diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/a.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/a.go
index 81c732001a..c5d8a2d789 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/a.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/a.go
@@ -6,22 +6,22 @@ package undeclared
 
 func x() int {
 	var z int
-	z = y // want "undeclared name: y"
+	z = y // want "(undeclared name|undefined): y"
 
-	if z == m { // want "undeclared name: m"
+	if z == m { // want "(undeclared name|undefined): m"
 		z = 1
 	}
 
 	if z == 1 {
 		z = 1
-	} else if z == n+1 { // want "undeclared name: n"
+	} else if z == n+1 { // want "(undeclared name|undefined): n"
 		z = 1
 	}
 
 	switch z {
 	case 10:
 		z = 1
-	case a: // want "undeclared name: a"
+	case a: // want "(undeclared name|undefined): a"
 		z = 1
 	}
 	return z
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/channels.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/channels.go
index ecf00ecfc2..76c7ba685e 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/channels.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/channels.go
@@ -5,7 +5,7 @@
 package undeclared
 
 func channels(s string) {
-	undefinedChannels(c()) // want "undeclared name: undefinedChannels"
+	undefinedChannels(c()) // want "(undeclared name|undefined): undefinedChannels"
 }
 
 func c() (<-chan string, chan string) {
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/consecutive_params.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/consecutive_params.go
index ab7b2ba5c1..73beace102 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/consecutive_params.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/consecutive_params.go
@@ -6,5 +6,5 @@ package undeclared
 
 func consecutiveParams() {
 	var s string
-	undefinedConsecutiveParams(s, s) // want "undeclared name: undefinedConsecutiveParams"
+	undefinedConsecutiveParams(s, s) // want "(undeclared name|undefined): undefinedConsecutiveParams"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/error_param.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/error_param.go
index 341a9d2a45..5de9254112 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/error_param.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/error_param.go
@@ -6,5 +6,5 @@ package undeclared
 
 func errorParam() {
 	var err error
-	undefinedErrorParam(err) // want "undeclared name: undefinedErrorParam"
+	undefinedErrorParam(err) // want "(undeclared name|undefined): undefinedErrorParam"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/literals.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/literals.go
index ab82463d00..c62174ec94 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/literals.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/literals.go
@@ -7,5 +7,5 @@ package undeclared
 type T struct{}
 
 func literals() {
-	undefinedLiterals("hey compiler", T{}, &T{}) // want "undeclared name: undefinedLiterals"
+	undefinedLiterals("hey compiler", T{}, &T{}) // want "(undeclared name|undefined): undefinedLiterals"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/operation.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/operation.go
index 9a543821ee..9396da4bd9 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/operation.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/operation.go
@@ -7,5 +7,5 @@ package undeclared
 import "time"
 
 func operation() {
-	undefinedOperation(10 * time.Second) // want "undeclared name: undefinedOperation"
+	undefinedOperation(10 * time.Second) // want "(undeclared name|undefined): undefinedOperation"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/selector.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/selector.go
index 9ed09a27f2..a4ed290d46 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/selector.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/selector.go
@@ -6,5 +6,5 @@ package undeclared
 
 func selector() {
 	m := map[int]bool{}
-	undefinedSelector(m[1]) // want "undeclared name: undefinedSelector"
+	undefinedSelector(m[1]) // want "(undeclared name|undefined): undefinedSelector"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/slice.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/slice.go
index d741c68f68..5cde299add 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/slice.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/slice.go
@@ -5,5 +5,5 @@
 package undeclared
 
 func slice() {
-	undefinedSlice([]int{1, 2}) // want "undeclared name: undefinedSlice"
+	undefinedSlice([]int{1, 2}) // want "(undeclared name|undefined): undefinedSlice"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/tuple.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/tuple.go
index 3148e8f4d4..9e91c59c25 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/tuple.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/tuple.go
@@ -5,7 +5,7 @@
 package undeclared
 
 func tuple() {
-	undefinedTuple(b()) // want "undeclared name: undefinedTuple"
+	undefinedTuple(b()) // want "(undeclared name|undefined): undefinedTuple"
 }
 
