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diff --git a/‎src/compiler/checker.ts
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+173-22Lines changed: 173 additions & 22 deletions b/‎src/compiler/checker.ts
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+173-22Lines changed: 173 additions & 22 deletions
diff --git a/‎src/compiler/commandLineParser.ts
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+7Lines changed: 7 additions & 0 deletions b/‎src/compiler/commandLineParser.ts
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+7Lines changed: 7 additions & 0 deletions
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/types.ts
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+18-4Lines changed: 18 additions & 4 deletions b/‎src/compiler/types.ts
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+18-4Lines changed: 18 additions & 4 deletions
diff --git a/‎src/lib/dom.generated.d.ts
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+1-21Lines changed: 1 addition & 21 deletions b/‎src/lib/dom.generated.d.ts
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+1-21Lines changed: 1 addition & 21 deletions
diff --git a/‎tests/baselines/reference/fuzzy.errors.txt
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+2Lines changed: 2 additions & 0 deletions b/‎tests/baselines/reference/fuzzy.errors.txt
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+2Lines changed: 2 additions & 0 deletions
diff --git a/‎tests/baselines/reference/strictFunctionTypes1.js
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+42Lines changed: 42 additions & 0 deletions b/‎tests/baselines/reference/strictFunctionTypes1.js
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+42Lines changed: 42 additions & 0 deletions
diff --git a/‎tests/baselines/reference/strictFunctionTypes1.symbols
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+96Lines changed: 96 additions & 0 deletions b/‎tests/baselines/reference/strictFunctionTypes1.symbols
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+96Lines changed: 96 additions & 0 deletions
@@ -269,6 +269,13 @@ namespace ts {
             category: Diagnostics.Strict_Type_Checking_Options,
             description: Diagnostics.Enable_strict_null_checks
         },
+        {
+            name: "strictFunctionTypes",
+            type: "boolean",
+            showInSimplifiedHelpView: true,
+            category: Diagnostics.Strict_Type_Checking_Options,
+            description: Diagnostics.Enable_strict_checking_of_function_types
+        },
         {
             name: "noImplicitThis",
             type: "boolean",
 
@@ -3314,6 +3314,10 @@
         "category": "Message",
         "code": 6185
     },
+    "Enable strict checking of function types.": {
+        "category": "Message",
+        "code": 6186
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
 
@@ -3215,6 +3215,7 @@ namespace ts {
         NonPrimitive            = 1 << 24,  // intrinsic object type
         /* @internal */
         JsxAttributes           = 1 << 25,  // Jsx attributes type
+        MarkerType              = 1 << 26,  // Marker type used for variance probing
 
         /* @internal */
         Nullable = Undefined | Null,
@@ -3343,10 +3344,21 @@ namespace ts {
         typeArguments?: Type[];  // Type reference type arguments (undefined if none)
     }
 
+    /* @internal */
+    export const enum Variance {
+        Invariant     = 0,  // Neither covariant nor contravariant
+        Covariant     = 1,  // Covariant
+        Contravariant = 2,  // Contravariant
+        Bivariant     = 3,  // Both covariant and contravariant
+        Independent   = 4,  // Unwitnessed type parameter
+    }
+
     // Generic class and interface types
     export interface GenericType extends InterfaceType, TypeReference {
         /* @internal */
-        instantiations: Map<TypeReference>;   // Generic instantiation cache
+        instantiations: Map<TypeReference>;  // Generic instantiation cache
+        /* @internal */
+        variances?: Variance[];  // Variance of each type parameter
     }
 
     export interface UnionOrIntersectionType extends Type {
@@ -3522,9 +3534,10 @@ namespace ts {
     }
 
     export const enum InferencePriority {
-        NakedTypeVariable = 1 << 0,  // Naked type variable in union or intersection type
-        MappedType        = 1 << 1,  // Reverse inference for mapped type
-        ReturnType        = 1 << 2,  // Inference made from return type of generic function
+        Contravariant     = 1 << 0,  // Inference from contravariant position
+        NakedTypeVariable = 1 << 1,  // Naked type variable in union or intersection type
+        MappedType        = 1 << 2,  // Reverse inference for mapped type
+        ReturnType        = 1 << 3,  // Inference made from return type of generic function
     }
 
     export interface InferenceInfo {
@@ -3707,6 +3720,7 @@ namespace ts {
         sourceMap?: boolean;
         sourceRoot?: string;
         strict?: boolean;
+        strictFunctionTypes?: boolean;  // Always combine with strict property
         strictNullChecks?: boolean;  // Always combine with strict property
         /* @internal */ stripInternal?: boolean;
         suppressExcessPropertyErrors?: boolean;
 
