cmd/compile: match Go 1.17 compiler error messages more closely

When being used by the compiler, fix up types2 error messages to be more
like Go 1.17 compiler errors. In particular:

  - add information about which method is missing when a type is not
    assignable/convertible/etc. to an interface.

  - add information about any existing method which has the same name,
    but wrong type.

  - add extra hint in the case that the source or destination type is a
    pointer to an interface, rather than an interface.

  - add extra hint "need type assertion" in the case that the source is
    an interface that is implemented by the destination.

  - the following change in the CL stack also adds information about any
    existing method with a different name that only differs in case.

Include much of the new logic in a new common function
(*Checker).missingMethodReason().

types2 still adds a little more information in some cases then the Go
1.17 compiler. For example, it typically says "(value of type T)",
rather than "(type T)", where "value" could also be "constant",
"variable", etc.

I kept the types2 error messages almost all the same when types2 is not
used by the compiler. The only change (to reduce amount of compatibility
code) was to change "M method" phrasing in one case to "method M"
phrasing in one error message (which is the phrasing it uses in all
other cases). That is the reason that there are a few small changes in
types2/testdata/check/*.src.

Added new test test/fixedbugs/issue48471.go to test that the added
information is appearing correctly.

Also adjusted the pattern matching in a bunch of other
test/fixedbugs/*.go, now that types2 is producing error messages closer
to Go 1.17. Was able to remove a couple test files from the types2
exception list in run.go.

Updated #48471

Change-Id: I8af1eae6eb8a5541d8ea20b66f494e2e795e1956
Reviewed-on: https://go-review.googlesource.com/c/go/+/363436
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/types2/assignments.go

@@ -85,7 +85,11 @@ func (check *Checker) assignment(x *operand, T Type, context string) {
 	reason := ""
 	if ok, _ := x.assignableTo(check, T, &reason); !ok {
 		if check.conf.CompilerErrorMessages {
-			check.errorf(x, "incompatible type: cannot use %s as %s value", x, T)
+			if reason != "" {
+				check.errorf(x, "cannot use %s as type %s in %s:\n\t%s", x, T, context, reason)
+			} else {
+				check.errorf(x, "cannot use %s as type %s in %s", x, T, context)
+			}
 		} else {
 			if reason != "" {
 				check.errorf(x, "cannot use %s as %s value in %s: %s", x, T, context, reason)

src/cmd/compile/internal/types2/conversions.go

@@ -69,9 +69,19 @@ func (check *Checker) conversion(x *operand, T Type) {
 
 	if !ok {
 		var err error_
-		err.errorf(x, "cannot convert %s to %s", x, T)
+		if check.conf.CompilerErrorMessages {
-		if cause != "" {
+			if cause != "" {
-			err.errorf(nopos, cause)
+				// Add colon at end of line if we have a following cause.
+				err.errorf(x, "cannot convert %s to type %s:", x, T)
+				err.errorf(nopos, cause)
+			} else {
+				err.errorf(x, "cannot convert %s to type %s", x, T)
+			}
+		} else {
+			err.errorf(x, "cannot convert %s to %s", x, T)
+			if cause != "" {
+				err.errorf(nopos, cause)
+			}
 		}
 		check.report(&err)
 		x.mode = invalid

src/cmd/compile/internal/types2/expr.go

@@ -1626,25 +1626,20 @@ func (check *Checker) typeAssertion(e syntax.Expr, x *operand, xtyp *Interface,
 		return
 	}
 
-	var msg string
-	if wrongType != nil {
-		if Identical(method.typ, wrongType.typ) {
-			msg = fmt.Sprintf("%s method has pointer receiver", method.name)
-		} else {
-			msg = fmt.Sprintf("wrong type for method %s: have %s, want %s", method.name, wrongType.typ, method.typ)
-		}
-	} else {
-		msg = fmt.Sprintf("missing %s method", method.name)
-	}
-
 	var err error_
+	var msg string
 	if typeSwitch {
 		err.errorf(e.Pos(), "impossible type switch case: %s", e)
-		err.errorf(nopos, "%s cannot have dynamic type %s (%s)", x, T, msg)
+		msg = check.sprintf("%s cannot have dynamic type %s %s", x, T,
+			check.missingMethodReason(T, x.typ, method, wrongType))
+
 	} else {
 		err.errorf(e.Pos(), "impossible type assertion: %s", e)
-		err.errorf(nopos, "%s does not implement %s (%s)", T, x.typ, msg)
+		msg = check.sprintf("%s does not implement %s %s", T, x.typ,
+			check.missingMethodReason(T, x.typ, method, wrongType))
+
 	}
+	err.errorf(nopos, msg)
 	check.report(&err)
 }
 

