclarationKind.None:
                 case AssignmentDeclarationKind.ThisProperty:
@@ -26161,7 +26184,7 @@ namespace ts {
                 case AssignmentDeclarationKind.ExportsProperty:
                 case AssignmentDeclarationKind.Prototype:
                 case AssignmentDeclarationKind.PrototypeProperty:
-                    let valueDeclaration = binaryExpression.left.symbol?.valueDeclaration;
+                    valueDeclaration = binaryExpression.left.symbol?.valueDeclaration;
                     // falls through
                 case AssignmentDeclarationKind.ModuleExports:
                     valueDeclaration ||= binaryExpression.symbol?.valueDeclaration;

src/compiler/diagnosticMessages.json

@@ -3361,6 +3361,11 @@
         "category": "Error",
         "code": 2836
     },
+    "Variable '{0}' is declared in a prior case block.": {
+        "category": "Error",
+        "code": 2837
+    },
+
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/switchCaseTdz.errors.txt

@@ -0,0 +1,135 @@
+tests/cases/compiler/switchCaseTdz.ts(3,9): error TS2448: Block-scoped variable 'x' used before its declaration.
+tests/cases/compiler/switchCaseTdz.ts(4,9): error TS2448: Block-scoped variable 'y' used before its declaration.
+tests/cases/compiler/switchCaseTdz.ts(5,9): error TS2448: Block-scoped variable 'z' used before its declaration.
+tests/cases/compiler/switchCaseTdz.ts(19,9): error TS2837: Variable 'x' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(20,9): error TS2837: Variable 'y' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(21,9): error TS2837: Variable 'z' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(24,13): error TS2837: Variable 'x' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(25,13): error TS2837: Variable 'y' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(26,13): error TS2837: Variable 'z' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(30,13): error TS2837: Variable 'x' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(31,13): error TS2837: Variable 'y' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(32,13): error TS2837: Variable 'z' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(39,9): error TS2837: Variable 'x' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(40,9): error TS2837: Variable 'y' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(41,9): error TS2837: Variable 'z' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(44,13): error TS2837: Variable 'x' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(45,13): error TS2837: Variable 'y' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(46,13): error TS2837: Variable 'z' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(50,13): error TS2837: Variable 'x' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(51,13): error TS2837: Variable 'y' is declared in a prior case block.
+tests/cases/compiler/switchCaseTdz.ts(52,13): error TS2837: Variable 'z' is declared in a prior case block.
+
+
+==== tests/cases/compiler/switchCaseTdz.ts (21 errors) ====
+    switch (1 + 1) {
+        case -1:
+            x;
+            ~
+!!! error TS2448: Block-scoped variable 'x' used before its declaration.
+!!! related TS2728 tests/cases/compiler/switchCaseTdz.ts:9:13: 'x' is declared here.
+            y;
+            ~
+!!! error TS2448: Block-scoped variable 'y' used before its declaration.
+!!! related TS2728 tests/cases/compiler/switchCaseTdz.ts:10:15: 'y' is declared here.
+            z;
+            ~
+!!! error TS2448: Block-scoped variable 'z' used before its declaration.
+!!! related TS2728 tests/cases/compiler/switchCaseTdz.ts:11:16: 'z' is declared here.
+
+        case 0:
+            var ok = 0;
+            let x = 0;
+            const y = 0;
+            const [z] = [0];
+
+            ok;
+            x;
+            y;
+            z;
+
+        case 1:
+            x = 0; // <- bad
+            ~
+!!! error TS2837: Variable 'x' is declared in a prior case block.
+            y; // <- bad
+            ~
+!!! error TS2837: Variable 'y' is declared in a prior case block.
+            z; // <- bad
+            ~
+!!! error TS2837: Variable 'z' is declared in a prior case block.
+            ok;
+            if (true) {
+                x = 0; // <- bad
+                ~
+!!! error TS2837: Variable 'x' is declared in a prior case block.
+                y; // <- bad
+                ~
+!!! error TS2837: Variable 'y' is declared in a prior case block.
+                z; // <- bad
+                ~
+!!! error TS2837: Variable 'z' is declared in a prior case block.
+                ok;
+            }
+            let f1 = function () {
+                x = 0; // <- bad
+                ~
+!!! error TS2837: Variable 'x' is declared in a prior case block.
+                y; // <- bad
+                ~
+!!! error TS2837: Variable 'y' is declared in a prior case block.
+                z; // <- bad
+                ~
+!!! error TS2837: Variable 'z' is declared in a prior case block.
+                ok;
+            }
+            break;
+
+        case 2:
+        case 3:
+            x; // <- bad
+            ~
+!!! error TS2837: Variable 'x' is declared in a prior case block.
+            y; // <- bad
+            ~
+!!! error TS2837: Variable 'y' is declared in a prior case block.
+            z; // <- bad
+            ~
+!!! error TS2837: Variable 'z' is declared in a prior case block.
+            ok;
+            if (true) {
+                x; // <- bad
+                ~
+!!! error TS2837: Variable 'x' is declared in a prior case block.
+                y; // <- bad
+                ~
+!!! error TS2837: Variable 'y' is declared in a prior case block.
+                z; // <- bad
+                ~
+!!! error TS2837: Variable 'z' is declared in a prior case block.
+                ok;
+            }
+            let f2 = function () {
+                x; // <- bad
+                ~
+!!! error TS2837: Variable 'x' is declared in a prior case block.
+                y; // <- bad
+                ~
+!!! error TS2837: Variable 'y' is declared in a prior case block.
+                z; // <- bad
+                ~
+!!! error TS2837: Variable 'z' is declared in a prior case block.
+                ok;
+            }
+
+    }
+
+    switch(2 + 2) {
+        case 0:
+            let x = 1;
+            switch(x + x) {
+                case 2:
+                    // Legal
+                    x;
+            }
+    }