@@ -4233,11 +4233,7 @@ interface HTMLBodyElement extends HTMLElement {
     onafterprint: (this: HTMLBodyElement, ev: Event) => any;
     onbeforeprint: (this: HTMLBodyElement, ev: Event) => any;
     onbeforeunload: (this: HTMLBodyElement, ev: BeforeUnloadEvent) => any;
-    onblur: (this: HTMLBodyElement, ev: FocusEvent) => any;
-    onerror: (this: HTMLBodyElement, ev: ErrorEvent) => any;
-    onfocus: (this: HTMLBodyElement, ev: FocusEvent) => any;
     onhashchange: (this: HTMLBodyElement, ev: HashChangeEvent) => any;
-    onload: (this: HTMLBodyElement, ev: Event) => any;
     onmessage: (this: HTMLBodyElement, ev: MessageEvent) => any;
     onoffline: (this: HTMLBodyElement, ev: Event) => any;
     ononline: (this: HTMLBodyElement, ev: Event) => any;
@@ -4246,7 +4242,6 @@ interface HTMLBodyElement extends HTMLElement {
     onpageshow: (this: HTMLBodyElement, ev: PageTransitionEvent) => any;
     onpopstate: (this: HTMLBodyElement, ev: PopStateEvent) => any;
     onresize: (this: HTMLBodyElement, ev: UIEvent) => any;
-    onscroll: (this: HTMLBodyElement, ev: UIEvent) => any;
     onstorage: (this: HTMLBodyElement, ev: StorageEvent) => any;
     onunload: (this: HTMLBodyElement, ev: Event) => any;
     text: any;
@@ -4901,10 +4896,6 @@ interface HTMLFrameElement extends HTMLElement, GetSVGDocument {
      * Sets or retrieves whether the user can resize the frame.
      */
     noResize: boolean;
-    /**
-     * Raised when the object has been completely received from the server.
-     */
-    onload: (this: HTMLFrameElement, ev: Event) => any;
     /**
      * Sets or retrieves whether the frame can be scrolled.
      */
@@ -4970,17 +4961,10 @@ interface HTMLFrameSetElement extends HTMLElement {
     onafterprint: (this: HTMLFrameSetElement, ev: Event) => any;
     onbeforeprint: (this: HTMLFrameSetElement, ev: Event) => any;
     onbeforeunload: (this: HTMLFrameSetElement, ev: BeforeUnloadEvent) => any;
-    /**
-     * Fires when the object loses the input focus.
-     */
-    onblur: (this: HTMLFrameSetElement, ev: FocusEvent) => any;
-    onerror: (this: HTMLFrameSetElement, ev: ErrorEvent) => any;
     /**
      * Fires when the object receives focus.
      */
-    onfocus: (this: HTMLFrameSetElement, ev: FocusEvent) => any;
     onhashchange: (this: HTMLFrameSetElement, ev: HashChangeEvent) => any;
-    onload: (this: HTMLFrameSetElement, ev: Event) => any;
     onmessage: (this: HTMLFrameSetElement, ev: MessageEvent) => any;
     onoffline: (this: HTMLFrameSetElement, ev: Event) => any;
     ononline: (this: HTMLFrameSetElement, ev: Event) => any;
@@ -4989,7 +4973,6 @@ interface HTMLFrameSetElement extends HTMLElement {
     onpageshow: (this: HTMLFrameSetElement, ev: PageTransitionEvent) => any;
     onpopstate: (this: HTMLFrameSetElement, ev: PopStateEvent) => any;
     onresize: (this: HTMLFrameSetElement, ev: UIEvent) => any;
-    onscroll: (this: HTMLFrameSetElement, ev: UIEvent) => any;
     onstorage: (this: HTMLFrameSetElement, ev: StorageEvent) => any;
     onunload: (this: HTMLFrameSetElement, ev: Event) => any;
     /**
@@ -5125,10 +5108,7 @@ interface HTMLIFrameElement extends HTMLElement, GetSVGDocument {
      * Sets or retrieves whether the user can resize the frame.
      */
     noResize: boolean;
-    /**
-     * Raised when the object has been completely received from the server.
-     */
-    onload: (this: HTMLIFrameElement, ev: Event) => any;
+
     readonly sandbox: DOMSettableTokenList;
     /**
      * Sets or retrieves whether the frame can be scrolled.
 