src/cmd/compile/internal/types2/lookup.go

@@ -6,6 +6,11 @@
 
 package types2
 
+import (
+	"fmt"
+	"strings"
+)
+
 // Internal use of LookupFieldOrMethod: If the obj result is a method
 // associated with a concrete (non-interface) type, the method's signature
 // may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
@@ -401,6 +406,59 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 	return
 }
 
+// missingMethodReason returns a string giving the detailed reason for a missing method m,
+// where m is missing from V, but required by T. It puts the reason in parentheses,
+// and may include more have/want info after that. If non-nil, wrongType is a relevant
+// method that matches in some way. It may have the correct name, but wrong type, or
+// it may have a pointer receiver.
+func (check *Checker) missingMethodReason(V, T Type, m, wrongType *Func) string {
+	var r string
+	var mname string
+	if check.conf.CompilerErrorMessages {
+		mname = m.Name() + " method"
+	} else {
+		mname = "method " + m.Name()
+	}
+	if wrongType != nil {
+		if Identical(m.typ, wrongType.typ) {
+			if m.Name() == wrongType.Name() {
+				r = fmt.Sprintf("(%s has pointer receiver)", mname)
+			} else {
+				r = fmt.Sprintf("(missing %s)\n\t\thave %s^^%s\n\t\twant %s^^%s",
+					mname, wrongType.Name(), wrongType.typ, m.Name(), m.typ)
+			}
+		} else {
+			if check.conf.CompilerErrorMessages {
+				r = fmt.Sprintf("(wrong type for %s)\n\t\thave %s^^%s\n\t\twant %s^^%s",
+					mname, wrongType.Name(), wrongType.typ, m.Name(), m.typ)
+			} else {
+				r = fmt.Sprintf("(wrong type for %s: have %s, want %s)",
+					mname, wrongType.typ, m.typ)
+			}
+		}
+		// This is a hack to print the function type without the leading
+		// 'func' keyword in the have/want printouts. We could change to have
+		// an extra formatting option for types2.Type that doesn't print out
+		// 'func'.
+		r = strings.Replace(r, "^^func", "", -1)
+	} else if IsInterface(T) {
+		if isInterfacePtr(V) {
+			r = fmt.Sprintf("(%s is pointer to interface, not interface)", V)
+		}
+	} else if isInterfacePtr(T) {
+		r = fmt.Sprintf("(%s is pointer to interface, not interface)", T)
+	}
+	if r == "" {
+		r = fmt.Sprintf("(missing %s)", mname)
+	}
+	return r
+}
+
+func isInterfacePtr(T Type) bool {
+	p, _ := under(T).(*Pointer)
+	return p != nil && IsInterface(p.base)
+}
+
 // assertableTo reports whether a value of type V can be asserted to have type T.
 // It returns (nil, false) as affirmative answer. Otherwise it returns a missing
 // method required by V and whether it is missing or just has the wrong type.

src/cmd/compile/internal/types2/operand.go

@@ -289,18 +289,21 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 
 	// T is an interface type and x implements T and T is not a type parameter
 	if Ti, ok := Tu.(*Interface); ok {
-		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
+		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* !Implements(V, Ti) */ {
 			if reason != nil {
-				// TODO(gri) the error messages here should follow the style in Checker.typeAssertion (factor!)
+				if check.conf.CompilerErrorMessages {
-				if wrongType != nil {
+					*reason = check.sprintf("%s does not implement %s %s", x.typ, T,
-					if Identical(m.typ, wrongType.typ) {
+						check.missingMethodReason(x.typ, T, m, wrongType))
-						*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)
-					} else {
-						*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
-					}
-
 				} else {
-					*reason = "missing method " + m.Name()
+					if wrongType != nil {
+						if Identical(m.typ, wrongType.typ) {
+							*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)
+						} else {
+							*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
+						}
+					} else {
+						*reason = "missing method " + m.Name()
+					}
 				}
 			}
 			return false, _InvalidIfaceAssign
@@ -308,6 +311,24 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 		return true, 0
 	}
 
