More specific inference for constrained 'infer' types in template literal types

src/compiler/checker.ts

@@ -22109,13 +22109,40 @@ namespace ts {
                 sourceEnd.slice(sourceEnd.length - endLen) !== targetEnd.slice(targetEnd.length - endLen);
         }
 
-        function isValidBigIntString(s: string): boolean {
+        /**
+         * Tests whether the provided string can be parsed as a number.
+         * @param s The string to test.
+         * @param roundTripOnly Indicates the resulting number matches the input when converted back to a string.
+         */
+        function isValidNumberString(s: string, roundTripOnly: boolean): boolean {
+            if (s === "") return false;
+            const n = +s;
+            return isFinite(n) && (!roundTripOnly || "" + n === s);
+        }
+
+        /**
+         * @param text a valid bigint string excluding a trailing `n`, but including a possible prefix `-`. Use `isValidBigIntString(text, roundTripOnly)` before calling this function.
+         */
+        function parseBigIntLiteralType(text: string) {
+            const negative = text.startsWith("-");
+            const base10Value = parsePseudoBigInt(`${negative ? text.slice(1) : text}n`);
+            return getBigIntLiteralType({ negative, base10Value });
+        }
+
+        /**
+         * Tests whether the provided string can be parsed as a bigint.
+         * @param s The string to test.
+         * @param roundTripOnly Indicates the resulting bigint matches the input when converted back to a string.
+         */
+        function isValidBigIntString(s: string, roundTripOnly: boolean): boolean {
+            if (s === "") return false;
             const scanner = createScanner(ScriptTarget.ESNext, /*skipTrivia*/ false);
             let success = true;
             scanner.setOnError(() => success = false);
             scanner.setText(s + "n");
             let result = scanner.scan();
-            if (result === SyntaxKind.MinusToken) {
+            const negative = result === SyntaxKind.MinusToken;
+            if (negative) {
                 result = scanner.scan();
             }
             const flags = scanner.getTokenFlags();
@@ -22124,7 +22151,8 @@ namespace ts {
             // * a bigint can be scanned, and that when it is scanned, it is
             // * the full length of the input string (so the scanner is one character beyond the augmented input length)
             // * it does not contain a numeric seperator (the `BigInt` constructor does not accept a numeric seperator in its input)
-            return success && result === SyntaxKind.BigIntLiteral && scanner.getTextPos() === (s.length + 1) && !(flags & TokenFlags.ContainsSeparator);
+            return success && result === SyntaxKind.BigIntLiteral && scanner.getTextPos() === (s.length + 1) && !(flags & TokenFlags.ContainsSeparator)
+                && (!roundTripOnly || s === pseudoBigIntToString({ negative, base10Value: parsePseudoBigInt(scanner.getTokenValue()) }));
         }
 