@@ -4,6 +4,7 @@ tests/cases/compiler/fuzzy.ts(21,13): error TS2322: Type '{ anything: number; on
   Types of property 'oneI' are incompatible.
     Type 'this' is not assignable to type 'I'.
       Type 'C' is not assignable to type 'I'.
+        Property 'alsoWorks' is missing in type 'C'.
 tests/cases/compiler/fuzzy.ts(25,20): error TS2352: Type '{ oneI: this; }' cannot be converted to type 'R'.
   Property 'anything' is missing in type '{ oneI: this; }'.
 
@@ -38,6 +39,7 @@ tests/cases/compiler/fuzzy.ts(25,20): error TS2352: Type '{ oneI: this; }' canno
 !!! error TS2322:   Types of property 'oneI' are incompatible.
 !!! error TS2322:     Type 'this' is not assignable to type 'I'.
 !!! error TS2322:       Type 'C' is not assignable to type 'I'.
+!!! error TS2322:         Property 'alsoWorks' is missing in type 'C'.
             }
 
             worksToo():R {
 
@@ -0,0 +1,42 @@
+//// [strictFunctionTypes1.ts]
+declare function f1<T>(f1: (x: T) => void, f2: (x: T) => void): (x: T) => void;
+declare function f2<T>(obj: T, f1: (x: T) => void, f2: (x: T) => void): T;
+declare function f3<T>(obj: T, f1: (x: T) => void, f2: (f: (x: T) => void) => void): T;
+
+interface Func<T> { (x: T): void }
+
+declare function f4<T>(f1: Func<T>, f2: Func<T>): Func<T>;
+
+declare function fo(x: Object): void;
+declare function fs(x: string): void;
+declare function fx(f: (x: "def") => void): void;
+
+const x1 = f1(fo, fs);  // (x: string) => void
+const x2 = f2("abc", fo, fs);  // "abc"
+const x3 = f3("abc", fo, fx);  // "abc" | "def"
+const x4 = f4(fo, fs);  // Func<string>
+
+
+//// [strictFunctionTypes1.js]
+"use strict";
+var x1 = f1(fo, fs); // (x: string) => void
+var x2 = f2("abc", fo, fs); // "abc"
+var x3 = f3("abc", fo, fx); // "abc" | "def"
+var x4 = f4(fo, fs); // Func<string>
+
+
+//// [strictFunctionTypes1.d.ts]
+declare function f1<T>(f1: (x: T) => void, f2: (x: T) => void): (x: T) => void;
+declare function f2<T>(obj: T, f1: (x: T) => void, f2: (x: T) => void): T;
+declare function f3<T>(obj: T, f1: (x: T) => void, f2: (f: (x: T) => void) => void): T;
+interface Func<T> {
+    (x: T): void;
+}
+declare function f4<T>(f1: Func<T>, f2: Func<T>): Func<T>;
+declare function fo(x: Object): void;
+declare function fs(x: string): void;
+declare function fx(f: (x: "def") => void): void;
+declare const x1: (x: string) => void;
+declare const x2 = "abc";
+declare const x3: string;
+declare const x4: Func<string>;
@@ -0,0 +1,96 @@
+=== tests/cases/compiler/strictFunctionTypes1.ts ===
+declare function f1<T>(f1: (x: T) => void, f2: (x: T) => void): (x: T) => void;
+>f1 : Symbol(f1, Decl(strictFunctionTypes1.ts, 0, 0))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 0, 20))
+>f1 : Symbol(f1, Decl(strictFunctionTypes1.ts, 0, 23))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 0, 28))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 0, 20))
+>f2 : Symbol(f2, Decl(strictFunctionTypes1.ts, 0, 42))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 0, 48))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 0, 20))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 0, 65))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 0, 20))
+
+declare function f2<T>(obj: T, f1: (x: T) => void, f2: (x: T) => void): T;
+>f2 : Symbol(f2, Decl(strictFunctionTypes1.ts, 0, 79))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 1, 20))
+>obj : Symbol(obj, Decl(strictFunctionTypes1.ts, 1, 23))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 1, 20))
+>f1 : Symbol(f1, Decl(strictFunctionTypes1.ts, 1, 30))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 1, 36))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 1, 20))
