Check for omitted expressions when checking const and let declaration names

src/compiler/checker.ts

@@ -12596,7 +12596,9 @@ module ts {
             else {
                 let elements = (<BindingPattern>name).elements;
                 for (let element of elements) {
-                    checkGrammarNameInLetOrConstDeclarations(element.name);
+                    if (element.kind !== SyntaxKind.OmittedExpression) {
+                        checkGrammarNameInLetOrConstDeclarations(element.name);
+                    }
                 }
             }
         }

tests/baselines/reference/arrayBindingPatternOmittedExpressions.js

@@ -0,0 +1,31 @@
+//// [arrayBindingPatternOmittedExpressions.ts]
+
+var results: string[];
+
+{
+    let [, b, , a] = results;
+    let x = {
+        a,
+        b
+    }
+}
+
+
+function f([, a, , b, , , , s, , , ] = results) {
+    a = s[1];
+    b = s[2];
+}
+
+//// [arrayBindingPatternOmittedExpressions.js]
+var results;
+{
+    let [, b, , a] = results;
+    let x = {
+        a,
+        b
+    };
+}
+function f([, a, , b, , , , s, , ,] = results) {
+    a = s[1];
+    b = s[2];
+}

tests/baselines/reference/arrayBindingPatternOmittedExpressions.types

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/arrayBindingPatternOmittedExpressions.ts ===
+
+var results: string[];
+>results : string[]
+
+{
+    let [, b, , a] = results;
+>b : string
+>a : string
+>results : string[]
+
+    let x = {
+>x : { a: string; b: string; }
+>{        a,        b    } : { a: string; b: string; }
+
+        a,
+>a : string
+
+        b
+>b : string
+    }
+}
+
+
+function f([, a, , b, , , , s, , , ] = results) {
+>f : ([, a, , b, , , , s, , , ]?: string[]) => void
+>a : string
+>b : string
+>s : string
+>results : string[]
+
+    a = s[1];
+>a = s[1] : string
+>a : string
+>s[1] : string
+>s : string
+
+    b = s[2];
+>b = s[2] : string
+>b : string
+>s[2] : string
+>s : string
+}

tests/cases/compiler/arrayBindingPatternOmittedExpressions.ts

@@ -0,0 +1,17 @@
+// @target: ES6
+
+var results: string[];
+
+{
+    let [, b, , a] = results;
+    let x = {
+        a,
+        b
+    }
+}
+
+
+function f([, a, , b, , , , s, , , ] = results) {
+    a = s[1];
+    b = s[2];
+}