gopls/internal/cache: reproduce and fix crash on if cond overflow

Through reverse engineering, I was able to reproduce the overflow of
golang/go#72026, and verify the fix of CL 653596.

Along the way, I incidentally reproduced golang/go#66766, which I think
we can safely ignore now that we understand it.

Updates golang/go#72026
Fixes golang/go#66766

Change-Id: I2131d771c13688c1ad47f6bc6285e524fb4c04a1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/654336
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

gopls/internal/cache/check.go

@@ -2005,15 +2005,14 @@ func typeErrorsToDiagnostics(pkg *syntaxPackage, inputs *typeCheckInputs, errs [
 			posn := safetoken.StartPosition(e.Fset, start)
 			if !posn.IsValid() {
 				// All valid positions produced by the type checker should described by
-				// its fileset.
+				// its fileset, yet since type checker errors are associated with
+				// positions in the AST, and AST nodes can overflow the file
+				// (golang/go#48300), we can't rely on this.
 				//
-				// Note: in golang/go#64488, we observed an error that was positioned
-				// over fixed syntax, which overflowed its file. So it's definitely
-				// possible that we get here (it's hard to reason about fixing up the
-				// AST). Nevertheless, it's a bug.
-				if pkg.hasFixedFiles() {
-					bug.Reportf("internal error: type checker error %q outside its Fset (fixed files)", e)
-				} else {
+				// We should fix the parser, but in the meantime type errors are not
+				// significant if there are parse errors, so we can safely ignore this
+				// case.
+				if len(pkg.parseErrors) == 0 {
 					bug.Reportf("internal error: type checker error %q outside its Fset", e)
 				}
 				continue

gopls/internal/cache/parsego/parse.go

@@ -532,6 +532,7 @@ func fixInitStmt(bad *ast.BadExpr, parent ast.Node, tok *token.File, src []byte)
 	if err != nil {
 		return false
 	}
+	assert(end <= len(src), "offset overflow") // golang/go#72026
 	stmtBytes := src[start:end]
 	stmt, err := parseStmt(tok, bad.Pos(), stmtBytes)
 	if err != nil {

gopls/internal/test/integration/completion/fixedbugs_test.go

@@ -38,3 +38,20 @@ package
 		}
 	})
 }
+
+func TestFixInitStatementCrash_Issue72026(t *testing.T) {
+	// This test checks that we don't crash when the if condition overflows the
+	// file (as is possible with a malformed struct type).
+
+	const files = `
+-- go.mod --
+module example.com
+
+go 1.18
+`
+
+	Run(t, files, func(t *testing.T, env *Env) {
+		env.CreateBuffer("p.go", "package p\nfunc _() {\n\tfor i := struct")
+		env.AfterChange()
+	})
+}