 func b() (string, error) {
diff --git a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/unique_params.go b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/unique_params.go
index 98f77a43cd..5b4241425e 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/unique_params.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/testdata/src/a/unique_params.go
@@ -7,5 +7,5 @@ package undeclared
 func uniqueArguments() {
 	var s string
 	var i int
-	undefinedUniqueArguments(s, i, s) // want "undeclared name: undefinedUniqueArguments"
+	undefinedUniqueArguments(s, i, s) // want "(undeclared name|undefined): undefinedUniqueArguments"
 }
diff --git a/gopls/internal/lsp/analysis/undeclaredname/undeclared.go b/gopls/internal/lsp/analysis/undeclaredname/undeclared.go
index faa14091ae..c06ae3538c 100644
--- a/gopls/internal/lsp/analysis/undeclaredname/undeclared.go
+++ b/gopls/internal/lsp/analysis/undeclaredname/undeclared.go
@@ -45,7 +45,7 @@ var Analyzer = &analysis.Analyzer{
 	RunDespiteErrors: true,
 }
 
-const undeclaredNamePrefix = "undeclared name: "
+var undeclaredNamePrefixes = []string{"undeclared name: ", "undefined: "}
 
 func run(pass *analysis.Pass) (interface{}, error) {
 	for _, err := range analysisinternal.GetTypeErrors(pass) {
@@ -55,10 +55,16 @@ func run(pass *analysis.Pass) (interface{}, error) {
 }
 
 func runForError(pass *analysis.Pass, err types.Error) {
-	if !strings.HasPrefix(err.Msg, undeclaredNamePrefix) {
+	var name string
+	for _, prefix := range undeclaredNamePrefixes {
+		if !strings.HasPrefix(err.Msg, prefix) {
+			continue
+		}
+		name = strings.TrimPrefix(err.Msg, prefix)
+	}
+	if name == "" {
 		return
 	}
-	name := strings.TrimPrefix(err.Msg, undeclaredNamePrefix)
 	var file *ast.File
 	for _, f := range pass.Files {
 		if f.Pos() <= err.Pos && err.Pos < f.End() {
diff --git a/gopls/internal/lsp/lsp_test.go b/gopls/internal/lsp/lsp_test.go
index bdf8a6b499..269fdaf3fb 100644
--- a/gopls/internal/lsp/lsp_test.go
+++ b/gopls/internal/lsp/lsp_test.go
@@ -513,7 +513,7 @@ func (r *runner) SuggestedFix(t *testing.T, spn span.Span, actionKinds []tests.S
 			break
 		}
 	}
-	codeActionKinds := []protocol.CodeActionKind{}
+	var codeActionKinds []protocol.CodeActionKind
 	for _, k := range actionKinds {
 		codeActionKinds = append(codeActionKinds, protocol.CodeActionKind(k.ActionKind))
 	}
@@ -541,12 +541,11 @@ func (r *runner) SuggestedFix(t *testing.T, spn span.Span, actionKinds []tests.S
 
 	}
 	if len(actions) != expectedActions {
-		// Hack: We assume that we only get one code action per range.
-		var cmds []string
+		var summaries []string
 		for _, a := range actions {
-			cmds = append(cmds, fmt.Sprintf("%s (%s)", a.Command, a.Title))
+			summaries = append(summaries, fmt.Sprintf("%q (%s)", a.Title, a.Kind))
 		}
-		t.Fatalf("unexpected number of code actions, want %d, got %d: %v", expectedActions, len(actions), cmds)
+		t.Fatalf("CodeAction(...): got %d code actions (%v), want %d", len(actions), summaries, expectedActions)
 	}
 	action := actions[0]
 	var match bool
@@ -557,7 +556,7 @@ func (r *runner) SuggestedFix(t *testing.T, spn span.Span, actionKinds []tests.S
 		}
 	}
 	if !match {
-		t.Fatalf("unexpected kind for code action %s, expected one of %v, got %v", action.Title, codeActionKinds, action.Kind)
+		t.Fatalf("unexpected kind for code action %s, got %v, want one of %v", action.Title, action.Kind, codeActionKinds)
 	}
 	var res map[span.URI]string
 	if cmd := action.Command; cmd != nil {
diff --git a/gopls/internal/lsp/protocol/span.go b/gopls/internal/lsp/protocol/span.go
index 23721ee703..58601a6b34 100644
--- a/gopls/internal/lsp/protocol/span.go
+++ b/gopls/internal/lsp/protocol/span.go
@@ -186,6 +186,18 @@ func Intersect(a, b Range) bool {
 		(a.End.Line == b.Start.Line) && a.End.Character < b.Start.Character)
 }
 