+	// Provide extra detail in compiler error messages in some cases when T is
+	// not an interface.
+	if check != nil && check.conf.CompilerErrorMessages {
+		if isInterfacePtr(Tu) {
+			*reason = check.sprintf("%s does not implement %s (%s is pointer to interface, not interface)", x.typ, T, T)
+			return false, _InvalidIfaceAssign
+		}
+		if Vi, _ := Vu.(*Interface); Vi != nil {
+			if m, _ := check.missingMethod(T, Vi, true); m == nil {
+				// T implements Vi, so give hint about type assertion.
+				if reason != nil {
+					*reason = check.sprintf("need type assertion")
+				}
+				return false, _IncompatibleAssign
+			}
+		}
+	}
+
 	// x is a bidirectional channel value, T is a channel
 	// type, x's type V and T have identical element types,
 	// and at least one of V or T is not a named type.

src/cmd/compile/internal/types2/testdata/check/expr3.src

@@ -459,9 +459,9 @@ func type_asserts() {
 
 	var t I
 	_ = t /* ERROR "use of .* outside type switch" */ .(type)
-	_ = t /* ERROR "m method has pointer receiver" */ .(T)
+	_ = t /* ERROR "method m has pointer receiver" */ .(T)
 	_ = t.(*T)
-	_ = t /* ERROR "missing m method" */ .(T1)
+	_ = t /* ERROR "missing method m" */ .(T1)
 	_ = t /* ERROR "wrong type for method m" */ .(T2)
 	_ = t /* STRICT "wrong type for method m" */ .(I2) // only an error in strict mode (issue 8561)
 

src/cmd/compile/internal/types2/testdata/check/issues.src

@@ -132,7 +132,7 @@ func issue10260() {
 
 	var x I1
 	x = T1 /* ERROR cannot use .*: missing method foo \(foo has pointer receiver\) */ {}
-	_ = x. /* ERROR impossible type assertion: x.\(T1\)\n\tT1 does not implement I1 \(foo method has pointer receiver\) */ (T1)
+	_ = x. /* ERROR impossible type assertion: x.\(T1\)\n\tT1 does not implement I1 \(method foo has pointer receiver\) */ (T1)
 
 	T1{}.foo /* ERROR cannot call pointer method foo on T1 */ ()
 	x.Foo /* ERROR "x.Foo undefined \(type I1 has no field or method Foo, but does have foo\)" */ ()

src/cmd/compile/internal/types2/testdata/check/stmt0.src

@@ -715,7 +715,7 @@ func typeswitches() {
 	var t I
 	switch t.(type) {
 	case T:
-	case T1 /* ERROR "missing m method" */ :
+	case T1 /* ERROR "missing method m" */ :
 	case T2 /* ERROR "wrong type for method m" */ :
 	case I2 /* STRICT "wrong type for method m" */ : // only an error in strict mode (issue 8561)
 	}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue49005.go

@@ -23,12 +23,12 @@ type T2 interface{ M() }
 
 func F2() T2
 
-var _ = F2(). /* ERROR impossible type assertion: F2\(\).\(\*X2\)\n\t\*X2 does not implement T2 \(missing M method\) */ (*X2)
+var _ = F2(). /* ERROR impossible type assertion: F2\(\).\(\*X2\)\n\t\*X2 does not implement T2 \(missing method M\) */ (*X2)
 
 type X2 struct{}
 
 func _() {
 	switch F2().(type) {
-	case * /* ERROR impossible type switch case: \*X2\n\tF2\(\) \(value of type T2\) cannot have dynamic type \*X2 \(missing M method\) */ X2:
+	case * /* ERROR impossible type switch case: \*X2\n\tF2\(\) \(value of type T2\) cannot have dynamic type \*X2 \(missing method M\) */ X2:
 	}
 }

