Control flow analysis for array construction

src/compiler/binder.ts

@@ -760,6 +760,15 @@ namespace ts {
             };
         }
 
+        function createFlowArrayMutation(antecedent: FlowNode, node: CallExpression | BinaryExpression): FlowNode {
+            setFlowNodeReferenced(antecedent);
+            return <FlowArrayMutation>{
+                flags: FlowFlags.ArrayMutation,
+                antecedent,
+                node
+            };
+        }
+
         function finishFlowLabel(flow: FlowLabel): FlowNode {
             const antecedents = flow.antecedents;
             if (!antecedents) {
@@ -1140,6 +1149,12 @@ namespace ts {
                 forEachChild(node, bind);
                 if (operator === SyntaxKind.EqualsToken && !isAssignmentTarget(node)) {
                     bindAssignmentTargetFlow(node.left);
+                    if (node.left.kind === SyntaxKind.ElementAccessExpression) {
+                        const elementAccess = <ElementAccessExpression>node.left;
+                        if (isNarrowableOperand(elementAccess.expression)) {
+                            currentFlow = createFlowArrayMutation(currentFlow, node);
+                        }
+                    }
                 }
             }
         }
@@ -1200,6 +1215,12 @@ namespace ts {
             else {
                 forEachChild(node, bind);
             }
+            if (node.expression.kind === SyntaxKind.PropertyAccessExpression) {
+                const propertyAccess = <PropertyAccessExpression>node.expression;
+                if (isNarrowableOperand(propertyAccess.expression) && isPushOrUnshiftIdentifier(propertyAccess.name)) {
+                    currentFlow = createFlowArrayMutation(currentFlow, node);
+                }
+            }
         }
 
         function getContainerFlags(node: Node): ContainerFlags {

src/compiler/core.ts

@@ -265,13 +265,33 @@ namespace ts {
         if (array) {
             result = [];
             for (let i = 0; i < array.length; i++) {
-                const v = array[i];
-                result.push(f(v, i));
+                result.push(f(array[i], i));
             }
         }
         return result;
     }
 
+    // Maps from T to T and avoids allocation if all elements map to themselves
+    export function sameMap<T>(array: T[], f: (x: T, i: number) => T): T[] {
+        let result: T[];
+        if (array) {
+            for (let i = 0; i < array.length; i++) {
+                if (result) {
+                    result.push(f(array[i], i));
+                }
+                else {
+                    const item = array[i];
+                    const mapped = f(item, i);
+                    if (item !== mapped) {
+                        result = array.slice(0, i);
+                        result.push(mapped);
+                    }
+                }
+            }
+        }
+        return result || array;
+    }
+
     /**
      * Flattens an array containing a mix of array or non-array elements.
      *

src/compiler/diagnosticMessages.json

@@ -2957,7 +2957,7 @@
         "category": "Error",
         "code": 7033
     },
-    "Variable '{0}' implicitly has type 'any' in some locations where its type cannot be determined.": {
+    "Variable '{0}' implicitly has type '{1}' in some locations where its type cannot be determined.": {
         "category": "Error",
         "code": 7034
     },

src/compiler/types.ts

@@ -1907,8 +1907,9 @@ namespace ts {
         TrueCondition  = 1 << 5,  // Condition known to be true
         FalseCondition = 1 << 6,  // Condition known to be false
         SwitchClause   = 1 << 7,  // Switch statement clause
-        Referenced     = 1 << 8,  // Referenced as antecedent once
-        Shared         = 1 << 9,  // Referenced as antecedent more than once
+        ArrayMutation  = 1 << 8,  // Potential array mutation
+        Referenced     = 1 << 9,  // Referenced as antecedent once
+        Shared         = 1 << 10, // Referenced as antecedent more than once
         Label = BranchLabel | LoopLabel,
         Condition = TrueCondition | FalseCondition
     }
@@ -1951,6 +1952,13 @@ namespace ts {
         antecedent: FlowNode;
     }
 
+    // FlowArrayMutation represents a node potentially mutates an array, i.e. an
+    // operation of the form 'x.push(value)', 'x.unshift(value)' or 'x[n] = value'.
+    export interface FlowArrayMutation extends FlowNode {
+        node: CallExpression | BinaryExpression;
+        antecedent: FlowNode;
+    }
+
     export type FlowType = Type | IncompleteType;
 
