Add related diagnostic to "used before defined" if type is a function that returns a union with undefined

src/compiler/checker.ts

@@ -20190,7 +20190,26 @@ namespace ts {
                 }
             }
             else if (!assumeInitialized && !(getFalsyFlags(type) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
-                error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
+                const diag = error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
+
+                // See GH:32846 - if the user is using a variable whose type is () => T1 | ... | undefined
+                // they may have meant to specify the type as (() => T1 | ...) | undefined
+                // This is assumed if: the type is a FunctionType, the return type is a Union, the last constituent of
+                // the union is `undefined`
+                if (type.symbol && type.symbol.declarations.length === 1 && isFunctionTypeNode(type.symbol.declarations[0])) {
+                    const funcTypeNode = <FunctionTypeNode>type.symbol.declarations[0];
+                    const returnType = getReturnTypeFromAnnotation(funcTypeNode);
+                    if (returnType && returnType.flags & TypeFlags.Union) {
+                        const unionTypes = (<UnionTypeNode>funcTypeNode.type).types;
+                        if (unionTypes && unionTypes[unionTypes.length - 1].kind === SyntaxKind.UndefinedKeyword) {
+                            const parenedFuncType = getMutableClone(funcTypeNode);
+                            // Highlight to the end of the second to last constituent of the union
+                            parenedFuncType.end = unionTypes[unionTypes.length - 2].end;
+                            addRelatedInfo(diag, createDiagnosticForNode(parenedFuncType, Diagnostics.Did_you_mean_to_parenthesize_this_function_type));
+                        }
+                    }
+                }
+
                 // Return the declared type to reduce follow-on errors
                 return type;
             }

src/compiler/diagnosticMessages.json

@@ -1047,6 +1047,10 @@
         "category": "Error",
         "code": 1359
     },
+    "Did you mean to parenthesize this function type?": {
+        "category": "Error",
+        "code": 1360
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",

tests/baselines/reference/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.errors.txt

@@ -0,0 +1,74 @@
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(2,1): error TS2454: Variable 'a' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(6,1): error TS2454: Variable 'c' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(10,1): error TS2454: Variable 'd' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(13,1): error TS2454: Variable 'e' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(16,1): error TS2454: Variable 'f' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(19,1): error TS2454: Variable 'g' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(24,1): error TS2454: Variable 'h' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(28,1): error TS2454: Variable 'i' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(32,1): error TS2454: Variable 'j' is used before being assigned.
+tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts(36,1): error TS2454: Variable 'j' is used before being assigned.
+
+
+==== tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts (10 errors) ====
+    let a: () => void | undefined;
+    a; // Error did you mean to paren this function type
+    ~
+!!! error TS2454: Variable 'a' is used before being assigned.
+!!! related TS1360 tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts:1:8: Did you mean to parenthesize this function type?
+
+    function b (): void | undefined {}
+    let c: typeof b;
+    c;
+    ~
+!!! error TS2454: Variable 'c' is used before being assigned.
+
+    type Undefined = undefined
+    let d: () => void | Undefined;
+    d;
+    ~
+!!! error TS2454: Variable 'd' is used before being assigned.
+
+    let e: () => string | void | number | object | undefined;
+    e; // Error did you mean to paren...
+    ~
+!!! error TS2454: Variable 'e' is used before being assigned.
+!!! related TS1360 tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts:12:8: Did you mean to parenthesize this function type?
+
+    let f: () => void;
+    f;
+    ~
+!!! error TS2454: Variable 'f' is used before being assigned.
+
+    let g: () => undefined;
+    g;
+    ~
+!!! error TS2454: Variable 'g' is used before being assigned.
+
+    type T1 = undefined | string;
+    type T2 = undefined | void;
+    let h: () => T1 & T2;
+    h;
+    ~
+!!! error TS2454: Variable 'h' is used before being assigned.
+
+    type T3 = void | undefined;
+    let i: () => T3;
+    i;
+    ~
+!!! error TS2454: Variable 'i' is used before being assigned.
+
+    type T4 = () => void | undefined;
+    let j: T4;
+    j; // Error did you mean to paren...
+    ~
+!!! error TS2454: Variable 'j' is used before being assigned.
+!!! related TS1360 tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts:30:11: Did you mean to parenthesize this function type?
+
+    type T5 = () => void
+    let k: T5 | undefined
+    j;
+    ~
+!!! error TS2454: Variable 'j' is used before being assigned.
+!!! related TS1360 tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts:30:11: Did you mean to parenthesize this function type?
+

tests/baselines/reference/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.js

@@ -0,0 +1,61 @@
+//// [functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts]
+let a: () => void | undefined;
+a; // Error did you mean to paren this function type
+
+function b (): void | undefined {}
+let c: typeof b;
+c;
+
+type Undefined = undefined
+let d: () => void | Undefined;
+d;
+
+let e: () => string | void | number | object | undefined;
+e; // Error did you mean to paren...
+
+let f: () => void;
+f;
+
+let g: () => undefined;
+g;
+
+type T1 = undefined | string;
+type T2 = undefined | void;
+let h: () => T1 & T2;
+h;
+
+type T3 = void | undefined;
+let i: () => T3;
+i;
+
+type T4 = () => void | undefined;
+let j: T4;
+j; // Error did you mean to paren...
+
+type T5 = () => void
+let k: T5 | undefined
+j;
+
+
+//// [functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.js]
+var a;
+a; // Error did you mean to paren this function type
+function b() { }
+var c;
+c;
+var d;
+d;
+var e;
+e; // Error did you mean to paren...
+var f;
+f;
+var g;
+g;
+var h;
+h;
+var i;
+i;
+var j;
+j; // Error did you mean to paren...
+var k;
+j;

tests/baselines/reference/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.symbols

