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diff --git a/‎src/compiler/checker.ts
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+13-1Lines changed: 13 additions & 1 deletion b/‎src/compiler/checker.ts
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+13-1Lines changed: 13 additions & 1 deletion
diff --git a/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions
diff --git a/‎src/compiler/parser.ts
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+15-2Lines changed: 15 additions & 2 deletions b/‎src/compiler/parser.ts
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+15-2Lines changed: 15 additions & 2 deletions
diff --git a/‎src/compiler/types.ts
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+8-4Lines changed: 8 additions & 4 deletions b/‎src/compiler/types.ts
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+8-4Lines changed: 8 additions & 4 deletions
diff --git a/‎tests/baselines/reference/api/tsserverlibrary.d.ts
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+5-1Lines changed: 5 additions & 1 deletion b/‎tests/baselines/reference/api/tsserverlibrary.d.ts
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+5-1Lines changed: 5 additions & 1 deletion
diff --git a/‎tests/baselines/reference/api/typescript.d.ts
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+5-1Lines changed: 5 additions & 1 deletion b/‎tests/baselines/reference/api/typescript.d.ts
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+5-1Lines changed: 5 additions & 1 deletion
diff --git a/‎tests/baselines/reference/definiteAssignmentAssertions.errors.txt
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+113Lines changed: 113 additions & 0 deletions b/‎tests/baselines/reference/definiteAssignmentAssertions.errors.txt
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+113Lines changed: 113 additions & 0 deletions
@@ -13123,8 +13123,10 @@ namespace ts {
             // the entire control flow graph from the variable's declaration (i.e. when the flow container and
             // declaration container are the same).
             const assumeInitialized = isParameter || isAlias || isOuterVariable ||
-                type !== autoType && type !== autoArrayType && (!strictNullChecks || (type.flags & TypeFlags.Any) !== 0 || isInTypeQuery(node) || node.parent.kind === SyntaxKind.ExportSpecifier) ||
+                type !== autoType && type !== autoArrayType && (!strictNullChecks || (type.flags & TypeFlags.Any) !== 0 ||
+                isInTypeQuery(node) || node.parent.kind === SyntaxKind.ExportSpecifier) ||
                 node.parent.kind === SyntaxKind.NonNullExpression ||
+                declaration.kind === SyntaxKind.VariableDeclaration && (<VariableDeclaration>declaration).exclamationToken ||
                 declaration.flags & NodeFlags.Ambient;
             const initialType = assumeInitialized ? (isParameter ? removeOptionalityFromDeclaredType(type, getRootDeclaration(declaration) as VariableLikeDeclaration) : type) :
                 type === autoType || type === autoArrayType ? undefinedType :
@@ -22671,6 +22673,7 @@ namespace ts {
         function isInstancePropertyWithoutInitializer(node: Node) {
             return node.kind === SyntaxKind.PropertyDeclaration &&
                 !hasModifier(node, ModifierFlags.Static | ModifierFlags.Abstract) &&
+                !(<PropertyDeclaration>node).exclamationToken &&
                 !(<PropertyDeclaration>node).initializer;
         }
 
@@ -26103,6 +26106,10 @@ namespace ts {
                 }
             }
 
+            if (node.exclamationToken && (node.parent.parent.kind !== SyntaxKind.VariableStatement || !node.type || node.initializer || node.flags & NodeFlags.Ambient)) {
+                return grammarErrorOnNode(node.exclamationToken, Diagnostics.A_definite_assignment_assertion_is_not_permitted_in_this_context);
+            }
+
             if (compilerOptions.module !== ModuleKind.ES2015 && compilerOptions.module !== ModuleKind.ESNext && compilerOptions.module !== ModuleKind.System && !compilerOptions.noEmit &&
                 !(node.parent.parent.flags & NodeFlags.Ambient) && hasModifier(node.parent.parent, ModifierFlags.Export)) {
                 checkESModuleMarker(node.name);
@@ -26266,6 +26273,11 @@ namespace ts {
             if (node.flags & NodeFlags.Ambient && node.initializer) {
                 return grammarErrorOnFirstToken(node.initializer, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
             }
+
+            if (node.exclamationToken && (!isClassLike(node.parent) || !node.type || node.initializer ||
+                node.flags & NodeFlags.Ambient || hasModifier(node, ModifierFlags.Static | ModifierFlags.Abstract))) {
+                return grammarErrorOnNode(node.exclamationToken, Diagnostics.A_definite_assignment_assertion_is_not_permitted_in_this_context);
+            }
         }
 
         function checkGrammarTopLevelElementForRequiredDeclareModifier(node: Node): boolean {
 