     // Incomplete types occur during control flow analysis of loops. An IncompleteType
@@ -2748,6 +2756,8 @@ namespace ts {
     export interface AnonymousType extends ObjectType {
         target?: AnonymousType;  // Instantiation target
         mapper?: TypeMapper;     // Instantiation mapper
+        elementType?: Type;      // Element expressions of evolving array type
+        finalArrayType?: Type;   // Final array type of evolving array type
     }
 
     /* @internal */

src/compiler/utilities.ts

@@ -1895,6 +1895,10 @@ namespace ts {
         return node.kind === SyntaxKind.Identifier && (<Identifier>node).text === "Symbol";
     }
 
+    export function isPushOrUnshiftIdentifier(node: Identifier) {
+        return node.text === "push" || node.text === "unshift";
+    }
+
     export function isModifierKind(token: SyntaxKind): boolean {
         switch (token) {
             case SyntaxKind.AbstractKeyword:

tests/baselines/reference/arrayLiterals2ES5.types

@@ -194,9 +194,9 @@ var d5 = [...temp3];
 
 var d6 = [...temp4];
 >d6 : any[]
->[...temp4] : any[]
->...temp4 : any
->temp4 : any[]
+>[...temp4] : undefined[]
+>...temp4 : undefined
+>temp4 : undefined[]
 
 var d7 = [...[...temp1]];
 >d7 : number[]

tests/baselines/reference/controlFlowArrayErrors.errors.txt

@@ -0,0 +1,99 @@
+tests/cases/compiler/controlFlowArrayErrors.ts(6,9): error TS7005: Variable 'y' implicitly has an 'any[]' type.
+tests/cases/compiler/controlFlowArrayErrors.ts(14,9): error TS7005: Variable 'y' implicitly has an 'any[]' type.
+tests/cases/compiler/controlFlowArrayErrors.ts(20,9): error TS7034: Variable 'x' implicitly has type 'any[]' in some locations where its type cannot be determined.
+tests/cases/compiler/controlFlowArrayErrors.ts(23,9): error TS7005: Variable 'x' implicitly has an 'any[]' type.
+tests/cases/compiler/controlFlowArrayErrors.ts(30,12): error TS2345: Argument of type 'true' is not assignable to parameter of type 'string | number'.
+tests/cases/compiler/controlFlowArrayErrors.ts(35,12): error TS2345: Argument of type 'true' is not assignable to parameter of type 'string | number'.
+tests/cases/compiler/controlFlowArrayErrors.ts(49,5): error TS2349: Cannot invoke an expression whose type lacks a call signature. Type '((...items: (string | number)[]) => number) | ((...items: boolean[]) => number)' has no compatible call signatures.
+tests/cases/compiler/controlFlowArrayErrors.ts(57,12): error TS2345: Argument of type '"hello"' is not assignable to parameter of type 'number'.
+tests/cases/compiler/controlFlowArrayErrors.ts(61,11): error TS7034: Variable 'x' implicitly has type 'any[]' in some locations where its type cannot be determined.
+tests/cases/compiler/controlFlowArrayErrors.ts(64,9): error TS7005: Variable 'x' implicitly has an 'any[]' type.
+
+
+==== tests/cases/compiler/controlFlowArrayErrors.ts (10 errors) ====
+
+    declare function cond(): boolean;
+
+    function f1() {
+        let x = [];
+        let y = x;  // Implicit any[] error
+            ~
+!!! error TS7005: Variable 'y' implicitly has an 'any[]' type.
+        x.push(5);
+        let z = x;
+    }
+
+    function f2() {
+        let x;
+        x = [];
+        let y = x;  // Implicit any[] error
+            ~
+!!! error TS7005: Variable 'y' implicitly has an 'any[]' type.
+        x.push(5);
+        let z = x;
+    }
+
+    function f3() {
+        let x = [];  // Implicit any[] error in some locations
+            ~
+!!! error TS7034: Variable 'x' implicitly has type 'any[]' in some locations where its type cannot be determined.
+        x.push(5);
+        function g() {
+            x;  // Implicit any[] error
+            ~
+!!! error TS7005: Variable 'x' implicitly has an 'any[]' type.
+        }
+    }
+
+    function f4() {
+        let x;
+        x = [5, "hello"];  // Non-evolving array
+        x.push(true);      // Error
+               ~~~~
+!!! error TS2345: Argument of type 'true' is not assignable to parameter of type 'string | number'.
+    }
+
+    function f5() {
+        let x = [5, "hello"];  // Non-evolving array
+        x.push(true);          // Error
+               ~~~~
+!!! error TS2345: Argument of type 'true' is not assignable to parameter of type 'string | number'.
+    }
+
+    function f6() {
+        let x;
+        if (cond()) {
+            x = [];
+            x.push(5);
+            x.push("hello");
+        }
+        else {
+            x = [true];  // Non-evolving array
+        }
+        x;           // boolean[] | (string | number)[]
+        x.push(99);  // Error
+        ~~~~~~~~~~
+!!! error TS2349: Cannot invoke an expression whose type lacks a call signature. Type '((...items: (string | number)[]) => number) | ((...items: boolean[]) => number)' has no compatible call signatures.
+    }
+
+    function f7() {
+        let x = [];       // x has evolving array value
+        x.push(5);
+        let y = x;        // y has non-evolving array value
+        x.push("hello");  // Ok
+        y.push("hello");  // Error
+               ~~~~~~~
+!!! error TS2345: Argument of type '"hello"' is not assignable to parameter of type 'number'.
+    }
+
+    function f8() {
+        const x = [];  // Implicit any[] error in some locations
+              ~
+!!! error TS7034: Variable 'x' implicitly has type 'any[]' in some locations where its type cannot be determined.
+        x.push(5);
+        function g() {
+            x;  // Implicit any[] error
+            ~
+!!! error TS7005: Variable 'x' implicitly has an 'any[]' type.
+        }
+    }