@@ -0,0 +1,89 @@
+=== tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts ===
+let a: () => void | undefined;
+>a : Symbol(a, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 0, 3))
+
+a; // Error did you mean to paren this function type
+>a : Symbol(a, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 0, 3))
+
+function b (): void | undefined {}
+>b : Symbol(b, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 1, 2))
+
+let c: typeof b;
+>c : Symbol(c, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 4, 3))
+>b : Symbol(b, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 1, 2))
+
+c;
+>c : Symbol(c, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 4, 3))
+
+type Undefined = undefined
+>Undefined : Symbol(Undefined, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 5, 2))
+
+let d: () => void | Undefined;
+>d : Symbol(d, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 8, 3))
+>Undefined : Symbol(Undefined, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 5, 2))
+
+d;
+>d : Symbol(d, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 8, 3))
+
+let e: () => string | void | number | object | undefined;
+>e : Symbol(e, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 11, 3))
+
+e; // Error did you mean to paren...
+>e : Symbol(e, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 11, 3))
+
+let f: () => void;
+>f : Symbol(f, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 14, 3))
+
+f;
+>f : Symbol(f, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 14, 3))
+
+let g: () => undefined;
+>g : Symbol(g, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 17, 3))
+
+g;
+>g : Symbol(g, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 17, 3))
+
+type T1 = undefined | string;
+>T1 : Symbol(T1, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 18, 2))
+
+type T2 = undefined | void;
+>T2 : Symbol(T2, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 20, 29))
+
+let h: () => T1 & T2;
+>h : Symbol(h, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 22, 3))
+>T1 : Symbol(T1, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 18, 2))
+>T2 : Symbol(T2, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 20, 29))
+
+h;
+>h : Symbol(h, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 22, 3))
+
+type T3 = void | undefined;
+>T3 : Symbol(T3, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 23, 2))
+
+let i: () => T3;
+>i : Symbol(i, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 26, 3))
+>T3 : Symbol(T3, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 23, 2))
+
+i;
+>i : Symbol(i, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 26, 3))
+
+type T4 = () => void | undefined;
+>T4 : Symbol(T4, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 27, 2))
+
+let j: T4;
+>j : Symbol(j, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 30, 3))
+>T4 : Symbol(T4, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 27, 2))
+
+j; // Error did you mean to paren...
+>j : Symbol(j, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 30, 3))
+
+type T5 = () => void
+>T5 : Symbol(T5, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 31, 2))
+
+let k: T5 | undefined
+>k : Symbol(k, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 34, 3))
+>T5 : Symbol(T5, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 31, 2))
+
+j;
+>j : Symbol(j, Decl(functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts, 30, 3))
+

tests/baselines/reference/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.types

@@ -0,0 +1,83 @@
+=== tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts ===
+let a: () => void | undefined;
+>a : () => void | undefined
+
+a; // Error did you mean to paren this function type
+>a : () => void | undefined
+
+function b (): void | undefined {}
+>b : () => void | undefined
+
+let c: typeof b;
+>c : () => void | undefined
+>b : () => void | undefined
+
+c;
+>c : () => void | undefined
+
+type Undefined = undefined
+>Undefined : undefined
+
+let d: () => void | Undefined;
+>d : () => void | undefined
+
+d;
+>d : () => void | undefined
+
+let e: () => string | void | number | object | undefined;
+>e : () => string | number | void | object | undefined
+
+e; // Error did you mean to paren...
+>e : () => string | number | void | object | undefined
+
+let f: () => void;
+>f : () => void
+
+f;
+>f : () => void
+
+let g: () => undefined;
+>g : () => undefined
+
+g;
+>g : () => undefined
+
+type T1 = undefined | string;
+>T1 : T1
+
+type T2 = undefined | void;
+>T2 : void | undefined
+
+let h: () => T1 & T2;
+>h : () => undefined
+
+h;
+>h : () => undefined
+
+type T3 = void | undefined;
+>T3 : void | undefined
+
+let i: () => T3;
+>i : () => void | undefined
+
+i;
+>i : () => void | undefined
+
+type T4 = () => void | undefined;
+>T4 : T4
+
+let j: T4;
+>j : T4
+
+j; // Error did you mean to paren...
+>j : T4
+
+type T5 = () => void
+>T5 : T5
+
+let k: T5 | undefined
+>k : T5 | undefined
+
+j;
+>j : T4
+

tests/cases/compiler/functionTypeReturnsUnionWithUndefinedWithStrictNullChecks.ts

@@ -0,0 +1,38 @@
+// @strictNullChecks: true
+
+let a: () => void | undefined;
+a; // Error did you mean to paren this function type
+
+function b (): void | undefined {}
+let c: typeof b;
+c;
+
+type Undefined = undefined
+let d: () => void | Undefined;
+d;
+
+let e: () => string | void | number | object | undefined;
+e; // Error did you mean to paren...
+
+let f: () => void;
+f;
+
+let g: () => undefined;
+g;
+
+type T1 = undefined | string;
+type T2 = undefined | void;
+let h: () => T1 & T2;
+h;
+
+type T3 = void | undefined;
+let i: () => T3;
+i;
+
+type T4 = () => void | undefined;
+let j: T4;
+j; // Error did you mean to paren...
+
+type T5 = () => void
+let k: T5 | undefined
+j;