test/alias2.go

@@ -46,8 +46,8 @@ var _ A0 = T0{}
 var _ T0 = A0{}
 
 // But aliases and original types cannot be used with new types based on them.
-var _ N0 = T0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|incompatible type"
+var _ N0 = T0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|cannot use T0{} \(value of type T0\) as type N0 in variable declaration"
-var _ N0 = A0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|incompatible type"
+var _ N0 = A0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|cannot use A0{} \(value of type T0\) as type N0 in variable declaration"
 
 var _ A5 = Value{}
 
@@ -82,10 +82,10 @@ func _() {
 	var _ A0 = T0{}
 	var _ T0 = A0{}
 
-	var _ N0 = T0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|incompatible type"
+	var _ N0 = T0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|cannot use T0{} \(value of type T0\) as type N0 in variable declaration"
-	var _ N0 = A0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|incompatible type"
+	var _ N0 = A0{} // ERROR "cannot use T0{} \(type T0\) as type N0 in assignment|cannot use A0{} \(value of type T0\) as type N0 in variable declaration"
 
-	var _ A5 = Value{} // ERROR "cannot use reflect\.Value{} \(type reflect.Value\) as type A5 in assignment|incompatible type"
+	var _ A5 = Value{} // ERROR "cannot use reflect\.Value{} \(type reflect.Value\) as type A5 in assignment|cannot use Value{} \(value of type reflect.Value\) as type A5 in variable declaration"
 }
 
 // Invalid type alias declarations.

test/append1.go

@@ -17,6 +17,6 @@ func main() {
 	_ = append(s...)       // ERROR "cannot use ... on first argument|not enough arguments in call to append"
 	_ = append(s, 2, s...) // ERROR "too many arguments to append|too many arguments in call to append"
 
-	_ = append(s, make([]int, 0))     // ERROR "cannot use make.* as type int in append|cannot use make.* as int value"
+	_ = append(s, make([]int, 0))     // ERROR "cannot use make.* as type int in append|cannot use make.* \(value of type \[\]int\) as type int in argument to append"
 	_ = append(s, make([]int, -1)...) // ERROR "negative len argument in make|index -1.* must not be negative"
 }

test/ddd1.go

@@ -19,7 +19,7 @@ var (
 	_ = sum(1.0, 2.0)
 	_ = sum(1.5)      // ERROR "1\.5 .untyped float constant. as int|integer"
 	_ = sum("hello")  // ERROR ".hello. (.untyped string constant. as int|.type untyped string. as type int)|incompatible"
-	_ = sum([]int{1}) // ERROR "\[\]int{...}.*as type int|incompatible"
+	_ = sum([]int{1}) // ERROR "\[\]int{.*}.*as type int"
 )
 
 func sum3(int, int, int) int { return 0 }

test/fixedbugs/bug389.go

@@ -9,4 +9,4 @@ package foo
 
 func fn(a float32) {}
 
-var f func(arg int) = fn  // ERROR "cannot use fn .type func.float32.. as type func.int. in assignment|different parameter types|incompatible type"
+var f func(arg int) = fn // ERROR "cannot use fn .type func.float32.. as type func.int. in assignment|different parameter types|cannot use fn .*type func.*float32.. as type func.*int. in variable declaration"

test/fixedbugs/issue41247.go

@@ -7,5 +7,5 @@
 package p
 
 func f() [2]int {
-	return [...]int{2: 0} // ERROR "cannot use \[\.\.\.\]int{...} \(type \[3\]int\)|incompatible type"
+	return [...]int{2: 0} // ERROR "cannot use \[\.\.\.\]int{.*} \(.*type \[3\]int\)"
 }