tests/baselines/reference/controlFlowArrayErrors.js

@@ -0,0 +1,125 @@
+//// [controlFlowArrayErrors.ts]
+
+declare function cond(): boolean;
+
+function f1() {
+    let x = [];
+    let y = x;  // Implicit any[] error
+    x.push(5);
+    let z = x;
+}
+
+function f2() {
+    let x;
+    x = [];
+    let y = x;  // Implicit any[] error
+    x.push(5);
+    let z = x;
+}
+
+function f3() {
+    let x = [];  // Implicit any[] error in some locations
+    x.push(5);
+    function g() {
+        x;  // Implicit any[] error
+    }
+}
+
+function f4() {
+    let x;
+    x = [5, "hello"];  // Non-evolving array
+    x.push(true);      // Error
+}
+
+function f5() {
+    let x = [5, "hello"];  // Non-evolving array
+    x.push(true);          // Error
+}
+
+function f6() {
+    let x;
+    if (cond()) {
+        x = [];
+        x.push(5);
+        x.push("hello");
+    }
+    else {
+        x = [true];  // Non-evolving array
+    }
+    x;           // boolean[] | (string | number)[]
+    x.push(99);  // Error
+}
+
+function f7() {
+    let x = [];       // x has evolving array value
+    x.push(5);
+    let y = x;        // y has non-evolving array value
+    x.push("hello");  // Ok
+    y.push("hello");  // Error
+}
+
+function f8() {
+    const x = [];  // Implicit any[] error in some locations
+    x.push(5);
+    function g() {
+        x;  // Implicit any[] error
+    }
+}
+
+//// [controlFlowArrayErrors.js]
+function f1() {
+    var x = [];
+    var y = x; // Implicit any[] error
+    x.push(5);
+    var z = x;
+}
+function f2() {
+    var x;
+    x = [];
+    var y = x; // Implicit any[] error
+    x.push(5);
+    var z = x;
+}
+function f3() {
+    var x = []; // Implicit any[] error in some locations
+    x.push(5);
+    function g() {
+        x; // Implicit any[] error
+    }
+}
+function f4() {
+    var x;
+    x = [5, "hello"]; // Non-evolving array
+    x.push(true); // Error
+}
+function f5() {
+    var x = [5, "hello"]; // Non-evolving array
+    x.push(true); // Error
+}
+function f6() {
+    var x;
+    if (cond()) {
+        x = [];
+        x.push(5);
+        x.push("hello");
+    }
+    else {
+        x = [true]; // Non-evolving array
+    }
+    x; // boolean[] | (string | number)[]
+    x.push(99); // Error
+}
+function f7() {
+    var x = []; // x has evolving array value
+    x.push(5);
+    var y = x; // y has non-evolving array value
+    x.push("hello"); // Ok
+    y.push("hello"); // Error
+}
+function f8() {
+    var x = []; // Implicit any[] error in some locations
+    x.push(5);
+    function g() {
+        x; // Implicit any[] error
+    }
+}