tests/baselines/reference/switchCaseTdz.js

@@ -0,0 +1,129 @@
+//// [switchCaseTdz.ts]
+switch (1 + 1) {
+    case -1:
+        x;
+        y;
+        z;
+
+    case 0:
+        var ok = 0;
+        let x = 0;
+        const y = 0;
+        const [z] = [0];
+
+        ok;
+        x;
+        y;
+        z;
+
+    case 1:
+        x = 0; // <- bad
+        y; // <- bad
+        z; // <- bad
+        ok;
+        if (true) {
+            x = 0; // <- bad
+            y; // <- bad
+            z; // <- bad
+            ok;
+        }
+        let f1 = function () {
+            x = 0; // <- bad
+            y; // <- bad
+            z; // <- bad
+            ok;
+        }
+        break;
+
+    case 2:
+    case 3:
+        x; // <- bad
+        y; // <- bad
+        z; // <- bad
+        ok;
+        if (true) {
+            x; // <- bad
+            y; // <- bad
+            z; // <- bad
+            ok;
+        }
+        let f2 = function () {
+            x; // <- bad
+            y; // <- bad
+            z; // <- bad
+            ok;
+        }
+
+}
+
+switch(2 + 2) {
+    case 0:
+        let x = 1;
+        switch(x + x) {
+            case 2:
+                // Legal
+                x;
+        }
+}
+
+//// [switchCaseTdz.js]
+switch (1 + 1) {
+    case -1:
+        x_1;
+        y_1;
+        z_1;
+    case 0:
+        var ok = 0;
+        var x_1 = 0;
+        var y_1 = 0;
+        var z_1 = [0][0];
+        ok;
+        x_1;
+        y_1;
+        z_1;
+    case 1:
+        x_1 = 0; // <- bad
+        y_1; // <- bad
+        z_1; // <- bad
+        ok;
+        if (true) {
+            x_1 = 0; // <- bad
+            y_1; // <- bad
+            z_1; // <- bad
+            ok;
+        }
+        var f1 = function () {
+            x_1 = 0; // <- bad
+            y_1; // <- bad
+            z_1; // <- bad
+            ok;
+        };
+        break;
+    case 2:
+    case 3:
+        x_1; // <- bad
+        y_1; // <- bad
+        z_1; // <- bad
+        ok;
+        if (true) {
+            x_1; // <- bad
+            y_1; // <- bad
+            z_1; // <- bad
+            ok;
+        }
+        var f2 = function () {
+            x_1; // <- bad
+            y_1; // <- bad
+            z_1; // <- bad
+            ok;
+        };
+}
+switch (2 + 2) {
+    case 0:
+        var x = 1;
+        switch (x + x) {
+            case 2:
+                // Legal
+                x;
+        }
+}