Fix validating non-square matrix compound multiplication

src/compiler/translator/IntermNode.cpp

@@ -57,8 +57,10 @@ bool ValidateMultiplication(TOperator op, const TType &left, const TType &right)
       case EOpMatrixTimesMatrix:
         return left.getCols() == right.getRows();
       case EOpMatrixTimesMatrixAssign:
-        return left.getCols() == right.getCols() &&
-               left.getRows() == right.getRows();
+          // We need to check two things:
+          // 1. The matrix multiplication step is valid.
+          // 2. The result will have the same number of columns as the lvalue.
+          return left.getCols() == right.getRows() && left.getCols() == right.getCols();
 
       default:
         UNREACHABLE();

src/tests/compiler_tests/MalformedShader_test.cpp

@@ -1572,3 +1572,41 @@ TEST_F(MalformedShaderTest, ESSL300BuiltInFunctionOverload)
         FAIL() << "Shader compilation succeeded, expecting failure " << mInfoLog;
     }
 }
+
+// Multiplying a 4x2 matrix with a 4x2 matrix should not work.
+TEST_F(MalformedShaderTest, CompoundMultiplyMatrixIdenticalNonSquareDimensions)
+{
+    const std::string &shaderString =
+        "#version 300 es\n"
+        "precision mediump float;\n"
+        "out vec4 my_FragColor;\n"
+        "void main()\n"
+        "{\n"
+        "   mat4x2 foo;\n"
+        "   foo *= mat4x2(4.0);\n"
+        "   my_FragColor = vec4(0.0);\n"
+        "}\n";
+    if (compile(shaderString))
+    {
+        FAIL() << "Shader compilation succeeded, expecting failure " << mInfoLog;
+    }
+}
+
+// Multiplying a matrix with 2 columns and 4 rows with a 2x2 matrix should work.
+TEST_F(MalformedShaderTest, CompoundMultiplyMatrixValidNonSquareDimensions)
+{
+    const std::string &shaderString =
+        "#version 300 es\n"
+        "precision mediump float;\n"
+        "out vec4 my_FragColor;\n"
+        "void main()\n"
+        "{\n"
+        "   mat2x4 foo;\n"
+        "   foo *= mat2x2(4.0);\n"
+        "   my_FragColor = vec4(0.0);\n"
+        "}\n";
+    if (!compile(shaderString))
+    {
+        FAIL() << "Shader compilation failed, expecting success " << mInfoLog;
+    }
+}