diff --git a/src/cmd/compile/internal/gc/const.go b/src/cmd/compile/internal/gc/const.go
index c5b8d816c6..569cab4390 100644
--- a/src/cmd/compile/internal/gc/const.go
+++ b/src/cmd/compile/internal/gc/const.go
@@ -413,16 +413,20 @@ func convlit1(n *Node, t *types.Type, explicit bool, reuse canReuseNode) *Node {
 	return n
 
 bad:
+	reportErr := false
 	if !n.Diag() {
-		if !t.Broke() {
-			yyerror("cannot convert %L to type %v", n, t)
-		}
+		reportErr = !t.Broke()
 		n.SetDiag(true)
 	}
 
 	if n.Type.IsUntyped() {
 		n = defaultlitreuse(n, nil, reuse)
 	}
+
+	if reportErr {
+		yyerror("cannot convert %L to type %v", n, t)
+	}
+
 	return n
 }
 
diff --git a/test/fixedbugs/issue21979.go b/test/fixedbugs/issue21979.go
new file mode 100644
index 0000000000..ac7c88f195
--- /dev/null
+++ b/test/fixedbugs/issue21979.go
@@ -0,0 +1,75 @@
+// errorcheck
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f() {
+	// old error: "(type untyped string)"
+	_ = bool("") // ERROR "cannot convert .. \(type string\) to type bool"
+
+	// old error: "(type untyped number)"
+	_ = bool(1) // ERROR "cannot convert 1 \(type int\) to type bool"
+
+	// old error: "(type untyped number)"
+	_ = bool(1.0) // ERROR "cannot convert 1 \(type float64\) to type bool"
+
+	// old error: "(type untyped number)"
+	_ = bool(-4 + 2i) // ERROR "cannot convert -4 \+ 2i \(type complex128\) to type bool"
+
+	// old error: "(type untyped bool)"
+	_ = string(true) // ERROR "cannot convert true \(type bool\) to type string"
+
+	_ = string(-1)
+
+	// old error: "(type untyped number)"
+	_ = string(1.0) // ERROR "cannot convert 1 \(type float64\) to type string"
+
+	// old error: "(type untyped number)"
+	_ = string(-4 + 2i) // ERROR "cannot convert -4 \+ 2i \(type complex128\) to type string"
+
+	// old error: "(type untyped string)"
+	_ = int("") // ERROR "cannot convert .. \(type string\) to type int"
+
+	// old error: "(type untyped bool)"
+	_ = int(true) // ERROR "cannot convert true \(type bool\) to type int"
+
+	_ = int(-1)
+	_ = int(1)
+	_ = int(1.0)
+	_ = int(-4 + 2i) // ERROR "truncated to integer"
+
+	// old error: "(type untyped string)"
+	_ = uint("") // ERROR "cannot convert .. \(type string\) to type uint"
+
+	// old error: "(type untyped bool)"
+	_ = uint(true) // ERROR "cannot convert true \(type bool\) to type uint"
+
+	_ = uint(-1) // ERROR "constant -1 overflows uint"
+	_ = uint(1)
+	_ = uint(1.0)
+	_ = uint(-4 + 2i) // ERROR "constant -4 overflows uint" "truncated to integer"
+
+	// old error: "(type untyped string)"
+	_ = float64("") // ERROR "cannot convert .. \(type string\) to type float64"
+
+	// old error: "(type untyped bool)"
+	_ = float64(true) // ERROR "cannot convert true \(type bool\) to type float64"
+
+	_ = float64(-1)
+	_ = float64(1)
+	_ = float64(1.0)
+	_ = float64(-4 + 2i) // ERROR "truncated to real"
+
+	// old error: "(type untyped string)"
+	_ = complex128("") // ERROR "cannot convert .. \(type string\) to type complex128"
+
+	// old error: "(type untyped bool)"
+	_ = complex128(true) // ERROR "cannot convert true \(type bool\) to type complex128"
+
+	_ = complex128(-1)
+	_ = complex128(1)
+	_ = complex128(1.0)
+}