         function isValidTypeForTemplateLiteralPlaceholder(source: Type, target: Type): boolean {
@@ -22133,8 +22161,8 @@ namespace ts {
             }
             if (source.flags & TypeFlags.StringLiteral) {
                 const value = (source as StringLiteralType).value;
-                return !!(target.flags & TypeFlags.Number && value !== "" && isFinite(+value) ||
-                    target.flags & TypeFlags.BigInt && value !== "" && isValidBigIntString(value) ||
+                return !!(target.flags & TypeFlags.Number && isValidNumberString(value, /*roundTripOnly*/ false) ||
+                    target.flags & TypeFlags.BigInt && isValidBigIntString(value, /*roundTripOnly*/ false) ||
                     target.flags & (TypeFlags.BooleanLiteral | TypeFlags.Nullable) && value === (target as IntrinsicType).intrinsicName);
             }
             if (source.flags & TypeFlags.TemplateLiteral) {
@@ -22712,7 +22740,60 @@ namespace ts {
                 // 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]);
+                        const source = matches ? matches[i] : neverType;
+                        const target = types[i];
+
+                        // If we are inferring from a string literal type to a type variable whose constraint includes one of the
+                        // allowed template literal placeholder types, infer from a literal type corresponding to the constraint.
+                        if (source.flags & TypeFlags.StringLiteral && target.flags & TypeFlags.TypeVariable) {
+                            const inferenceContext = getInferenceInfoForType(target);
+                            const constraint = inferenceContext ? getBaseConstraintOfType(inferenceContext.typeParameter) : undefined;
+                            if (constraint && !isTypeAny(constraint)) {
+                                const constraintTypes = constraint.flags & TypeFlags.Union ? (constraint as UnionType).types : [constraint];
+                                let allTypeFlags: TypeFlags = reduceLeft(constraintTypes, (flags, t) => flags | t.flags, 0 as TypeFlags);
+
+                                // If the constraint contains `string`, we don't need to look for a more preferred type
+                                if (!(allTypeFlags & TypeFlags.String)) {
+                                    const str = (source as StringLiteralType).value;
+
+                                    // If the type contains `number` or a number literal and the string isn't a valid number, exclude numbers
+                                    if (allTypeFlags & TypeFlags.NumberLike && !isValidNumberString(str, /*roundTripOnly*/ true)) {
+                                        allTypeFlags &= ~TypeFlags.NumberLike;
+                                    }
+
+                                    // If the type contains `bigint` or a bigint literal and the string isn't a valid bigint, exclude bigints
+                                    if (allTypeFlags & TypeFlags.BigIntLike && !isValidBigIntString(str, /*roundTripOnly*/ true)) {
+                                        allTypeFlags &= ~TypeFlags.BigIntLike;
+                                    }
+
+                                    // for each type in the constraint, find the highest priority matching type
+                                    const matchingType = reduceLeft(constraintTypes, (left, right) =>
+                                        !(right.flags & allTypeFlags) ? left :
+                                        left.flags & TypeFlags.String ? left : right.flags & TypeFlags.String ? source :
+                                        left.flags & TypeFlags.TemplateLiteral ? left : right.flags & TypeFlags.TemplateLiteral && isTypeMatchedByTemplateLiteralType(source, right as TemplateLiteralType) ? source :
+                                        left.flags & TypeFlags.StringMapping ? left : right.flags & TypeFlags.StringMapping && str === applyStringMapping(right.symbol, str) ? source :
+                                        left.flags & TypeFlags.StringLiteral ? left : right.flags & TypeFlags.StringLiteral && (right as StringLiteralType).value === str ? right :
+                                        left.flags & TypeFlags.Number ? left : right.flags & TypeFlags.Number ? getNumberLiteralType(+str) :
+                                        left.flags & TypeFlags.Enum ? left : right.flags & TypeFlags.Enum ? getNumberLiteralType(+str) :
+                                        left.flags & TypeFlags.NumberLiteral ? left : right.flags & TypeFlags.NumberLiteral && (right as NumberLiteralType).value === +str ? right :
+                                        left.flags & TypeFlags.BigInt ? left : right.flags & TypeFlags.BigInt ? parseBigIntLiteralType(str) :
+                                        left.flags & TypeFlags.BigIntLiteral ? left : right.flags & TypeFlags.BigIntLiteral && pseudoBigIntToString((right as BigIntLiteralType).value) === str ? right :
+                                        left.flags & TypeFlags.Boolean ? left : right.flags & TypeFlags.Boolean ? str === "true" ? trueType : str === "false" ? falseType : booleanType :
+                                        left.flags & TypeFlags.BooleanLiteral ? left : right.flags & TypeFlags.BooleanLiteral && (right as IntrinsicType).intrinsicName === str ? right :
+                                        left.flags & TypeFlags.Undefined ? left : right.flags & TypeFlags.Undefined && (right as IntrinsicType).intrinsicName === str ? right :
+                                        left.flags & TypeFlags.Null ? left : right.flags & TypeFlags.Null && (right as IntrinsicType).intrinsicName === str ? right :
+                                        left,
+                                        neverType as Type);
+
+                                    if (!(matchingType.flags & TypeFlags.Never)) {
+                                        inferFromTypes(matchingType, target);
+                                        continue;
+                                    }
+                                }
+                            }
+                        }
+
+                        inferFromTypes(source, target);
                     }
                 }
             }

src/compiler/types.ts

@@ -5870,7 +5870,7 @@ namespace ts {
         AlwaysStrict                 = 1 << 10,  // Always use strict rules for contravariant inferences
         MaxValue                     = 1 << 11, // Seed for inference priority tracking
 
-        PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof,  // These priorities imply that the resulting type should be a combination of all candidates
+        PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof, // These priorities imply that the resulting type should be a combination of all candidates
         Circularity = -1,  // Inference circularity (value less than all other priorities)
     }