+// Format implements fmt.Formatter.
+//
+// Note: Formatter is implemented instead of Stringer (presumably) for
+// performance reasons, though it is not clear that it matters in practice.
 func (r Range) Format(f fmt.State, _ rune) {
-	fmt.Fprintf(f, "%v:%v-%v:%v", r.Start.Line, r.Start.Character, r.End.Line, r.End.Character)
+	fmt.Fprintf(f, "%v-%v", r.Start, r.End)
+}
+
+// Format implements fmt.Formatter.
+//
+// See Range.Format for discussion of why the Formatter interface is
+// implemented rather than Stringer.
+func (p Position) Format(f fmt.State, _ rune) {
+	fmt.Fprintf(f, "%v:%v", p.Line, p.Character)
 }
diff --git a/gopls/internal/lsp/testdata/bad/bad1.go b/gopls/internal/lsp/testdata/bad/bad1.go
index 512f2d9869..04472feae4 100644
--- a/gopls/internal/lsp/testdata/bad/bad1.go
+++ b/gopls/internal/lsp/testdata/bad/bad1.go
@@ -1,3 +1,4 @@
+//go:build go1.11
 // +build go1.11
 
 package bad
@@ -5,7 +6,7 @@ package bad
 // See #36637
 type stateFunc func() stateFunc //@item(stateFunc, "stateFunc", "func() stateFunc", "type")
 
-var a unknown //@item(global_a, "a", "unknown", "var"),diag("unknown", "compiler", "undeclared name: unknown", "error")
+var a unknown //@item(global_a, "a", "unknown", "var"),diag("unknown", "compiler", "(undeclared name|undefined): unknown", "error")
 
 func random() int { //@item(random, "random", "func() int", "func")
 	//@complete("", global_a, bob, random, random2, random3, stateFunc, stuff)
@@ -14,8 +15,8 @@ func random() int { //@item(random, "random", "func() int", "func")
 
 func random2(y int) int { //@item(random2, "random2", "func(y int) int", "func"),item(bad_y_param, "y", "int", "var")
 	x := 6       //@item(x, "x", "int", "var"),diag("x", "compiler", "x declared but not used", "error")
-	var q blah   //@item(q, "q", "blah", "var"),diag("q", "compiler", "q declared but not used", "error"),diag("blah", "compiler", "undeclared name: blah", "error")
-	var t **blob //@item(t, "t", "**blob", "var"),diag("t", "compiler", "t declared but not used", "error"),diag("blob", "compiler", "undeclared name: blob", "error")
+	var q blah   //@item(q, "q", "blah", "var"),diag("q", "compiler", "q declared but not used", "error"),diag("blah", "compiler", "(undeclared name|undefined): blah", "error")
+	var t **blob //@item(t, "t", "**blob", "var"),diag("t", "compiler", "t declared but not used", "error"),diag("blob", "compiler", "(undeclared name|undefined): blob", "error")
 	//@complete("", q, t, x, bad_y_param, global_a, bob, random, random2, random3, stateFunc, stuff)
 
 	return y
@@ -24,10 +25,10 @@ func random2(y int) int { //@item(random2, "random2", "func(y int) int", "func")
 func random3(y ...int) { //@item(random3, "random3", "func(y ...int)", "func"),item(y_variadic_param, "y", "[]int", "var")
 	//@complete("", y_variadic_param, global_a, bob, random, random2, random3, stateFunc, stuff)
 
