Make never inferences with template literal types only in special cases

src/compiler/checker.ts

@@ -21890,8 +21890,16 @@ namespace ts {
             function inferToTemplateLiteralType(source: Type, target: TemplateLiteralType) {
                 const matches = inferTypesFromTemplateLiteralType(source, target);
                 const types = target.types;
-                for (let i = 0; i < types.length; i++) {
-                    inferFromTypes(matches ? matches[i] : neverType, types[i]);
+                // When the target template literal contains only placeholders (meaning that inference is intended to extract
+                // single characters and remainder strings) and inference fails to produce matches, we want to infer 'never' for
+                // each placeholder such that instantiation with the inferred value(s) produces 'never', a type for which an
+                // assignment check will fail. If we make no inferences, we'll likely end up with the constraint 'string' which,
+                // upon instantiation, would collapse all the placeholders to just 'string', and an assignment check might
+                // succeed. That would be a pointless and confusing outcome.
+                if (matches || every(target.texts, s => s.length === 0)) {
+                    for (let i = 0; i < types.length; i++) {
+                        inferFromTypes(matches ? matches[i] : neverType, types[i]);
+                    }
                 }
             }
 

tests/baselines/reference/templateLiteralTypes1.types

@@ -419,7 +419,7 @@ type Foo<T> = T extends `*${infer S}*` ? S : never;
 >Foo : Foo<T>
 
 type TF1 = Foo<any>;      // never
->TF1 : never
+>TF1 : string
 
 type TF2 = Foo<string>;   // never
 >TF2 : never

tests/baselines/reference/templateLiteralTypes3.errors.txt

@@ -1,4 +1,4 @@
-tests/cases/conformance/types/literal/templateLiteralTypes3.ts(20,19): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+tests/cases/conformance/types/literal/templateLiteralTypes3.ts(20,19): error TS2345: Argument of type '"hello"' is not assignable to parameter of type '`*${string}*`'.
 tests/cases/conformance/types/literal/templateLiteralTypes3.ts(57,5): error TS2322: Type '"hello"' is not assignable to type '`*${string}*`'.
 tests/cases/conformance/types/literal/templateLiteralTypes3.ts(69,5): error TS2322: Type '"123"' is not assignable to type '`*${number}*`'.
 tests/cases/conformance/types/literal/templateLiteralTypes3.ts(71,5): error TS2322: Type '"**123**"' is not assignable to type '`*${number}*`'.
@@ -29,7 +29,7 @@ tests/cases/conformance/types/literal/templateLiteralTypes3.ts(74,5): error TS23
     function f1<T extends string>(s: string, n: number, b: boolean, t: T) {
         let x1 = foo1('hello');  // Error
                       ~~~~~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"hello"' is not assignable to parameter of type '`*${string}*`'.
         let x2 = foo1('*hello*');
         let x3 = foo1('**hello**');
         let x4 = foo1(`*${s}*` as const);
@@ -138,4 +138,12 @@ tests/cases/conformance/types/literal/templateLiteralTypes3.ts(74,5): error TS23
     interface ITest<P extends Prefixes, E extends AllPrefixData = PrefixData<P>> {
         blah: string;
     }
+
+    // Repro from #45906
+
+    type Schema = { a: { b: { c: number } } };
+
+    declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;
+
+    chain("a");
 

tests/baselines/reference/templateLiteralTypes3.js

@@ -116,6 +116,14 @@ type PrefixData<P extends Prefixes> = `${P}:baz`;
 interface ITest<P extends Prefixes, E extends AllPrefixData = PrefixData<P>> {
     blah: string;
 }
+
+// Repro from #45906
+
+type Schema = { a: { b: { c: number } } };
+
+declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;
+
+chain("a");
 
 
 //// [templateLiteralTypes3.js]
@@ -168,6 +176,7 @@ var templated1 = value1 + " abc";
 // Type '`${string} abc`' is not assignable to type '`${string} ${string}`'.
 var value2 = "abc";
 var templated2 = value2 + " abc";
+chain("a");
 
 
 //// [templateLiteralTypes3.d.ts]
@@ -216,3 +225,11 @@ declare type PrefixData<P extends Prefixes> = `${P}:baz`;
 interface ITest<P extends Prefixes, E extends AllPrefixData = PrefixData<P>> {
     blah: string;
 }
+declare type Schema = {
+    a: {
+        b: {
+            c: number;
+        };
+    };
+};
+declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;

tests/baselines/reference/templateLiteralTypes3.symbols

@@ -385,3 +385,23 @@ interface ITest<P extends Prefixes, E extends AllPrefixData = PrefixData<P>> {
 >blah : Symbol(ITest.blah, Decl(templateLiteralTypes3.ts, 114, 78))
 }
 
+// Repro from #45906
+
+type Schema = { a: { b: { c: number } } };
+>Schema : Symbol(Schema, Decl(templateLiteralTypes3.ts, 116, 1))
+>a : Symbol(a, Decl(templateLiteralTypes3.ts, 120, 15))
+>b : Symbol(b, Decl(templateLiteralTypes3.ts, 120, 20))
+>c : Symbol(c, Decl(templateLiteralTypes3.ts, 120, 25))
+
+declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;
+>chain : Symbol(chain, Decl(templateLiteralTypes3.ts, 120, 42))
+>F : Symbol(F, Decl(templateLiteralTypes3.ts, 122, 23))
+>Schema : Symbol(Schema, Decl(templateLiteralTypes3.ts, 116, 1))
+>field : Symbol(field, Decl(templateLiteralTypes3.ts, 122, 47))
+>F : Symbol(F, Decl(templateLiteralTypes3.ts, 122, 23))
+>F : Symbol(F, Decl(templateLiteralTypes3.ts, 122, 23))
+>F : Symbol(F, Decl(templateLiteralTypes3.ts, 122, 23))
+
+chain("a");
+>chain : Symbol(chain, Decl(templateLiteralTypes3.ts, 120, 42))
+

tests/baselines/reference/templateLiteralTypes3.types

@@ -49,8 +49,8 @@ function f1<T extends string>(s: string, n: number, b: boolean, t: T) {
 >t : T
 
     let x1 = foo1('hello');  // Error
->x1 : never
->foo1('hello') : never
+>x1 : string
+>foo1('hello') : string
 >foo1 : <V extends string>(arg: `*${V}*`) => V
 >'hello' : "hello"
 
@@ -379,3 +379,20 @@ interface ITest<P extends Prefixes, E extends AllPrefixData = PrefixData<P>> {
 >blah : string
 }
 
+// Repro from #45906
+
+type Schema = { a: { b: { c: number } } };
+>Schema : Schema
+>a : { b: {    c: number;}; }
+>b : { c: number; }
+>c : number
+
+declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;
+>chain : <F extends "a">(field: F | `${F}.${F}`) => void
+>field : F | `${F}.${F}`
+
+chain("a");
+>chain("a") : void
+>chain : <F extends "a">(field: F | `${F}.${F}`) => void
+>"a" : "a"
+

tests/cases/conformance/types/literal/templateLiteralTypes3.ts

@@ -118,3 +118,11 @@ type PrefixData<P extends Prefixes> = `${P}:baz`;
 interface ITest<P extends Prefixes, E extends AllPrefixData = PrefixData<P>> {
     blah: string;
 }
+
+// Repro from #45906
+
+type Schema = { a: { b: { c: number } } };
+
+declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;
+
+chain("a");