go/types, types2: only report version errors if new(expr) is ok otherwise

If new(expr) is used before Go 1.26, don't report version errors if there
are other problems with the expression.

While at it, implement multiple missing type checks for new(expr) and
add corresponding test cases that were missed in CL 704935 (tests for
no value expressions, generic types, untyped nil).

Reorganize/rename builtins0.go tests for new to match existing test case
patterns again.

Fixes #75986.
For #45624.

Change-Id: I39e5516d3f8d191cc390a4d8b9911c312bbb177c
Reviewed-on: https://go-review.googlesource.com/c/go/+/713241
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

3 files changed, 59 insertions, 16 deletions

diff --git a/src/internal/types/testdata/check/builtins0.go b/src/internal/types/testdata/check/builtins0.go
index e326b92ac7..9b99a890ac 100644
--- a/src/internal/types/testdata/check/builtins0.go
+++ b/src/internal/types/testdata/check/builtins0.go
@@ -609,36 +609,38 @@ func min2() {
 	)
 }
 
-func newInvalid() {
-	f2 := func() (x, y int) { return }
 
+func new1() {
 	_ = new()     // ERROR "not enough arguments"
 	_ = new(1, 2) // ERROR "too many arguments"
+	_ = new(unsafe /* ERROR "use of package unsafe not in selector" */ )
+
+	_ = new(struct{ x, y int })
+	p := new(float64)
+	q := new(*float64)
+	_ = *p == **q
+
+	type G[P any] struct{}
+	_ = new(G[int])
+	_ = new(G /* ERROR "cannot use generic type G without instantiation" */ )
+
 	new /* ERROR "not used" */ (int)
 	_ = &new /* ERROR "cannot take address" */ (int)
 	_ = new(int... /* ERROR "invalid use of ..." */)
 	_ = new(f0 /* ERROR "f0() (no value) used as value or type" */ ())
 	_ = new(len /* ERROR "len (built-in) must be called" */)
 	_ = new(1 /* ERROR "argument to new (overflows)" */ << 70)
-	_ = new(f2 /* ERRORx "multiple-value.*in single-value context" */ ())
 }
 
-// new(T)
-func newType() {
-	_ = new(struct{ x, y int })
-
-	p := new(float64)
-	q := new(*float64)
-	_ = *p == **q
-}
-
-// new(expr), added in go1.26
-func newExpr() {
-	f1 := func() (x []int) { return }
+func new2() {
+	// new(expr), added in go1.26
+	f1 := func() []int { panic(0) }
+	f2 := func() (int, int) { panic(0) }
 	var (
 		_ *[]int        = new(f1())
 		_ *func() []int = new(f1)
 		_ *bool         = new(false)
+		_ *bool         = new(1 < 2)
 		_ *int          = new(123)
 		_ *float64      = new(1.0)
 		_ *uint         = new(uint(3))
@@ -647,6 +649,14 @@ func newExpr() {
 		_ *struct{}     = new(struct{}{})
 		_ *any          = new(any)
 
+		_ = new(f2 /* ERRORx "multiple-value.*in single-value context" */ ())
+		_ = new(1 << /* ERROR "constant shift overflow" */ 1000)
+		_ = new(1e10000 /* ERROR "cannot use 1e10000 (untyped float constant 1e+10000) as float64 value in argument to new (overflows)" */ )
+		_ = new(nil /* ERROR "use of untyped nil in argument to new" */ )
+		_ = new(comparable /* ERROR "cannot use type comparable outside a type constraint" */ )
+		_ = new(new /* ERROR "new (built-in) must be called" */ )
+		_ = new(panic /* ERROR "panic(0) (no value) used as value or type" */ (0))
+
 		// from issue 43125
 		_ = new(-1)
 		_ = new(1 + 1)
diff --git a/src/internal/types/testdata/check/go1_25.go b/src/internal/types/testdata/check/go1_25.go
index b2ace83343..3799bc02b4 100644
--- a/src/internal/types/testdata/check/go1_25.go
+++ b/src/internal/types/testdata/check/go1_25.go
@@ -10,4 +10,9 @@
 
 package p
 
-var _ = new /* ERROR "new(expr) requires go1.26 or later" */ (123)
+func f(x int) {
+	_ = new /* ERROR "new(123) requires go1.26 or later" */ (123)
+	_ = new /* ERROR "new(x) requires go1.26 or later" */ (x)
+	_ = new /* ERROR "new(f) requires go1.26 or later" */ (f)
+	_ = new /* ERROR "new(1 < 2) requires go1.26 or later" */ (1 < 2)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue75986.go b/src/internal/types/testdata/fixedbugs/issue75986.go
new file mode 100644
index 0000000000..b2b1509e03
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue75986.go
@@ -0,0 +1,28 @@
+// -lang=go1.25
+
+// Copyright 2025 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
+
+import "strings"
+
+type T int
+type G[P any] struct{}
+
+var x T
+
+// Verify that we don't get a version error when there's another error present in new(expr).
+
+func f() {
+	_ = new(U /* ERROR "undefined: U" */)
+	_ = new(strings.BUILDER /* ERROR "undefined: strings.BUILDER (but have Builder)" */)
+	_ = new(T)      // ok
+	_ = new(G[int]) // ok
+	_ = new(G /* ERROR "cannot use generic type G without instantiation" */)
+	_ = new(nil /* ERROR "use of untyped nil in argument to new" */)
+	_ = new(comparable /* ERROR "cannot use type comparable outside a type constraint" */)
+	_ = new(new /* ERROR "new (built-in) must be called" */)
+	_ = new(panic /* ERROR "panic(0) (no value) used as value or type" */ (0))
+}