Fix panic when inferring [...rest, ...T] from tuple shorter than fixed-arity constraint (#3917)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>
Co-authored-by: ahejlsberg <4226954+ahejlsberg@users.noreply.github.com>

Author: 3 people

internal/checker/inference.go

@@ -727,11 +727,13 @@ func (c *Checker) inferFromObjectTypes(n *InferenceState, source *Type, target *
 								impliedArity := constraint.TargetTupleType().fixedLength
 								endIndex := sourceArity - getEndElementCount(target.TargetTupleType(), ElementFlagsFixed)
 								startIndex := endIndex - impliedArity
-								trailingSlice := c.createTupleTypeEx(c.getTypeArguments(source)[startIndex:endIndex], source.TargetTupleType().elementInfos[startIndex:endIndex], false /*readonly*/)
-								if restType := c.getElementTypeOfSliceOfTupleType(source, startLength, endLength+impliedArity, false, false); restType != nil {
-									c.inferFromTypes(n, restType, elementTypes[startLength])
+								if startIndex >= startLength {
+									trailingSlice := c.createTupleTypeEx(c.getTypeArguments(source)[startIndex:endIndex], source.TargetTupleType().elementInfos[startIndex:endIndex], false /*readonly*/)
+									if restType := c.getElementTypeOfSliceOfTupleType(source, startLength, endLength+impliedArity, false, false); restType != nil {
+										c.inferFromTypes(n, restType, elementTypes[startLength])
+									}
+									c.inferFromTypes(n, trailingSlice, elementTypes[startLength+1])
 								}
-								c.inferFromTypes(n, trailingSlice, elementTypes[startLength+1])
 							}
 						}
 					}

testdata/baselines/reference/compiler/inferRestTupleShortSource.errors.txt

@@ -0,0 +1,16 @@
+inferRestTupleShortSource.ts(7,3): error TS2345: Argument of type '[]' is not assignable to parameter of type '[...string[], string]'.
+  Source has 0 element(s) but target requires 1.
+
+
+==== inferRestTupleShortSource.ts (1 errors) ====
+    // Regression test: tsgo panics when inferring [...rest, ...T] from a tuple shorter than fixed-arity constraint
+    
+    function f<T extends [string]>(args: [...string[], ...T]) {
+      // ...
+    }
+    
+    f([])
+      ~~
+!!! error TS2345: Argument of type '[]' is not assignable to parameter of type '[...string[], string]'.
+!!! error TS2345:   Source has 0 element(s) but target requires 1.
+    

testdata/baselines/reference/compiler/inferRestTupleShortSource.js

@@ -0,0 +1,19 @@
+//// [tests/cases/compiler/inferRestTupleShortSource.ts] ////
+
+//// [inferRestTupleShortSource.ts]
+// Regression test: tsgo panics when inferring [...rest, ...T] from a tuple shorter than fixed-arity constraint
+
+function f<T extends [string]>(args: [...string[], ...T]) {
+  // ...
+}
+
+f([])
+
+
+//// [inferRestTupleShortSource.js]
+"use strict";
+// Regression test: tsgo panics when inferring [...rest, ...T] from a tuple shorter than fixed-arity constraint
+function f(args) {
+    // ...
+}
+f([]);

testdata/baselines/reference/compiler/inferRestTupleShortSource.symbols

@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/inferRestTupleShortSource.ts] ////
+
+=== inferRestTupleShortSource.ts ===
+// Regression test: tsgo panics when inferring [...rest, ...T] from a tuple shorter than fixed-arity constraint
+
+function f<T extends [string]>(args: [...string[], ...T]) {
+>f : Symbol(f, Decl(inferRestTupleShortSource.ts, 0, 0))
+>T : Symbol(T, Decl(inferRestTupleShortSource.ts, 2, 11))
+>args : Symbol(args, Decl(inferRestTupleShortSource.ts, 2, 31))
+>T : Symbol(T, Decl(inferRestTupleShortSource.ts, 2, 11))
+
+  // ...
+}
+
+f([])
+>f : Symbol(f, Decl(inferRestTupleShortSource.ts, 0, 0))
+

testdata/baselines/reference/compiler/inferRestTupleShortSource.types

@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/inferRestTupleShortSource.ts] ////
+
+=== inferRestTupleShortSource.ts ===
+// Regression test: tsgo panics when inferring [...rest, ...T] from a tuple shorter than fixed-arity constraint
+
+function f<T extends [string]>(args: [...string[], ...T]) {
+>f : <T extends [string]>(args: [...string[], ...T]) => void
+>args : [...string[], ...T]
+
+  // ...
+}
+
+f([])
+>f([]) : void
+>f : <T extends [string]>(args: [...string[], ...T]) => void
+>[] : []
+

testdata/tests/cases/compiler/inferRestTupleShortSource.ts

@@ -0,0 +1,9 @@
+// @strict: true
+
+// Regression test: tsgo panics when inferring [...rest, ...T] from a tuple shorter than fixed-arity constraint
+
+function f<T extends [string]>(args: [...string[], ...T]) {
+  // ...
+}
+
+f([])