Improvements to type relation errors (#4357)

fix #3291

Changes: 
1. Figure out the most accurate location for the diagnostic
2. If diagnostic target a child node of the base diagnostic target then
emit diagnostic directly there
3. Otherwise emit back at the root(or closest child node) and build
stack of error message

Example the following would now emit the error on a
```ts
 const b = #{ prop: #{a: "abc"}};
 const a: {prop: {}} = b;
```

```
Type '{ prop: { a: "abc" } }' is not assignable to type '{ prop: {} }'
  Type '{ a: "abc" }' is not assignable to type '{}'
    Object value may only specify known properties, and 'a' does not exist in type '{}'.
```

Previously the error would have been in the complete wrong place 
<img width="271" alt="image"
src="https://github.com/user-attachments/assets/c403d1ec-3611-4ad6-87b0-2e0a075dc1c5">

.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": {