-	var ch chan (favType1)   //@item(ch, "ch", "chan (favType1)", "var"),diag("ch", "compiler", "ch declared but not used", "error"),diag("favType1", "compiler", "undeclared name: favType1", "error")
-	var m map[keyType]int    //@item(m, "m", "map[keyType]int", "var"),diag("m", "compiler", "m declared but not used", "error"),diag("keyType", "compiler", "undeclared name: keyType", "error")
-	var arr []favType2       //@item(arr, "arr", "[]favType2", "var"),diag("arr", "compiler", "arr declared but not used", "error"),diag("favType2", "compiler", "undeclared name: favType2", "error")
-	var fn1 func() badResult //@item(fn1, "fn1", "func() badResult", "var"),diag("fn1", "compiler", "fn1 declared but not used", "error"),diag("badResult", "compiler", "undeclared name: badResult", "error")
-	var fn2 func(badParam)   //@item(fn2, "fn2", "func(badParam)", "var"),diag("fn2", "compiler", "fn2 declared but not used", "error"),diag("badParam", "compiler", "undeclared name: badParam", "error")
+	var ch chan (favType1)   //@item(ch, "ch", "chan (favType1)", "var"),diag("ch", "compiler", "ch declared but not used", "error"),diag("favType1", "compiler", "(undeclared name|undefined): favType1", "error")
+	var m map[keyType]int    //@item(m, "m", "map[keyType]int", "var"),diag("m", "compiler", "m declared but not used", "error"),diag("keyType", "compiler", "(undeclared name|undefined): keyType", "error")
+	var arr []favType2       //@item(arr, "arr", "[]favType2", "var"),diag("arr", "compiler", "arr declared but not used", "error"),diag("favType2", "compiler", "(undeclared name|undefined): favType2", "error")
+	var fn1 func() badResult //@item(fn1, "fn1", "func() badResult", "var"),diag("fn1", "compiler", "fn1 declared but not used", "error"),diag("badResult", "compiler", "(undeclared name|undefined): badResult", "error")
+	var fn2 func(badParam)   //@item(fn2, "fn2", "func(badParam)", "var"),diag("fn2", "compiler", "fn2 declared but not used", "error"),diag("badParam", "compiler", "(undeclared name|undefined): badParam", "error")
 	//@complete("", arr, ch, fn1, fn2, m, y_variadic_param, global_a, bob, random, random2, random3, stateFunc, stuff)
 }
diff --git a/gopls/internal/lsp/testdata/godef/a/a_x_test.go b/gopls/internal/lsp/testdata/godef/a/a_x_test.go
index 4631eba2c0..f166f05508 100644
--- a/gopls/internal/lsp/testdata/godef/a/a_x_test.go
+++ b/gopls/internal/lsp/testdata/godef/a/a_x_test.go
@@ -5,5 +5,5 @@ import (
 )
 
 func TestA2(t *testing.T) { //@TestA2,godef(TestA2, TestA2)
-	Nonexistant() //@diag("Nonexistant", "compiler", "undeclared name: Nonexistant", "error")
+	Nonexistant() //@diag("Nonexistant", "compiler", "(undeclared name|undefined): Nonexistant", "error")
 }
diff --git a/gopls/internal/lsp/testdata/undeclared/var.go b/gopls/internal/lsp/testdata/undeclared/var.go
index e27a733d94..3fda582ce1 100644
--- a/gopls/internal/lsp/testdata/undeclared/var.go
+++ b/gopls/internal/lsp/testdata/undeclared/var.go
@@ -1,14 +1,14 @@
 package undeclared
 