@@ -831,6 +831,10 @@
         "category": "Error",
         "code": 1254
     },
+    "A definite assignment assertion '!' is not permitted in this context.": {
+        "category": "Error",
+        "code": 1255
+    },
     "'with' statements are not allowed in an async function block.": {
         "category": "Error",
         "code": 1300
 
@@ -99,6 +99,7 @@ namespace ts {
                     visitNode(cbNode, (<VariableLikeDeclaration>node).dotDotDotToken) ||
                     visitNode(cbNode, (<VariableLikeDeclaration>node).name) ||
                     visitNode(cbNode, (<VariableLikeDeclaration>node).questionToken) ||
+                    visitNode(cbNode, (<VariableLikeDeclaration>node).exclamationToken) ||
                     visitNode(cbNode, (<VariableLikeDeclaration>node).type) ||
                     visitNode(cbNode, (<VariableLikeDeclaration>node).initializer);
             case SyntaxKind.FunctionType:
@@ -5251,9 +5252,17 @@ namespace ts {
             return parseIdentifier();
         }
 
-        function parseVariableDeclaration(): VariableDeclaration {
+        function parseVariableDeclarationAllowExclamation() {
+            return parseVariableDeclaration(/*allowExclamation*/ true);
+        }
+
+        function parseVariableDeclaration(allowExclamation?: boolean): VariableDeclaration {
             const node = <VariableDeclaration>createNode(SyntaxKind.VariableDeclaration);
             node.name = parseIdentifierOrPattern();
+            if (allowExclamation && node.name.kind === SyntaxKind.Identifier &&
+                token() === SyntaxKind.ExclamationToken && !scanner.hasPrecedingLineBreak()) {
+                node.exclamationToken = parseTokenNode();
+            }
             node.type = parseTypeAnnotation();
             if (!isInOrOfKeyword(token())) {
                 node.initializer = parseInitializer();
@@ -5295,7 +5304,8 @@ namespace ts {
                 const savedDisallowIn = inDisallowInContext();
                 setDisallowInContext(inForStatementInitializer);
 
-                node.declarations = parseDelimitedList(ParsingContext.VariableDeclarations, parseVariableDeclaration);
+                node.declarations = parseDelimitedList(ParsingContext.VariableDeclarations,
+                    inForStatementInitializer ? parseVariableDeclaration : parseVariableDeclarationAllowExclamation);
 
                 setDisallowInContext(savedDisallowIn);
             }
@@ -5346,6 +5356,9 @@ namespace ts {
 
         function parsePropertyDeclaration(node: PropertyDeclaration): PropertyDeclaration {
             node.kind = SyntaxKind.PropertyDeclaration;
+            if (!node.questionToken && token() === SyntaxKind.ExclamationToken && !scanner.hasPrecedingLineBreak()) {
+                node.exclamationToken = parseTokenNode();
+            }
             node.type = parseTypeAnnotation();
 
             // For instance properties specifically, since they are evaluated inside the constructor,
 
@@ -598,6 +598,7 @@ namespace ts {
 
     export type DotDotDotToken = Token<SyntaxKind.DotDotDotToken>;
     export type QuestionToken = Token<SyntaxKind.QuestionToken>;
+    export type ExclamationToken = Token<SyntaxKind.ExclamationToken>;
     export type ColonToken = Token<SyntaxKind.ColonToken>;
     export type EqualsToken = Token<SyntaxKind.EqualsToken>;
     export type AsteriskToken = Token<SyntaxKind.AsteriskToken>;
@@ -761,9 +762,10 @@ namespace ts {
     export interface VariableDeclaration extends NamedDeclaration {
         kind: SyntaxKind.VariableDeclaration;
         parent?: VariableDeclarationList | CatchClause;
-        name: BindingName;                  // Declared variable name
-        type?: TypeNode;                    // Optional type annotation
-        initializer?: Expression;           // Optional initializer
+        name: BindingName;                    // Declared variable name
+        exclamationToken?: ExclamationToken;  // Optional definite assignment assertion
+        type?: TypeNode;                      // Optional type annotation
+        initializer?: Expression;             // Optional initializer
     }
 
     export interface VariableDeclarationList extends Node {
@@ -801,8 +803,9 @@ namespace ts {
 
     export interface PropertyDeclaration extends ClassElement, JSDocContainer {
         kind: SyntaxKind.PropertyDeclaration;
-        questionToken?: QuestionToken;      // Present for use with reporting a grammar error
         name: PropertyName;
+        questionToken?: QuestionToken;      // Present for use with reporting a grammar error
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;           // Optional initializer
     }
@@ -860,6 +863,7 @@ namespace ts {
         dotDotDotToken?: DotDotDotToken;
         name: DeclarationName;
         questionToken?: QuestionToken;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
 