+>f2 : Symbol(f2, Decl(strictFunctionTypes1.ts, 1, 50))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 1, 56))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 1, 20))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 1, 20))
+
+declare function f3<T>(obj: T, f1: (x: T) => void, f2: (f: (x: T) => void) => void): T;
+>f3 : Symbol(f3, Decl(strictFunctionTypes1.ts, 1, 74))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 2, 20))
+>obj : Symbol(obj, Decl(strictFunctionTypes1.ts, 2, 23))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 2, 20))
+>f1 : Symbol(f1, Decl(strictFunctionTypes1.ts, 2, 30))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 2, 36))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 2, 20))
+>f2 : Symbol(f2, Decl(strictFunctionTypes1.ts, 2, 50))
+>f : Symbol(f, Decl(strictFunctionTypes1.ts, 2, 56))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 2, 60))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 2, 20))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 2, 20))
+
+interface Func<T> { (x: T): void }
+>Func : Symbol(Func, Decl(strictFunctionTypes1.ts, 2, 87))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 4, 15))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 4, 21))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 4, 15))
+
+declare function f4<T>(f1: Func<T>, f2: Func<T>): Func<T>;
+>f4 : Symbol(f4, Decl(strictFunctionTypes1.ts, 4, 34))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 6, 20))
+>f1 : Symbol(f1, Decl(strictFunctionTypes1.ts, 6, 23))
+>Func : Symbol(Func, Decl(strictFunctionTypes1.ts, 2, 87))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 6, 20))
+>f2 : Symbol(f2, Decl(strictFunctionTypes1.ts, 6, 35))
+>Func : Symbol(Func, Decl(strictFunctionTypes1.ts, 2, 87))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 6, 20))
+>Func : Symbol(Func, Decl(strictFunctionTypes1.ts, 2, 87))
+>T : Symbol(T, Decl(strictFunctionTypes1.ts, 6, 20))
+
+declare function fo(x: Object): void;
+>fo : Symbol(fo, Decl(strictFunctionTypes1.ts, 6, 58))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 8, 20))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+
+declare function fs(x: string): void;
+>fs : Symbol(fs, Decl(strictFunctionTypes1.ts, 8, 37))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 9, 20))
+
+declare function fx(f: (x: "def") => void): void;
+>fx : Symbol(fx, Decl(strictFunctionTypes1.ts, 9, 37))
+>f : Symbol(f, Decl(strictFunctionTypes1.ts, 10, 20))
+>x : Symbol(x, Decl(strictFunctionTypes1.ts, 10, 24))
+
+const x1 = f1(fo, fs);  // (x: string) => void
+>x1 : Symbol(x1, Decl(strictFunctionTypes1.ts, 12, 5))
+>f1 : Symbol(f1, Decl(strictFunctionTypes1.ts, 0, 0))
+>fo : Symbol(fo, Decl(strictFunctionTypes1.ts, 6, 58))
+>fs : Symbol(fs, Decl(strictFunctionTypes1.ts, 8, 37))
+
+const x2 = f2("abc", fo, fs);  // "abc"
+>x2 : Symbol(x2, Decl(strictFunctionTypes1.ts, 13, 5))
+>f2 : Symbol(f2, Decl(strictFunctionTypes1.ts, 0, 79))
+>fo : Symbol(fo, Decl(strictFunctionTypes1.ts, 6, 58))
+>fs : Symbol(fs, Decl(strictFunctionTypes1.ts, 8, 37))
+
+const x3 = f3("abc", fo, fx);  // "abc" | "def"
+>x3 : Symbol(x3, Decl(strictFunctionTypes1.ts, 14, 5))
+>f3 : Symbol(f3, Decl(strictFunctionTypes1.ts, 1, 74))
+>fo : Symbol(fo, Decl(strictFunctionTypes1.ts, 6, 58))
+>fx : Symbol(fx, Decl(strictFunctionTypes1.ts, 9, 37))
+
+const x4 = f4(fo, fs);  // Func<string>
+>x4 : Symbol(x4, Decl(strictFunctionTypes1.ts, 15, 5))
+>f4 : Symbol(f4, Decl(strictFunctionTypes1.ts, 4, 34))
+>fo : Symbol(fo, Decl(strictFunctionTypes1.ts, 6, 58))
+>fs : Symbol(fs, Decl(strictFunctionTypes1.ts, 8, 37))
+