Improvements to type relation errors

.chronus/changes/type-relation-improvements-2024-8-6-13-13-15.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Improvements to type relation errors: Show stack when it happens in a nested property otherwise show up in the correct location.

packages/compiler/src/core/checker.ts

@@ -5649,7 +5649,7 @@ export function createChecker(program: Program): Checker {
   function checkArgumentAssignable(
     argumentType: Type | Value | IndeterminateEntity,
     parameterType: Entity,
-    diagnosticTarget: DiagnosticTarget
+    diagnosticTarget: Entity | Node
   ): boolean {
     const [valid] = relation.isTypeAssignableTo(argumentType, parameterType, diagnosticTarget);
     if (!valid) {
@@ -7420,7 +7420,7 @@ export function createChecker(program: Program): Checker {
   function checkTypeOfValueMatchConstraint(
     source: Entity,
     constraint: CheckValueConstraint,
-    diagnosticTarget: DiagnosticTarget
+    diagnosticTarget: Entity | Node
   ): boolean {
     const [related, diagnostics] = relation.isTypeAssignableTo(
       source,
@@ -7455,7 +7455,7 @@ export function createChecker(program: Program): Checker {
   function checkTypeAssignable(
     source: Entity | IndeterminateEntity,
     target: Entity,
-    diagnosticTarget: DiagnosticTarget
+    diagnosticTarget: Entity | Node
   ): boolean {
     const [related, diagnostics] = relation.isTypeAssignableTo(source, target, diagnosticTarget);
     if (!related) {
@@ -7464,11 +7464,7 @@ export function createChecker(program: Program): Checker {
     return related;
   }
 
-  function checkValueOfType(
-    source: Value,
-    target: Type,
-    diagnosticTarget: DiagnosticTarget
-  ): boolean {
+  function checkValueOfType(source: Value, target: Type, diagnosticTarget: Entity | Node): boolean {
     const [related, diagnostics] = relation.isValueOfType(source, target, diagnosticTarget);
     if (!related) {
       reportCheckerDiagnostics(diagnostics);

packages/compiler/src/core/messages.ts

@@ -444,8 +444,13 @@ const diagnostics = {
   unassignable: {
     severity: "error",
     messages: {
-      default: paramMessage`Type '${"value"}' is not assignable to type '${"targetType"}'`,
-      withDetails: paramMessage`Type '${"sourceType"}' is not assignable to type '${"targetType"}'\n  ${"details"}`,
+      default: paramMessage`Type '${"sourceType"}' is not assignable to type '${"targetType"}'`,
+    },
+  },
+  "property-unassignable": {
+    severity: "error",
+    messages: {
+      default: paramMessage`Types of property '${"propName"}' are incompatible`,
     },
   },
   "property-required": {