 func m() int {
-	z, _ := 1+y, 11 //@diag("y", "compiler", "undeclared name: y", "error"),suggestedfix("y", "quickfix", "")
+	z, _ := 1+y, 11 //@diag("y", "compiler", "(undeclared name|undefined): y", "error"),suggestedfix("y", "quickfix", "")
 	if 100 < 90 {
 		z = 1
-	} else if 100 > n+2 { //@diag("n", "compiler", "undeclared name: n", "error"),suggestedfix("n", "quickfix", "")
+	} else if 100 > n+2 { //@diag("n", "compiler", "(undeclared name|undefined): n", "error"),suggestedfix("n", "quickfix", "")
 		z = 4
 	}
-	for i < 200 { //@diag("i", "compiler", "undeclared name: i", "error"),suggestedfix("i", "quickfix", "")
+	for i < 200 { //@diag("i", "compiler", "(undeclared name|undefined): i", "error"),suggestedfix("i", "quickfix", "")
 	}
-	r() //@diag("r", "compiler", "undeclared name: r", "error")
+	r() //@diag("r", "compiler", "(undeclared name|undefined): r", "error")
 	return z
 }
diff --git a/gopls/internal/lsp/testdata/undeclared/var.go.golden b/gopls/internal/lsp/testdata/undeclared/var.go.golden
index a266df7c0c..de5cbb42fb 100644
--- a/gopls/internal/lsp/testdata/undeclared/var.go.golden
+++ b/gopls/internal/lsp/testdata/undeclared/var.go.golden
@@ -2,16 +2,16 @@
 package undeclared
 
 func m() int {
-	z, _ := 1+y, 11 //@diag("y", "compiler", "undeclared name: y", "error"),suggestedfix("y", "quickfix", "")
+	z, _ := 1+y, 11 //@diag("y", "compiler", "(undeclared name|undefined): y", "error"),suggestedfix("y", "quickfix", "")
 	if 100 < 90 {
 		z = 1
-	} else if 100 > n+2 { //@diag("n", "compiler", "undeclared name: n", "error"),suggestedfix("n", "quickfix", "")
+	} else if 100 > n+2 { //@diag("n", "compiler", "(undeclared name|undefined): n", "error"),suggestedfix("n", "quickfix", "")
 		z = 4
 	}
 	i := 
-	for i < 200 { //@diag("i", "compiler", "undeclared name: i", "error"),suggestedfix("i", "quickfix", "")
+	for i < 200 { //@diag("i", "compiler", "(undeclared name|undefined): i", "error"),suggestedfix("i", "quickfix", "")
 	}
-	r() //@diag("r", "compiler", "undeclared name: r", "error")
+	r() //@diag("r", "compiler", "(undeclared name|undefined): r", "error")
 	return z
 }
 
@@ -20,15 +20,15 @@ package undeclared
 
 func m() int {
 	y := 
-	z, _ := 1+y, 11 //@diag("y", "compiler", "undeclared name: y", "error"),suggestedfix("y", "quickfix", "")
+	z, _ := 1+y, 11 //@diag("y", "compiler", "(undeclared name|undefined): y", "error"),suggestedfix("y", "quickfix", "")
 	if 100 < 90 {
 		z = 1
-	} else if 100 > n+2 { //@diag("n", "compiler", "undeclared name: n", "error"),suggestedfix("n", "quickfix", "")
+	} else if 100 > n+2 { //@diag("n", "compiler", "(undeclared name|undefined): n", "error"),suggestedfix("n", "quickfix", "")
 		z = 4
 	}
-	for i < 200 { //@diag("i", "compiler", "undeclared name: i", "error"),suggestedfix("i", "quickfix", "")
+	for i < 200 { //@diag("i", "compiler", "(undeclared name|undefined): i", "error"),suggestedfix("i", "quickfix", "")
 	}
-	r() //@diag("r", "compiler", "undeclared name: r", "error")
+	r() //@diag("r", "compiler", "(undeclared name|undefined): r", "error")
 	return z
 }
 
