Merge pull request #14324 from mihailik/master

Special-case (0,eval) for side-effect-free 0 left of comma

Author: mhegazy

Diff for: src/compiler/checker.ts

@@ -15929,12 +15929,16 @@ namespace ts {
                     checkAssignmentOperator(rightType);
                     return getRegularTypeOfObjectLiteral(rightType);
                 case SyntaxKind.CommaToken:
-                    if (!compilerOptions.allowUnreachableCode && isSideEffectFree(left)) {
+                    if (!compilerOptions.allowUnreachableCode && isSideEffectFree(left) && !isEvalNode(right)) {
                         error(left, Diagnostics.Left_side_of_comma_operator_is_unused_and_has_no_side_effects);
                     }
                     return rightType;
             }
 
+            function isEvalNode(node: Expression) {
+                return node.kind === SyntaxKind.Identifier && (node as Identifier).text === "eval";
+            }
+
             // Return true if there was no error, false if there was an error.
             function checkForDisallowedESSymbolOperand(operator: SyntaxKind): boolean {
                 const offendingSymbolOperand =

Diff for: tests/baselines/reference/evalAfter0.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/compiler/evalAfter0.ts(4,2): error TS2695: Left side of comma operator is unused and has no side effects.
+
+
+==== tests/cases/compiler/evalAfter0.ts (1 errors) ====
+    (0,eval)("10"); // fine: special case for eval
+    
+    declare var eva;
+    (0,eva)("10"); // error: no side effect left of comma (suspect of missing method name or something)
+     ~
+!!! error TS2695: Left side of comma operator is unused and has no side effects.

Diff for: tests/baselines/reference/evalAfter0.js

@@ -0,0 +1,9 @@
+//// [evalAfter0.ts]
+(0,eval)("10"); // fine: special case for eval
+
+declare var eva;
+(0,eva)("10"); // error: no side effect left of comma (suspect of missing method name or something)
+
+//// [evalAfter0.js]
+(0, eval)("10"); // fine: special case for eval
+(0, eva)("10"); // error: no side effect left of comma (suspect of missing method name or something)

Diff for: tests/cases/compiler/evalAfter0.ts

@@ -0,0 +1,4 @@
+(0,eval)("10"); // fine: special case for eval
+
+declare var eva;
+(0,eva)("10"); // error: no side effect left of comma (suspect of missing method name or something)