cmd/compile: better error message for language version errors

Fixes #33753.
Updates #31747.

Change-Id: Icc42b23405ead4f7f17b0ffa3611405454b6b271
Reviewed-on: https://go-review.googlesource.com/c/go/+/198491
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/noder.go

@@ -1327,7 +1327,7 @@ func checkLangCompat(lit *syntax.BasicLit) {
 	}
 	// len(s) > 2
 	if strings.Contains(s, "_") {
-		yyerror("underscores in numeric literals only supported as of -lang=go1.13")
+		yyerrorv("go1.13", "underscores in numeric literals")
 		return
 	}
 	if s[0] != '0' {
@@ -1335,15 +1335,15 @@ func checkLangCompat(lit *syntax.BasicLit) {
 	}
 	base := s[1]
 	if base == 'b' || base == 'B' {
-		yyerror("binary literals only supported as of -lang=go1.13")
+		yyerrorv("go1.13", "binary literals")
 		return
 	}
 	if base == 'o' || base == 'O' {
-		yyerror("0o/0O-style octal literals only supported as of -lang=go1.13")
+		yyerrorv("go1.13", "0o/0O-style octal literals")
 		return
 	}
 	if lit.Kind != syntax.IntLit && (base == 'x' || base == 'X') {
-		yyerror("hexadecimal floating-point literals only supported as of -lang=go1.13")
+		yyerrorv("go1.13", "hexadecimal floating-point literals")
 	}
 }
 

src/cmd/compile/internal/gc/subr.go

@@ -154,6 +154,11 @@ func yyerrorl(pos src.XPos, format string, args ...interface{}) {
 	}
 }
 
+func yyerrorv(lang string, format string, args ...interface{}) {
+	what := fmt.Sprintf(format, args...)
+	yyerrorl(lineno, "%s requires %s or later (-lang was set to %s; check go.mod)", what, lang, flag_lang)
+}
+
 func yyerror(format string, args ...interface{}) {
 	yyerrorl(lineno, format, args...)
 }

src/cmd/compile/internal/gc/typecheck.go

@@ -604,7 +604,7 @@ func typecheck1(n *Node, top int) (res *Node) {
 				return n
 			}
 			if t.IsSigned() && !langSupported(1, 13) {
-				yyerror("invalid operation: %v (signed shift count type %v, only supported as of -lang=go1.13)", n, r.Type)
+				yyerrorv("go1.13", "invalid operation: %v (signed shift count type %v)", n, r.Type)
 				n.Type = nil
 				return n
 			}

test/fixedbugs/issue31747.go

@@ -8,11 +8,11 @@ package p
 
 // numeric literals
 const (
-	_ = 1_000 // ERROR "underscores in numeric literals only supported as of -lang=go1.13"
-	_ = 0b111 // ERROR "binary literals only supported as of -lang=go1.13"
-	_ = 0o567 // ERROR "0o/0O-style octal literals only supported as of -lang=go1.13"
+	_ = 1_000 // ERROR "underscores in numeric literals requires go1.13 or later \(-lang was set to go1.12; check go.mod\)"
+	_ = 0b111 // ERROR "binary literals requires go1.13 or later"
+	_ = 0o567 // ERROR "0o/0O-style octal literals requires go1.13 or later"
 	_ = 0xabc // ok
-	_ = 0x0p1 // ERROR "hexadecimal floating-point literals only supported as of -lang=go1.13"
+	_ = 0x0p1 // ERROR "hexadecimal floating-point literals requires go1.13 or later"
 
 	_ = 0B111 // ERROR "binary"
 	_ = 0O567 // ERROR "octal"
@@ -29,6 +29,6 @@ const (
 // signed shift counts
 var (
 	s int
-	_ = 1 << s // ERROR "signed shift count type int, only supported as of -lang=go1.13"
+	_ = 1 << s // ERROR "invalid operation: 1 << s \(signed shift count type int\) requires go1.13 or later"
 	_ = 1 >> s // ERROR "signed shift count"
 )