Keep a give-up statement's first-attempt diagnostics

packages/compiler/src/frontend/lowering/lower-stmts.ts

@@ -759,6 +759,11 @@ export function lowerStmts(lowerer: Lowerer, stmts: readonly ts.Statement[]): Ir
       // out of tdzPredeclared; a retry after a later declarator degrades
       // must initialize the same box again.
       const tdzBefore = new Map(lowerer.ctx.tdzPredeclared);
+      // The first attempt's diagnostics: a statement that still fails
+      // after degrading reports them, since a retry does not repeat what
+      // lowered once (a poisoned generic instance) and its own diagnostics
+      // can miss the failure's cause.
+      let firstAttempt: ScrDiagnostic[] | null = null;
       try {
         for (let attempt = 0; ; attempt++) {
           const hits = new Set<number>();
@@ -773,7 +778,12 @@ export function lowerStmts(lowerer: Lowerer, stmts: readonly ts.Statement[]): Ir
             // global degrades that type and the statement lowers again.
             // A declaration this statement flushed inside a probe (where
             // no retry runs) can retry here too.
-            if (attempt >= MAX_DEGRADE_RETRIES || deferFences) throw e;
+            firstAttempt ??= lowerer.diags.slice(diagsBefore);
+            const giveUp = (): unknown => {
+              if (attempt > 0) lowerer.restoreFirstAttemptDiags(diagsBefore, firstAttempt!);
+              return e;
+            };
+            if (attempt >= MAX_DEGRADE_RETRIES || deferFences) throw giveUp();
             const slot = lowerer.degradeAtCommit(e, STATEMENT_SLOTS);
             // Pushed-down types the failed attempt mapped widen to the
             // whole slot before anything else is given up.
@@ -782,7 +792,7 @@ export function lowerStmts(lowerer: Lowerer, stmts: readonly ts.Statement[]): Ir
               !lowerer.reopenFlushedAtCommit(diagsBefore) &&
               !lowerer.widenPartials(hits)
             )
-              throw e;
+              throw giveUp();
             // A failed attempt adds nothing to `out` but the TDZ boxes
             // forward captures declared (they stay declared), so the
             // retry appends after them.

packages/compiler/src/frontend/lowering/lowerer.ts

@@ -9853,6 +9853,19 @@ export class Lowerer {
     this.diags.push(...kept);
   }
 
+  /** A unit that still fails after its retries commits its first
+   * attempt's own diagnostics (`first`, taken before the first
+   * degradation) in place of the last attempt's. The diagnostics the
+   * retries carry (dropAttemptDiags) stay as they are now: a declaration
+   * reopened since may have registered. */
+  restoreFirstAttemptDiags(diagsBefore: number, first: readonly ScrDiagnostic[]): void {
+    const carried = (d: ScrDiagnostic): boolean =>
+      d.rejects !== undefined || this.loweredOnceDiags.has(d);
+    const kept = this.diags.slice(diagsBefore).filter(carried);
+    this.diags.length = diagsBefore;
+    this.diags.push(...first.filter((d) => !carried(d)), ...kept);
+  }
+
   /** Lowers one unit whose body-level poison skips it (a generic
    * instance), with localized failure's commit point: a degradable type
    * failure at one of its declaration slots degrades the type and the unit