Support inferring an index signature from 'any'

src/compiler/checker.ts

@@ -7137,6 +7137,7 @@ namespace ts {
                     return getUnionType(propTypes, UnionReduction.Subtype);
                 }
             }
+            if (isTypeAny(type)) return type;
             return undefined;
         }
 
@@ -12714,6 +12715,9 @@ namespace ts {
                             inferFromObjectTypes(source, target);
                         }
                     }
+                    else if (isTypeAny(source)) {
+                        inferFromIndexTypes(source, target);
+                    }
                 }
             }
 

tests/baselines/reference/inferIndexSignatureFromAny.js

@@ -0,0 +1,7 @@
+//// [inferIndexSignatureFromAny.ts]
+declare function f<T>(x: { [key: string]: T }): T;
+const a = f(<any>null);
+
+
+//// [inferIndexSignatureFromAny.js]
+var a = f(null);

tests/baselines/reference/inferIndexSignatureFromAny.symbols

@@ -0,0 +1,13 @@
+=== tests/cases/compiler/inferIndexSignatureFromAny.ts ===
+declare function f<T>(x: { [key: string]: T }): T;
+>f : Symbol(f, Decl(inferIndexSignatureFromAny.ts, 0, 0))
+>T : Symbol(T, Decl(inferIndexSignatureFromAny.ts, 0, 19))
+>x : Symbol(x, Decl(inferIndexSignatureFromAny.ts, 0, 22))
+>key : Symbol(key, Decl(inferIndexSignatureFromAny.ts, 0, 28))
+>T : Symbol(T, Decl(inferIndexSignatureFromAny.ts, 0, 19))
+>T : Symbol(T, Decl(inferIndexSignatureFromAny.ts, 0, 19))
+
+const a = f(<any>null);
+>a : Symbol(a, Decl(inferIndexSignatureFromAny.ts, 1, 5))
+>f : Symbol(f, Decl(inferIndexSignatureFromAny.ts, 0, 0))
+

tests/baselines/reference/inferIndexSignatureFromAny.types

@@ -0,0 +1,16 @@
+=== tests/cases/compiler/inferIndexSignatureFromAny.ts ===
+declare function f<T>(x: { [key: string]: T }): T;
+>f : <T>(x: { [key: string]: T; }) => T
+>T : T
+>x : { [key: string]: T; }
+>key : string
+>T : T
+>T : T
+
+const a = f(<any>null);
+>a : any
+>f(<any>null) : any
+>f : <T>(x: { [key: string]: T; }) => T
+><any>null : any
+>null : null
+

tests/cases/compiler/inferIndexSignatureFromAny.ts

@@ -0,0 +1,2 @@
+declare function f<T>(x: { [key: string]: T }): T;
+const a = f(<any>null);