test/fixedbugs/issue48471.go

@@ -0,0 +1,41 @@
+// errorcheck
+
+// Copyright 2021 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
+
+type I interface{ M(int) }
+
+type T struct{}
+
+type T2 struct{}
+
+func (*T2) m(int)
+
+type T3 struct{}
+
+func (*T3) M(string) {}
+
+type T4 struct{}
+
+func (*T4) M(int)
+
+func f(I)
+
+func g() {
+	f(new(T)) // ERROR "cannot use new\(T\) \(.*type \*T\) as type I in argument to f:\n\t\*T does not implement I \(missing M method\)"
+	var i I
+	i = new(T)    // ERROR "cannot use new\(T\) \(.*type \*T\) as type I in assignment:\n\t\*T does not implement I \(missing M method\)"
+	i = I(new(T)) // ERROR "cannot convert new\(T\) \(.*type \*T\) to type I:\n\t\*T does not implement I \(missing M method\)"
+	i = new(T2)   // ERROR "cannot use new\(T2\) \(.*type \*T2\) as type I in assignment:\n\t\*T2 does not implement I \(missing M method\)"
+	i = new(T3)   // ERROR "cannot use new\(T3\) \(.*type \*T3\) as type I in assignment:\n\t\*T3 does not implement I \(wrong type for M method\)\n\t\thave M\(string\)\n\t\twant M\(int\)"
+	i = T4{}      // ERROR "cannot use T4\{\} \(.*type T4\) as type I in assignment:\n\tT4 does not implement I \(M method has pointer receiver\)"
+	i = new(I)    // ERROR "cannot use new\(I\) \(.*type \*I\) as type I in assignment:\n\t\*I does not implement I \(\*I is pointer to interface, not interface\)"
+	_ = i.(*T2)   // ERROR "impossible type assertion: i.\(\*T2\)\n\t\*T2 does not implement I \(missing M method\)"
+	_ = i.(*T3)   // ERROR "impossible type assertion: i.\(\*T3\)\n\t\*T3 does not implement I \(wrong type for M method\)\n\t\thave M\(string\)\n\t\twant M\(int\)"
+	var t *T4
+	t = i // ERROR "cannot use i \(variable of type I\) as type \*T4 in assignment:\n\tneed type assertion"
+	_ = i
+}

test/fixedbugs/issue6572.go

@@ -17,6 +17,6 @@ func bar() (T, string, T) { // ERROR "undefined"
 func main() {
 	var x, y, z int
 	x, y = foo()
-	x, y, z = bar() // ERROR "cannot (use type|assign) string|incompatible type"
+	x, y, z = bar() // ERROR "cannot (use type|assign|use.*type) string|"
 	_, _, _ = x, y, z
 }

test/fixedbugs/issue9521.go

@@ -13,6 +13,6 @@ func f() (_, _ []int)         { return }
 func g() (x []int, y float64) { return }
 
 func main() {
-	_ = append(f()) // ERROR "cannot use \[\]int value as type int in append|incompatible type"
+	_ = append(f()) // ERROR "cannot use \[\]int value as type int in append|cannot use.*type \[\]int.*to append"
-	_ = append(g()) // ERROR "cannot use float64 value as type int in append|incompatible type"
+	_ = append(g()) // ERROR "cannot use float64 value as type int in append|cannot use.*type float64.*to append"
 }

test/interface/explicit.go

@@ -38,7 +38,7 @@ var e E
 
 func main() {
 	e = t // ok
-	t = e // ERROR "need explicit|need type assertion|incompatible type"
+	t = e // ERROR "need explicit|need type assertion"
 
 	// neither of these can work,
 	// because i has an extra method

test/run.go

@@ -2125,14 +2125,11 @@ var types2Failures = setOf(
 	"shift1.go",       // issue #42989
 	"typecheck.go",    // invalid function is not causing errors when called
 
-	"interface/private.go", // types2 phrases errors differently (doesn't use non-spec "private" term)
-
 	"fixedbugs/bug176.go", // types2 reports all errors (pref: types2)
 	"fixedbugs/bug195.go", // types2 reports slightly different (but correct) bugs
 	"fixedbugs/bug228.go", // types2 doesn't run when there are syntax errors
 	"fixedbugs/bug231.go", // types2 bug? (same error reported twice)
 	"fixedbugs/bug255.go", // types2 reports extra errors
-	"fixedbugs/bug374.go", // types2 reports extra errors
 	"fixedbugs/bug388.go", // types2 not run due to syntax errors
 	"fixedbugs/bug412.go", // types2 produces a follow-on error