@@ -461,6 +461,7 @@ declare namespace ts {
     }
     type DotDotDotToken = Token<SyntaxKind.DotDotDotToken>;
     type QuestionToken = Token<SyntaxKind.QuestionToken>;
+    type ExclamationToken = Token<SyntaxKind.ExclamationToken>;
     type ColonToken = Token<SyntaxKind.ColonToken>;
     type EqualsToken = Token<SyntaxKind.EqualsToken>;
     type AsteriskToken = Token<SyntaxKind.AsteriskToken>;
@@ -537,6 +538,7 @@ declare namespace ts {
         kind: SyntaxKind.VariableDeclaration;
         parent?: VariableDeclarationList | CatchClause;
         name: BindingName;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
@@ -571,8 +573,9 @@ declare namespace ts {
     }
     interface PropertyDeclaration extends ClassElement, JSDocContainer {
         kind: SyntaxKind.PropertyDeclaration;
-        questionToken?: QuestionToken;
         name: PropertyName;
+        questionToken?: QuestionToken;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
@@ -606,6 +609,7 @@ declare namespace ts {
         dotDotDotToken?: DotDotDotToken;
         name: DeclarationName;
         questionToken?: QuestionToken;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
 
@@ -461,6 +461,7 @@ declare namespace ts {
     }
     type DotDotDotToken = Token<SyntaxKind.DotDotDotToken>;
     type QuestionToken = Token<SyntaxKind.QuestionToken>;
+    type ExclamationToken = Token<SyntaxKind.ExclamationToken>;
     type ColonToken = Token<SyntaxKind.ColonToken>;
     type EqualsToken = Token<SyntaxKind.EqualsToken>;
     type AsteriskToken = Token<SyntaxKind.AsteriskToken>;
@@ -537,6 +538,7 @@ declare namespace ts {
         kind: SyntaxKind.VariableDeclaration;
         parent?: VariableDeclarationList | CatchClause;
         name: BindingName;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
@@ -571,8 +573,9 @@ declare namespace ts {
     }
     interface PropertyDeclaration extends ClassElement, JSDocContainer {
         kind: SyntaxKind.PropertyDeclaration;
-        questionToken?: QuestionToken;
         name: PropertyName;
+        questionToken?: QuestionToken;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
@@ -606,6 +609,7 @@ declare namespace ts {
         dotDotDotToken?: DotDotDotToken;
         name: DeclarationName;
         questionToken?: QuestionToken;
+        exclamationToken?: ExclamationToken;
         type?: TypeNode;
         initializer?: Expression;
     }
 
@@ -0,0 +1,113 @@
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(5,5): error TS2564: Property 'b' has no initializer and is not definitely assigned in the constructor.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(20,6): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(21,6): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(22,13): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(28,6): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(34,15): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(68,10): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(69,10): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(70,10): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(75,15): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts(76,15): error TS1255: A definite assignment assertion '!' is not permitted in this context.
+
+
+==== tests/cases/conformance/controlFlow/definiteAssignmentAssertions.ts (11 errors) ====
+    // Suppress strict property initialization check
+    
+    class C1 {
+        a!: number;
+        b: string;  // Error
+        ~
+!!! error TS2564: Property 'b' has no initializer and is not definitely assigned in the constructor.
+    }
+    
+    // Suppress definite assignment check in constructor
+    
+    class C2 {
+        a!: number;
+        constructor() {
+            let x = this.a;
+        }
+    }
+    
+    // Definite assignment assertion requires type annotation, no initializer, no static modifier
+    
+    class C3 {
+        a! = 1;
+         ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+        b!: number = 1;
+         ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+        static c!: number;
+                ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+    }
+    
+    // Definite assignment assertion not permitted in ambient context
+    
+    declare class C4 {
+        a!: number;
+         ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+    }
+    
+    // Definite assignment assertion not permitted on abstract property
+    
+    abstract class C5 {
+        abstract a!: number;
+                  ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+    }
+    
+    // Suppress definite assignment check for variable
+    
+    function f1() {
+        let x!: number;
+        let y = x;
+        var a!: number;
+        var b = a;
+    }
+    
+    function f2() {
+        let x!: string | number;
+        if (typeof x === "string") {
+            let s: string = x;
+        }
+        else {
+            let n: number = x;
+        }
+    }
+    
+    function f3() {
+        let x!: number;
+        const g = () => {
+            x = 1;
+        }
+        g();
+        let y = x;
+    }
+    
+    // Definite assignment assertion requires type annotation and no initializer
+    
+    function f4() {
+        let a!;
+             ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+        let b! = 1;
+             ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+        let c!: number = 1;
+             ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+    }
+    
+    // Definite assignment assertion not permitted in ambient context
+    
+    declare let v1!: number;
+                  ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+    declare var v2!: number;
+                  ~
+!!! error TS1255: A definite assignment assertion '!' is not permitted in this context.
+