Improve type inference for coalesce

src/Analyser/TypeSpecifier.php

@@ -1281,11 +1281,13 @@ private function createForExpr(
 
 		if (
 			$scope !== null
-			&& $context->true()
+			&& !$context->null()
 			&& $expr instanceof Expr\BinaryOp\Coalesce
-			&& $type->isSuperTypeOf($scope->getType($expr->right))->no()
 		) {
-			$expr = $expr->left;
+			$rightIsSuperType = $type->isSuperTypeOf($scope->getType($expr->right));
+			if (($context->true() && $rightIsSuperType->no()) || ($context->false() && $rightIsSuperType->yes())) {
+				$expr = $expr->left;
+			}
 		}
 
 		if (

tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php

@@ -1157,4 +1157,10 @@ public function testBug7156(): void
 		$this->analyse([__DIR__ . '/data/bug-7156.php'], []);
 	}
 
+	public function testBug7973(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->analyse([__DIR__ . '/data/bug-7973.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-7973.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types = 1);
+
+namespace Bug7973;
+
+use DateTime;
+use DateTimeZone;
+use Exception;
+use InvalidArgumentException;
+
+function createFromString(string $timespec): DateTime {
+	try {
+		return new DateTime($timespec, new DateTimeZone('UTC'));
+	} catch (Exception) {
+		throw new InvalidArgumentException(sprintf('Invalid timespec "%s"', $timespec));
+	}
+}
+
+function () {
+	$rows = [
+		['timespec'=>null],
+		['timespec'=>'2020-01-01T01:02:03+08:00'],
+	];
+
+	$result = [];
+	foreach($rows as $row) {
+		$result[] = ($row['timespec'] ?? null) !== null ? createFromString($row['timespec']) : null;
+	}
+};