gopls/internal/analysis/modernize: fix minmax bug

The matcher for pattern 2 forgot to check that the IfStmt.Else
subtree was nil, leading to unsound fixes.

Updates golang/go#71847

Change-Id: I0919076c1af38012cedf3072ef5d1117e96a64b9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/651375
Reviewed-by: Jonathan Amsterdam <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

gopls/internal/analysis/modernize/minmax.go

@@ -95,7 +95,7 @@ func minmax(pass *analysis.Pass) {
 				})
 			}
 
-		} else if prev, ok := curIfStmt.PrevSibling(); ok && isSimpleAssign(prev.Node()) {
+		} else if prev, ok := curIfStmt.PrevSibling(); ok && isSimpleAssign(prev.Node()) && ifStmt.Else == nil {
 			fassign := prev.Node().(*ast.AssignStmt)
 
 			// Have: lhs0 = rhs0; if a < b { lhs = rhs }

gopls/internal/analysis/modernize/testdata/src/minmax/minmax.go

@@ -103,3 +103,15 @@ func nopeNotAMinimum(x, y int) int {
 	}
 	return y
 }
+
+// Regression test for https://github.com/golang/go/issues/71847#issuecomment-2673491596
+func nopeHasElseBlock(x int) int {
+	y := x
+	// Before, this was erroneously reduced to y = max(x, 0)
+	if y < 0 {
+		y = 0
+	} else {
+		y += 2
+	}
+	return y
+}

gopls/internal/analysis/modernize/testdata/src/minmax/minmax.go.golden

@@ -80,3 +80,15 @@ func nopeNotAMinimum(x, y int) int {
 	}
 	return y
 }
+
+// Regression test for https://github.com/golang/go/issues/71847#issuecomment-2673491596
+func nopeHasElseBlock(x int) int {
+	y := x
+	// Before, this was erroneously reduced to y = max(x, 0)
+	if y < 0 {
+		y = 0
+	} else {
+		y += 2
+	}
+	return y
+}