@@ -36,16 +36,16 @@ func m() int {
 package undeclared
 
 func m() int {
-	z, _ := 1+y, 11 //@diag("y", "compiler", "undeclared name: y", "error"),suggestedfix("y", "quickfix", "")
+	z, _ := 1+y, 11 //@diag("y", "compiler", "(undeclared name|undefined): y", "error"),suggestedfix("y", "quickfix", "")
 	n := 
 	if 100 < 90 {
 		z = 1
-	} else if 100 > n+2 { //@diag("n", "compiler", "undeclared name: n", "error"),suggestedfix("n", "quickfix", "")
+	} else if 100 > n+2 { //@diag("n", "compiler", "(undeclared name|undefined): n", "error"),suggestedfix("n", "quickfix", "")
 		z = 4
 	}
-	for i < 200 { //@diag("i", "compiler", "undeclared name: i", "error"),suggestedfix("i", "quickfix", "")
+	for i < 200 { //@diag("i", "compiler", "(undeclared name|undefined): i", "error"),suggestedfix("i", "quickfix", "")
 	}
-	r() //@diag("r", "compiler", "undeclared name: r", "error")
+	r() //@diag("r", "compiler", "(undeclared name|undefined): r", "error")
 	return z
 }
 
diff --git a/gopls/internal/lsp/tests/tests.go b/gopls/internal/lsp/tests/tests.go
index 689fcca94f..253e28aaf9 100644
--- a/gopls/internal/lsp/tests/tests.go
+++ b/gopls/internal/lsp/tests/tests.go
@@ -1404,6 +1404,8 @@ func uriName(uri span.URI) string {
 	return filepath.Base(strings.TrimSuffix(uri.Filename(), ".go"))
 }
 
+// TODO(golang/go#54845): improve the formatting here to match standard
+// line:column position formatting.
 func SpanName(spn span.Span) string {
 	return fmt.Sprintf("%v_%v_%v", uriName(spn.URI()), spn.Start().Line(), spn.Start().Column())
 }
diff --git a/gopls/internal/lsp/tests/util.go b/gopls/internal/lsp/tests/util.go
index 8a08b044b5..6f2d01cd8d 100644
--- a/gopls/internal/lsp/tests/util.go
+++ b/gopls/internal/lsp/tests/util.go
@@ -86,21 +86,20 @@ func DiffDiagnostics(uri span.URI, want, got []*source.Diagnostic) string {
 	}
 	for i, w := range want {
 		g := got[i]
-		if match, err := regexp.MatchString(w.Message, g.Message); err != nil {
-			return summarizeDiagnostics(i, uri, want, got, "invalid regular expression %q: %v", w.Message, err)
-
-		} else if !match {
-			return summarizeDiagnostics(i, uri, want, got, "got Message %q, want match for pattern %q", g.Message, w.Message)
-		}
-		if w.Severity != g.Severity {
-			return summarizeDiagnostics(i, uri, want, got, "got Severity %v, want %v", g.Severity, w.Severity)
-		}
-		if w.Source != g.Source {
-			return summarizeDiagnostics(i, uri, want, got, "got Source %v, want %v", g.Source, w.Source)
-		}
 		if !rangeOverlaps(g.Range, w.Range) {
 			return summarizeDiagnostics(i, uri, want, got, "got Range %v, want overlap with %v", g.Range, w.Range)
 		}
+		if match, err := regexp.MatchString(w.Message, g.Message); err != nil {
+			return summarizeDiagnostics(i, uri, want, got, "%s: invalid regular expression %q: %v", w.Range.Start, w.Message, err)
+		} else if !match {
+			return summarizeDiagnostics(i, uri, want, got, "%s: got Message %q, want match for pattern %q", g.Range.Start, g.Message, w.Message)
+		}
+		if w.Severity != g.Severity {
+			return summarizeDiagnostics(i, uri, want, got, "%s: got Severity %v, want %v", g.Range.Start, g.Severity, w.Severity)
+		}
+		if w.Source != g.Source {
+			return summarizeDiagnostics(i, uri, want, got, "%s: got Source %v, want %v", g.Range.Start, g.Source, w.Source)
+		}
 	}
 	return ""
 }